- Email: [email protected]
Fruit thinning in ‘Conference’ pear grown under deficit irrigation: Implications for fruit quality at harvest and after cold storage
Scientia Horticulturae 129 (2011) 64–70
Contents lists available at ScienceDirect
Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti
Fruit thinning in ‘Conference’ pear grown under deficit irrigation: Implications for fruit quality at harvest and after cold storage Gerardo Lopez a,∗ , Christian Larrigaudière b , Joan Girona a , M. Hossein Behboudian c , Jordi Marsal a a
Irrigation Technology, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), Centre UdL-IRTA, Rovira Roure 191, 25198 Lleida, Spain Postharvest, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), Centre UdL-IRTA, Rovira Roure 191, 25198 Lleida, Spain c Institute of Natural Resources, Massey University, Private Bag 11122, Palmerston North, New Zealand b
a r t i c l e
i n f o
Article history: Received 13 September 2010 Received in revised form 28 February 2011 Accepted 4 March 2011 Keywords: Ethylene production Drought Pyrus communis Water stress Weight loss in storage
a b s t r a c t Fruit thinning in pear is feasible for mitigation of water stress effects. However, it is not well known how fruit quality at harvest and after cold storage is affected by pre-harvest water stress. Even less is known about the effects of fruit thinning on quality under these circumstances. To elucidate these, we applied deficit irrigation (DI) and fruit thinning treatments to ‘Conference’ pear over the growing seasons of 2008 and 2009. At the onset of Stage II (80 and 67 days before harvest in 2008 and 2009, respectively), two irrigation treatments were applied: full irrigation (FI) and DI. FI trees received 100% of crop evapotranspiration (ETc). DI trees received no irrigation during the first three weeks of Stage II to induce water stress, but then received 20% of ETc to ensure tree survival. From bud-break until the onset of Stage II and during post-harvest, FI and DI trees received 100% of ETc. Each irrigation treatment received two thinning levels: no thinning leaving commercial crop load (∼180 fruits tree−1 ), and handthinning at the onset of Stage II leaving a light crop load (∼85 fruits tree−1 ). Under commercial crop loads, DI trees were moderately water-stressed and this had some positive effects on fruit quality. DI increased fruit firmness (FF), soluble solids concentrations (SSC) and acidity at harvest while no changes were observed in fruit maturity (based on ethylene production). Differences in FF and acidity at harvest between FI and DI fruit were maintained during cold storage. DI also reduced fruit weight loss during storage. But fruit size was reduced under DI. Fruit thinning under DI resulted in better fruit composition with no detrimental effect on fresh-market yield compared to un-thinned fruit. Fruit size at harvest and SSC values after five months of cold storage were higher in fruit from thinned trees than fruit from unthinned trees. Fruit thinning increased fruit ethylene production, indicating advanced maturity. This may lead to earlier harvest which is desirable in years with impending drought. Fruit thinning is therefore a useful technique to enhance pear marketability under water shortage. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Water shortage is becoming increasingly frequent in fruit producing areas with Mediterranean climates. The major pearproducing areas in Spain are not exempt from this problem and several restrictions on irrigation have been experienced in the past few years. These restrictions are usually imposed during midsummer. Water stress therefore develops when pears are in their cell-expansion growth period (Stage II). Water stress in Stage II reduces the potential of achieving commercial fruit size at harvest (Marsal et al., 2008, 2010). Size is an important quality attribute for fresh-market pear. Fruit thinning has been proposed as a contingency plan for reducing water-stress, partially compensating for
∗ Corresponding author. E-mail address: [email protected] (G. Lopez). 0304-4238/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2011.03.007
the negative effects on fruit size (Mpelasoka et al., 2001; Lopez et al., 2006; Marsal et al., 2008, 2010; Intrigliolo and Castel, 2010). The benefit of fruit thinning on pear growth was not solely related to a reduction in fruit competition for photoassimilates. It has been also associated with improvements in tree water status (Marsal et al., 2008). Although pricing depends on fruit size, consumer acceptance depends on eating quality that should be maintained after a period of cold storage. It is not known how pear quality responds to fruit thinning under water limited conditions in Stage II. We hypothesised that fruit from trees with a lower crop load could have a better quality than fruit from trees with a normal crop load. Fruit thinning has generally resulted in higher firmness, soluble solids concentration (SSC) and acidity in apple (Wünsche and Ferguson, 2005) and in higher SSC in peach (Crisosto et al., 1994) and plum (Intrigliolo and Castel, 2010). Gaps of knowledge also exist in how pear quality responds to pre-harvest water stress, especially for
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
fruit cold-stored after harvest. Although pear was one of the first fruit crops undergoing research on deficit irrigation, information on how fruit quality is affected is scant compared to other crops such as apple and peach (Behboudian et al., 2011; Gelly et al., 2003, 2004). We therefore studied, over two growing seasons, the combined effects of pre-harvest water stress and fruit thinning on important fruit quality attributes in pear, both at harvest and after cold storage. We expected that our findings will enable growers to adopt suitable fruit thinning strategies for production of better quality fruit under water limited conditions. Besides being the most popular pear cultivar in Spain, ‘Conference’ was used because it is exposed to the increasingly frequent episodes of water shortage in midsummer and is kept in cold storage for several months. 2. Materials and methods 2.1. Pear orchard The experiment was conducted over two years (2008–2009) in a ‘Conference’ pear (Pyrus communis L.) orchard located at the IRTA-Estaciò Experimental de Lleida (41◦ 37 N; 0◦ 52 E; 260 m a.s.l.); Spain. The orchard was planted in 1999 with 4.0 m between rows and 1.6 m within rows. The trees were grafted onto dwarfing quince rootstock (M–A). The orchard was managed according to local commercial practices, including fruit thinning when commercial crop load exceeded 200 fruit per tree. 2.2. Phenological stages of ‘Conference’ pear Pear growth consists of two linear stages: Stage I and Stage II (Bain, 1961). Stage I is characterised by slow growth, lasting for approximately 30 days in ‘Conference’ pear. Fruit growth is faster in Stage II, being almost linear up to harvest. To confirm these growth stages in our experiment, throughout the season destructive fruit samples were taken from experimental trees, dried and weighed. In 2008 Stage II occurred between 2 June and 21 August. In 2009 it did between 12 June and 18 August. The experimental fruit were harvested in one pick, on 21 August in 2008 and on 18 August in 2009. 2.3. Irrigation and fruit thinning treatments Irrigation season began at bud-break (late March) and finished before leaf fall was complete (late October). Trees were irrigated on a daily basis by drip irrigation system with two drippers per tree (4 l h−1 per dripper). There was a single pipeline per tree row which passed close to the trunks of the trees. Irrigation requirements were calculated on a weekly basis by a water balance technique to replace crop evapotranspiration (ETc). ETc was calculated as (ETo × Kc) – Rainfall (Allen et al., 1998), where ETo and Kc represent the reference evapotranspiration and crop coefficient, respectively. ETo was obtained from the Catalan Agrometeorological Network (XAC) for the ‘El Poal’ weather station, located 7 km away from the experimental orchard. Kc values were obtained from a weighting lysimeter located in the centre of the pear orchard (Girona et al., 2011). All trees received full irrigation (100% of ETc) from bud-break until the onset of Stage II. At the onset of Stage II (80 and 67 days before harvest in 2008 and 2009, respectively), two irrigation treatments were applied: full irrigation (FI) and deficit irrigation (DI). FI trees received 100% of ETc. DI trees received no irrigation during the first three weeks of Stage II to induce some degree of water stress. However, after the first three weeks of Stage II, DI trees were irri-
65
gated with 20% of ETc to ensure tree survival. After harvest all trees received again 100% of ETc. Each irrigation treatment received two fruit thinning levels: no thinning (NT) leaving commercial crop load, and hand-thinning (T) at the onset of Stage II leaving a light crop load (about 45% of the crop load of NT trees). Hereafter we use the treatment designation ‘T’ to refer to the fruit remained on the tree after fruit thinning of the tree. 2.4. Experimental design To avoid carry-over effects of the irrigation and fruit thinning treatments, different trees were used in 2008 and 2009. A randomized complete block design was used in both experimental years. In 2008 the experiment had three block replicates and in 2009 it had two. Each block housed four plots each designated to one of the following treatments: FI–T, FI–NT, DI–T, and DI–NT. Each plot had three rows of six trees with the four central trees of the middle row being considered as experimental and all the others as guard trees. 2.5. Measurements of applied water and tree water status The amount of water applied in each experimental plot was measured with digital water meters (CZ2000-3M, Contazara, Zaragoza, Spain). Midday stem water potential (SWP) was measured weekly during Stage II. This was done with a pressure chamber (Model 3005; Soil Moisture Equipment, Santa Barbara, CA, USA.). Measurements were taken at solar noon ± 30 min from leaves located near the bases of the two central trees in each plot (one leaf per tree). To ensure equilibrium between the leaf and the stem attached to it, selected leaves were bagged with aluminium foil 1 h before measurement (McCutchan and Shackel, 1992). 2.6. Determination of fruit quality All fruit from each experimental tree were harvested, counted and weighed. Mean fruit fresh weight was then calculated. In 2008, fruits were graded immediately after harvest using an electronic fruit grader (Model S2010; SAMMO s.r.l., Cesena, Italy). The freshmarket yield for each tree was considered to be the weight of all fruit with a maximum cheek diameter equal to or higher than 65 mm. In 2008, twenty fruit per plot were randomly selected for the following measurements: fruit firmness (FF), juice soluble solids concentration (SSC) and titratable acidity (TA). Firmness for each fruit was evaluated on two opposite peeled surfaces using a manual penetrometer with a tip of 0.5 cm2 and fixed in a drill stand (Penefel, Copa-Technology, CTIFL, France). SSC and acidity were measured on a mixture of juice from 10 pears (2 replicates) from each plot. SSC was determined using a digital refractometer (PR-32␣ Palette Series, Atago Co. Ltd., Tokyo, Japan). Acidity was determined by titrating the juice with 0.1 N NaOH to an end-point of pH 8.2. In 2009, six fruit per tree were used for measurement of firmness, SSC and acidity. These determinations were carried out three times before harvest, at harvest, and twice during cold storage. The cold storage was at 0 ◦ C and 90% relative humidity and lasted for either two months or five months after harvest depending on the experiment. In 2009, eighteen fruit per experimental tree were selected, their total fresh weight measured, and cold-stored. Their total weight was measured again after two and five months of cold storage to determine weight loss. Results were expressed as weight loss per gram of fruit.
66
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
Table 1 Effects of irrigation and fruit thinning treatments on crop load, fruit weight at harvest, total yield and fresh-market yield. Data were subjected to analysis of variance. Treatments
2008 FI–T FI–NT DI–T DI–NT 2009 FI–T FI–NT DI–T DI–NT
Crop load (fruits tree−1 )
Fruit weight (g)
Total yield (kg tree−1 )
Fresh-market yield (kg tree−1 )
104 cd 187 a 77 d 163 ab
189 a 165 bc 159 c 120 d
19.7 b 30.2 a 12.7 c 19.9 b
18.2 b 22.5 a 10.1 c 8.7 c
71 c 186 a 87 bc 198 a
185 a 156 c 168 bc 128 d
13.1 c 28.9 a 14.6 c 25.3 b
Means followed by different letters in the same column for each year are significantly different at 5% according to Duncans test. The fresh-market yield for each tree was considered to be the weight of all fruit with a maximum cheek diameter equal to or higher than 65 mm. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T).
2.7. Determination of fruit ethylene production In 2009, fruit ethylene production was determined at harvest and after cold storage. It was measured on a daily basis over a period of 8–12 days using a flow-through system located in a room at 20 ◦ C. The system consisted of 1.5-l jars continuously aerated with humidified air at a flow rate of approximately 1.5 l h−1 . Four replicates (2 fruits per replicate) from each treatment were placed into the jars. Gas samples were taken of the effluent air from the jars using a 1-ml syringe. The gas was injected into a gas chromatograph (Hewlett-Packard 5890 Series II, Barcelona, Spain) equipped with a FID detector and an alumina column (1.5 m × 3.0 mm). Gas analyses were conducted isothermally at 100 ◦ C with the injector and detector kept at 120 ◦ C and 180 ◦ C, respectively. The flow rates used for the N2 carrier gas, air and H2 , were, respectively, 45, 400 and 45 ml min−1 . Ethylene production was determined under atmospheric pressure at 20 ◦ C and expressed in l kg−1 h−1 .
onset of Stage II until harvest (full data not shown). On average, firmness values decreased from 92 to 64 N and acidity values from 4.8 to 2.1 g malic acid l−1 . The opposite was observed for SSC that increased from 8.8% to 13.8%. 3.2. Differences in crop load, fruit weight and yield at harvest The crop loads (fruit number tree−1 ) were light in trees that received fruit thinning (T) and at the commercial level in trees that received no thinning (NT) (Table 1). No significant differences in crop load were found between irrigation treatments.
2.8. Analysis of data The effect of the different irrigation and fruit thinning treatment combinations on midday SWP was analyzed by repeated measure analysis of variance (ANOVA). The effect of the treatment combinations on fruit quality was analyzed by ANOVA. The effect of crop load on fruit quality and ethylene production was evaluated by regression analysis. Statistical analyses were performed using the SAS software package (SAS Institute, Cary, NC). Statistical significance was established for P < 0.05. Duncan’s test was applied for separation of the least square means (LSM) of treatment combinations that differed significantly. 3. Results 3.1. Applied irrigation, variations in stem water potential and quality during pre-harvest All trees received the same amount of irrigation from bud-break until the onset of Stage II (100 mm in 2008 and 130 mm in 2009). In 2008, from the onset of Stage II until harvest, FI received 315 mm while DI received 66 mm (20% of FI). The corresponding values for 2009 were 367 and 63 with DI receiving 17% of FI. From harvest until leaf fall, all trees received the same amount of irrigation, 96 mm in 2008 and 160 mm in 2009. FI trees had a higher SWP than DI trees in both years (Fig. 1). Under DI, SWP was affected by the extent of thinning in both years. Fruit thinning led to less negative SWP (Fig. 1). The seasonal patterns of fruit quality obtained in 2009 revealed that firmness and acidity values of the fruit decreased from the
Fig. 1. Seasonal variations in midday stem water potential in response to irrigation and fruit thinning treatments. Data were subjected to repeated measure analysis of variance. Series followed by different letters for each year are significantly different at 5% according to Duncan’s test. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T).
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
67
Irrespective of fruit thinning treatment, DI trees had lower fruit weight, total yield and fresh-market yield at harvest than FI trees, except for the total yield in 2009 (Table 1). Under FI conditions, T trees had lower total yield and fresh-market yield than NT trees but higher fruit weight (Table 1). Under DI conditions, T trees had lower total yield at harvest than NT trees but higher fruit weight and equal fresh-market yield. 3.3. Effects of irrigation and fruit thinning on quality parameters at harvest and after cold storage Lower firmness and acidity values were found with two and five months of cold storage compared to values at harvest (Table 2). SSC values increased during cold storage (Table 2). In 2009, DI–NT fruit had higher FF and TA than FI–NT fruit at harvest and with two and five months of cold storage (Table 2). DI–NT fruit had also higher SSC than FI–NT fruit at harvest. This was also observed in 2008. DI–NT fruit had a SSC of 17.4% while this was 15.5% for FI–NT fruit. With two and five months of cold storage, DI fruit had higher FF and TA than FI fruit irrespective of fruit thinning treatment, except for FF in FI–T and DI–T with five months of cold storage (Table 2). No clear effect of fruit thinning on fruit quality was found at harvest and with two months of cold storage. However, T trees had higher SSC than NT trees with five months of cold storage (Table 2). Fruit weight loss in storage increased with time (Table 2). For NT it was lower in DI fruit than in FI fruit (Table 2). With two months in cold storage, the percentage weight loss for DI–NT and FI–NT were, respectively, 1.7% and 2.2%. The corresponding values with five months in cold storage were 2.3% and 2.8%. In contrast, when fruit thinning was applied, no significant differences in weight loss during cold storage were observed between irrigation treatments (Table 2). 3.4. Effects of irrigation and fruit thinning on ethylene production At harvest fruit ethylene production was between 2 and 8 l kg−1 h−1 and no peak in production was observed after 8 days in jars for any of the treatments (results not shown). Ethylene production increased during the cold storage period (Fig. 2). In general, fruit from thinned trees showed a stronger trend of producing more ethylene at 20 ◦ C than those from un-thinned trees especially after having been in cold storage for two months (Fig. 2). Maximum differences were found from day 5 to day 8 in jars. Ethylene production by fruit from thinned trees then decreased to reach that observed for NT fruit. However, due to the inherent variability in ethylene production and crop loads among replicates, the differences between thinning treatments were not significant. The effect of crop load on ethylene production was clearer when performing a regression analysis. In fruit left either for two months or for five months in storage, there was a significantly negative linear correlation between crop load and ethylene production (Fig. 3). Higher crop load led to lower ethylene production. 4. Discussion The average crop load is approximately 200 fruit per tree in the area of this experiment. In this area commercial irrigation replaces 100% of ETc when no fruit thinning is performed. These local growing conditions were represented by our FI–NT treatment. However, under water shortage, FI is not allowed and trees will have to grow under deficit irrigation. If no contingency plan to reduce waterstress is available, deficit irrigated trees will not undergo fruit thinning. This situation was represented by the DI–NT treatment. Choosing fruit thinning as a mitigation technique was represented by the DI–T treatment. In the following discussion the effects of
Fig. 2. Ethylene production of Conference pear fruit after cold storage in response to irrigation and fruit thinning treatments in 2009. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T). Separate bars denote the LSD for the means at the peak of ethylene production.
water stress for commercial crop load (FI–NT vs. DI–NT) and fruit thinning under water stress conditions (DI–NT vs. DI–T) are discussed into different sections to facilitate the presentation. 4.1. Effect of deficit irrigation on fruit quality at harvest DI–NT fruit had a higher SSC at harvest than FI–NT fruit (Table 2). This is consistent with previous research on pear (Ramos et al., 1994), peach (Li et al., 1989; Mercier et al., 2009), apple (Mills et al., 1994; Mpelasoka et al., 2000) and plum (Intrigliolo and Castel, 2010). Significant linear relationships have been found between tree water status and fruit SSC in ‘Barlett’ pear (Ramos et al., 1994) and in ‘O’Henry’ peach (Lopez et al., 2010), with higher levels of water stress leading to higher SSC. The increased SSC values for DI–NT fruit may indicate partial dehydration of fruit under water stress. Accordingly, higher fruit dry matter concentration (fruit dry weight/fresh weight) was found in DI–NT (0.20) fruit than in FI–NT (0.18) fruit at harvest. Increased SSC under DI could have a positive impact on fruit taste. For pear, Jackson (2003) reported that fruit taste is mainly related to the sugar concentration in the juice if acidity were low. Since our ‘Conference’ pear had low levels of acidity (about 2–3 g malic acid l−1 ) and DI–NT increased SSC values, DI–NT fruit could be perceived positively by consumers. This could justify not using pear acidity as a quality indicator under deficit irrigation. Moreover acidity results were not consistent between years. No effect on acidity was observed in 2008 (results not shown) while in 2009 DI–NT fruit were more acidic than FI–NT fruit (Table 2). In numerous studies reviewed by Behboudian et al. (2011) the effects of deficit irrigation on pear fruit acidity were not conclusive either.
68
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
Table 2 Effects of irrigation and fruit thinning treatments on fruit firmness (FF), soluble solids concentration (SSC), titratable acidity (TA) and weight loss at harvest and after cold storage in 2009. Data were subjected to analysis of variance. Treatments Harvest FI–T FI–NT DI–T DI–NT 2 months of cold storage FI–T FI–NT DI–T DI–NT 5 months of cold storage FI–T FI–NT DI–T DI–NT
Weight loss (g g−1 fruit)
FF (N)
SSC (%)
TA (g malic acid l−1 )
62.4 b 61.7 b 64.7 ab 67.2 a
13.9 ab 13.3 b 14.2 a 14.2 a
2.0 ab 1.6 b 2.0 a 2.1 a
57.3 b 58.1 b 63.5 a 61.2 a
15.4 a 14.4 b 15.7 a 15.1 ab
1.5 b 1.5 b 1.9 a 2.1 a
0.018 b 0.022 a 0.017 b 0.017 b
54.2 a 51.3 b 55.9 a 56.4 a
15.7 a 14.8 b 16.1 a 14.8 b
1.5 b 1.3 b 1.8 a 1.8 a
0.026 ab 0.029 a 0.024 b 0.023 b
Means followed by different letters in the same column for each year are significantly different at 5% according to Duncans test. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T).
In 2009, DI–NT fruit were firmer than FI–NT fruit at harvest. Higher firmness under DI has also been reported for apple (Kilili et al., 1996b; Leib et al., 2006). Higher FF under DI might be because of lower fruit size. Water stress reduces fruit size and smaller fruit tend to be firmer than larger fruit. However, Mpelasoka et al. (2000) found higher FF in apple fruit produced under DI irrespective of the size.
4.2. Effect of deficit irrigation on fruit quality in cold storage DI had no effect on fruit maturity based on ethylene production (Fig. 2). Hence, any effect of DI on fruit quality may be associated with water stress and not mediated through ethylene production. After two and five months of cold storage, higher firmness and acidity values were found in DI–NT fruit compared to FI–NT (Table 2). This indicates that the significant differences in FF and TA values at harvest between DI–NT and FI–NT fruit were maintained during the subsequent cold storage (Intrigliolo and Castel, 2010; Behboudian et al., 2011). This was not true for the SSC. The differences in SSC between DI–NT and FI–NT were not observed after two and five months of cold storage (Table 2). Therefore, SSC of ‘Conference’ pear subjected to deficit irrigation seems to have a different behavior during cold storage than FF and TA. Weight loss in ‘Conference’ pear increases with time in storage (Nguyen et al., 2007). However, lower weight loss was observed in DI–NT fruit than in FI–NT fruit. This has not been reported for pear but our finding is consistent with those observed in apple (Kilili et al., 1996a; Mpelasoka et al., 2000), apricot (Perez-Pastor et al., 2007) and peach (Mercier et al., 2009). The difference in weight loss between DI–NT and FI–NT fruit could be due to differences in the structure and/or composition of the skin or the epicuticular waxes covering the skin. Cuticle modification by water stress has been reported by Crisosto et al. (1994) in peach fruit where the lower rate of water loss under DI was explained by a thicker cuticle and a higher density of trichomes on the skin surface. A possible reason here could be the lower water content of DI pear. Differences in weight loss in ‘Conference’ pear are function of differences in the original water content (Nguyen et al., 2006). Irrespective of the mechanism involved, DI fruit could have better quality after cold storage than FI fruit due to a reduction in weight loss. The reduction in weight loss in DI fruit could prolong the cold storage life, facilitating marketing for a longer period of time. 4.3. Effect of fruit thinning on yield and fruit quality at harvest
Fig. 3. Relationships between crop load and ethylene production in relation to day of assessment after removal from cold storage in 2009. Relationships were adjusted to a simple linear equation for each day of evaluation. Each point represents a replicate (2 fruits per replicate) from each treatment. Day represents the number of days in jars and asterisks (*) indicate that the slope of the relationship is not equal to zero for P < 0.05.
DI–NT improved important quality attributes at harvest and after cold storage, but it reduced fruit size and fresh-market yield compared to FI–NT (Table 1). This indicates that under DI conditions ‘Conference’ pear cannot produce their maximum yield. It would be therefore necessary to lower the crop load by fruit thinning. Although T trees had lower total yield and fresh-market yield than NT under FI, no detrimental effects on fresh-market yield
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
was observed under DI (Table 1). Therefore, if full irrigation is not allowed fruit thinning can maintain the fresh-market yield (Marsal et al., 2010). Moreover, fruit weight at harvest was always higher in T than in NT trees (Table 1). DI–T trees had similar fruit weights at harvest to FI–NT trees, confirming that fruit size can be improved by fruit thinning as exemplified for peach (Lopez et al., 2006; Marsal et al., 2006) and pear (Marsal et al., 2008). Fruit size in pear is one of the most important quality attributes and fruit thinning under deficit irrigation can improve it thereby enhancing marketability. Fruit thinning did not affect fruit firmness, acidity, and SSC at harvest. But there was a trend, especially in 2008, for higher values of these parameters with fruit thinning (results from regression analysis not shown). Factors such as the timing of fruit thinning may explain the lack of significant responses. Under our conditions, thinning was performed late in the season to coincide with the timing of water shortage. There might not have been enough time available to the expected higher amounts of photoassimilates to have a desirable influence on fruit quality changes in the remaining fruit. A similar conclusion was obtained in another study in peach (Lopez et al., 2010). However, early season fruit thinning has increased firmness, SSC and acidity in apple (Wünsche and Ferguson, 2005) and in peach SSC (Crisosto et al., 1994). 4.4. Effect of fruit thinning on fruit quality in cold storage FI–T and DI–T fruit had higher values of SSC after five months of cold storage than FI–NT and DI–NT fruit (Table 2). The presumably higher photoassimilates arising from fruit thinning, that caused the trend for a higher SSC in the fruit at harvest, might have led to significantly higher SSC accumulated during cold storage. Ethylene production was used as an indicator of physiological maturity. It was measured in T and NT fruit at harvest and immediately after storage and during a 2-week period of shelf life at 20 ◦ C. After removal from cold storage, a significant negative linear relationship was found between crop load and the fruit capability to produce ethylene (Fig. 3). Fruit from trees with light crop load produced more ethylene than fruit from trees with a normal crop load. Low crop load therefore significantly enhanced the physiological maturity of the fruit. T fruit, even when picked at the same firmness value of NT fruit, were more mature. The more advanced physiological maturity with lower crop load has not been reported for pear. Our findings are in agreement with some experiments on apple reviewed by Wünsche and Ferguson (2005). 5. Conclusions This work aimed at introducing a contingency plan for growing pear under water shortage. We applied DI to trees with commercial load (∼180 fruits tree−1 ) which caused a moderate water stress. Under these circumstances, DI increased the fruit firmness, acidity and soluble solids concentrations at harvest while no changes were observed in fruit maturity. The increased firmness and acidity under DI were maintained during the subsequent cold storage. DI also reduced weight loss during storage but as expected also reduced fruit size. To counteract this expected effect we included another treatment, fruit thinning. This treatment, which reduced crop load by 45%, maintained the improved fruit composition associated to DI. Additionally, fruit thinning improved fruit size at harvest and the SSC values after five months of cold storage. Fruit thinning also advanced physiological fruit maturity. This may lead to earlier harvests which could be desirable in years with drought because fruit’s exposure to water stress will be reduced. If moderate water stress is imposed as a consequence of irrigation shortage, fruit quality
69
could improve in ‘Conference’ pear. Lowering crop load by applying fruit thinning produced more benefits compared to leaving a commercial crop load. We therefore recommend fruit thinning as a mitigation technique under pre-harvest water stress conditions. Acknowledgements We thank M. Mata, J. del Campo, B. Basile, C. Paris, and N. Bonastre for their help with the field work. We also thank D. Ubach for the assistance during ethylene determination. This study was funded by The Spanish Ministry of Research and Technology under project AGL2005-00538 and CONSOLIDER-INGENIO 2010 (CSD2006-00067). References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. Guidelines for computing crop water requirements. FAO irrigation and drainage paper n◦ . 56. Bain, J.M., 1961. Some morphological, anatomical, and physiological changes in pear fruit (Pyrus communis var. Williams Bon Chrétien) during development and following harvest. Aust. J. Bot. 9, 99–123. Behboudian, M.H., Marsal, J., Girona, J., Lopez, G., 2011. Quality and yield responses of deciduous fruits to reduced irrigation. Hortic. Rev. 38, 149–189. Crisosto, C.H., Johnson, R.S., Luza, J.G., Crisosto, G.M., 1994. Irrigation regimes affect fruit soluble solids concentration and rate of water loss of ‘O’Henry’ peaches. HortScience 29, 169–1171. Gelly, M., Recasens, I., Mata, M., Arbones, A., Rufat, J., Girona, J., Marsal, J., 2003. Effects of water deficit during stage II of peach fruit development and postharvest on fruit quality and ethylene production. J. Hortic. Sci. Biotechnol. 78, 324–330. Gelly, M., Recasens, I., Girona, J., Mata, M., Arbones, A., Rufat, J., Marsal, J., 2004. Effects of stage II and postharvest deficit irrigation on peach quality during maturation and after cold storage. J. Sci. Food Agric. 84, 561–568. Girona, J., del Campo, J., Mata, M., Lopez, G., Marsal, J., 2011. A comparative study of apple and pear tree water consumption measured with two weighing lysimeters. Irrig. Sci. 29, 55–63. Intrigliolo, D.S., Castel, J.R., 2010. Response of plum trees to deficit irrigation under two crop levels: tree growth, yield and fruit quality. Irrig. Sci. 28, 525–534. Jackson, J.E. (Ed.), 2003. Biology of Apple and Pears. University of Cambridge, United Kingdom. Kilili, A.W., Behboudian, M.H., Mills, T.M., 1996a. Composition and quality of ‘Braeburn’ apples under reduced irrigation. Sci. Hortic. 67, 1–11. Kilili, A.W., Behboudian, M.H., Mills, T.M., 1996b. Postharvest performance of ‘Braeburn’ apples in relation to withholding of irrigation at different stages of the growing season. J. Hortic. Sci. 71, 693–701. Leib, B.G., Caspari, H.W., Redulla, C.A., Andrews, P.K., Jabro, J.J., 2006. Partial rootzone drying and deficit irrigation of ‘Fugiˇı apples in a semi-arid climate. Irrig. Sci. 24, 85–99. Li, S.H., Huguet, J.G., Schoch, P.G., Orlando, P., 1989. Response of peach-tree growth and cropping to soil water deficit at various phenological stages of fruit development. J. Hortic. Sci. 64, 541–552. Lopez, G., Mata, M., Arbones, A., Solans, J.R., Girona, J., Marsal, J., 2006. Mitigation of effects of extreme drought during stage III of peach fruit development by summer pruning and fruit thinning. Tree Physiol. 26, 469–477. Lopez, G., Behboudian, M.H., Vallverdu, X., Mata, M., Girona, J., Marsal, J., 2010. Mitigation of severe water stress by fruit thinning in ‘O’Henry’ peach: implications for fruit quality. Sci. Hortic. 125, 294–300, doi:10.1016/j.scienta.2010.04.003. Marsal, J., Lopez, G., Mata, M., Girona, J., 2006. Branch removal and defruiting for the amelioration of water stress effects on fruit growth during stage III of peach fruit development. Sci. Hortic. 108, 55–60. Marsal, J., Mata, M., Arbones, A., Del Campo, J., Girona, J., Lopez, G., 2008. Factors involved in alleviating water stress by partial crop removal in pear trees. Tree Physiol. 28, 1375–1382. Marsal, J., Behboudian, M.H., Mata, M., Basile, B., del Campo, J., Girona, J., Lopez, G., 2010. Fruit thinning in ‘Conference’ pear grown under deficit irrigation to optimise yield and to improve tree water status. J. Hortic. Sci. Biotechnol. 85, 125–130. McCutchan, H., Shackel, K.A., 1992. Stem water potential as a sensitive indicator of water stress in prune trees (Prunus Domestica L. cv French). J. Am. Soc. Hortic. Sci. 117, 607–611. Mercier, V., Bussi, C., Lescourret, F., Genard, M., 2009. Effects of different irrigation regimes applied during the final stage of rapid fruit growth on an early maturing peach cultivar. Irrig. Sci. 27, 297–306. Mills, T.M., Behboudian, M.H., Tan, P.Y., Clothier, B.E., 1994. Plant water status and fruit quality in ‘Braeburn’ apples. HortScience 29, 1274–1278. Mpelasoka, B.S., Behboudian, M.H., Dixon, J., Neal, S.M., Caspari, H.W., 2000. Improvement of fruit quality and storage potential of ‘Braeburn’ apple through deficit irrigation. J. Hortic. Sci. Biotechnol. 75, 615–621.
70
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
Mpelasoka, B.S., Behboudian, M.H., Mills, T.M., 2001. Water relations, photosynthesis, growth, yield and fruit size of ‘Braeburn’ apple: responses to deficit irrigation and to crop load. J. Hortic. Sci. Biotechnol. 76, 150–156. Nguyen, T.A., Dresselaers, T., Verboven, P., D’hallewin, G., Culeddu, N., Van Hecke, P., Nicolai, B.M., 2006. Finite element modelling and MRI validation of 3D transient water profiles in pears during postharvest storage. J. Sci. Food Agric. 86, 745–756. Nguyen, T.A., Verboven, P., Schenk, A., Nicolaï, B.M., 2007. Prediction of water loss from pears (Pyrus communis cv. Conference) during controlled atmosphere storage as affected by relative humidity. J. Food Eng. 83, 149–155.
Perez-Pastor, A., Ruiz-Sanchez, M.C., Martinez, J.A., Nortes, P.A., Artes, F., Domingo, R., 2007. Effect of deficit irrigation on apricot fruit quality at harvest and during storage. J. Sci. Food Agric. 87, 2409–2415. Ramos, D.E., Weinbaum, S.A., Shackel, K.A., Shcwankl, L.J., Mitcham, E.J., Mitchell, F.G., Snyder, R.G., Mayer, G., McGourty, G., 1994. Influence of tree water status and canopy position on fruit size and quality of Bartlett pears. Acta Hortic. 367, 192–200. Wünsche, J.N., Ferguson, I.B., 2005. Crop load interactions in apple. Hortic. Rev. 31, 231–290.
Contents lists available at ScienceDirect
Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti
Fruit thinning in ‘Conference’ pear grown under deficit irrigation: Implications for fruit quality at harvest and after cold storage Gerardo Lopez a,∗ , Christian Larrigaudière b , Joan Girona a , M. Hossein Behboudian c , Jordi Marsal a a
Irrigation Technology, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), Centre UdL-IRTA, Rovira Roure 191, 25198 Lleida, Spain Postharvest, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), Centre UdL-IRTA, Rovira Roure 191, 25198 Lleida, Spain c Institute of Natural Resources, Massey University, Private Bag 11122, Palmerston North, New Zealand b
a r t i c l e
i n f o
Article history: Received 13 September 2010 Received in revised form 28 February 2011 Accepted 4 March 2011 Keywords: Ethylene production Drought Pyrus communis Water stress Weight loss in storage
a b s t r a c t Fruit thinning in pear is feasible for mitigation of water stress effects. However, it is not well known how fruit quality at harvest and after cold storage is affected by pre-harvest water stress. Even less is known about the effects of fruit thinning on quality under these circumstances. To elucidate these, we applied deficit irrigation (DI) and fruit thinning treatments to ‘Conference’ pear over the growing seasons of 2008 and 2009. At the onset of Stage II (80 and 67 days before harvest in 2008 and 2009, respectively), two irrigation treatments were applied: full irrigation (FI) and DI. FI trees received 100% of crop evapotranspiration (ETc). DI trees received no irrigation during the first three weeks of Stage II to induce water stress, but then received 20% of ETc to ensure tree survival. From bud-break until the onset of Stage II and during post-harvest, FI and DI trees received 100% of ETc. Each irrigation treatment received two thinning levels: no thinning leaving commercial crop load (∼180 fruits tree−1 ), and handthinning at the onset of Stage II leaving a light crop load (∼85 fruits tree−1 ). Under commercial crop loads, DI trees were moderately water-stressed and this had some positive effects on fruit quality. DI increased fruit firmness (FF), soluble solids concentrations (SSC) and acidity at harvest while no changes were observed in fruit maturity (based on ethylene production). Differences in FF and acidity at harvest between FI and DI fruit were maintained during cold storage. DI also reduced fruit weight loss during storage. But fruit size was reduced under DI. Fruit thinning under DI resulted in better fruit composition with no detrimental effect on fresh-market yield compared to un-thinned fruit. Fruit size at harvest and SSC values after five months of cold storage were higher in fruit from thinned trees than fruit from unthinned trees. Fruit thinning increased fruit ethylene production, indicating advanced maturity. This may lead to earlier harvest which is desirable in years with impending drought. Fruit thinning is therefore a useful technique to enhance pear marketability under water shortage. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Water shortage is becoming increasingly frequent in fruit producing areas with Mediterranean climates. The major pearproducing areas in Spain are not exempt from this problem and several restrictions on irrigation have been experienced in the past few years. These restrictions are usually imposed during midsummer. Water stress therefore develops when pears are in their cell-expansion growth period (Stage II). Water stress in Stage II reduces the potential of achieving commercial fruit size at harvest (Marsal et al., 2008, 2010). Size is an important quality attribute for fresh-market pear. Fruit thinning has been proposed as a contingency plan for reducing water-stress, partially compensating for
∗ Corresponding author. E-mail address: [email protected] (G. Lopez). 0304-4238/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2011.03.007
the negative effects on fruit size (Mpelasoka et al., 2001; Lopez et al., 2006; Marsal et al., 2008, 2010; Intrigliolo and Castel, 2010). The benefit of fruit thinning on pear growth was not solely related to a reduction in fruit competition for photoassimilates. It has been also associated with improvements in tree water status (Marsal et al., 2008). Although pricing depends on fruit size, consumer acceptance depends on eating quality that should be maintained after a period of cold storage. It is not known how pear quality responds to fruit thinning under water limited conditions in Stage II. We hypothesised that fruit from trees with a lower crop load could have a better quality than fruit from trees with a normal crop load. Fruit thinning has generally resulted in higher firmness, soluble solids concentration (SSC) and acidity in apple (Wünsche and Ferguson, 2005) and in higher SSC in peach (Crisosto et al., 1994) and plum (Intrigliolo and Castel, 2010). Gaps of knowledge also exist in how pear quality responds to pre-harvest water stress, especially for
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
fruit cold-stored after harvest. Although pear was one of the first fruit crops undergoing research on deficit irrigation, information on how fruit quality is affected is scant compared to other crops such as apple and peach (Behboudian et al., 2011; Gelly et al., 2003, 2004). We therefore studied, over two growing seasons, the combined effects of pre-harvest water stress and fruit thinning on important fruit quality attributes in pear, both at harvest and after cold storage. We expected that our findings will enable growers to adopt suitable fruit thinning strategies for production of better quality fruit under water limited conditions. Besides being the most popular pear cultivar in Spain, ‘Conference’ was used because it is exposed to the increasingly frequent episodes of water shortage in midsummer and is kept in cold storage for several months. 2. Materials and methods 2.1. Pear orchard The experiment was conducted over two years (2008–2009) in a ‘Conference’ pear (Pyrus communis L.) orchard located at the IRTA-Estaciò Experimental de Lleida (41◦ 37 N; 0◦ 52 E; 260 m a.s.l.); Spain. The orchard was planted in 1999 with 4.0 m between rows and 1.6 m within rows. The trees were grafted onto dwarfing quince rootstock (M–A). The orchard was managed according to local commercial practices, including fruit thinning when commercial crop load exceeded 200 fruit per tree. 2.2. Phenological stages of ‘Conference’ pear Pear growth consists of two linear stages: Stage I and Stage II (Bain, 1961). Stage I is characterised by slow growth, lasting for approximately 30 days in ‘Conference’ pear. Fruit growth is faster in Stage II, being almost linear up to harvest. To confirm these growth stages in our experiment, throughout the season destructive fruit samples were taken from experimental trees, dried and weighed. In 2008 Stage II occurred between 2 June and 21 August. In 2009 it did between 12 June and 18 August. The experimental fruit were harvested in one pick, on 21 August in 2008 and on 18 August in 2009. 2.3. Irrigation and fruit thinning treatments Irrigation season began at bud-break (late March) and finished before leaf fall was complete (late October). Trees were irrigated on a daily basis by drip irrigation system with two drippers per tree (4 l h−1 per dripper). There was a single pipeline per tree row which passed close to the trunks of the trees. Irrigation requirements were calculated on a weekly basis by a water balance technique to replace crop evapotranspiration (ETc). ETc was calculated as (ETo × Kc) – Rainfall (Allen et al., 1998), where ETo and Kc represent the reference evapotranspiration and crop coefficient, respectively. ETo was obtained from the Catalan Agrometeorological Network (XAC) for the ‘El Poal’ weather station, located 7 km away from the experimental orchard. Kc values were obtained from a weighting lysimeter located in the centre of the pear orchard (Girona et al., 2011). All trees received full irrigation (100% of ETc) from bud-break until the onset of Stage II. At the onset of Stage II (80 and 67 days before harvest in 2008 and 2009, respectively), two irrigation treatments were applied: full irrigation (FI) and deficit irrigation (DI). FI trees received 100% of ETc. DI trees received no irrigation during the first three weeks of Stage II to induce some degree of water stress. However, after the first three weeks of Stage II, DI trees were irri-
65
gated with 20% of ETc to ensure tree survival. After harvest all trees received again 100% of ETc. Each irrigation treatment received two fruit thinning levels: no thinning (NT) leaving commercial crop load, and hand-thinning (T) at the onset of Stage II leaving a light crop load (about 45% of the crop load of NT trees). Hereafter we use the treatment designation ‘T’ to refer to the fruit remained on the tree after fruit thinning of the tree. 2.4. Experimental design To avoid carry-over effects of the irrigation and fruit thinning treatments, different trees were used in 2008 and 2009. A randomized complete block design was used in both experimental years. In 2008 the experiment had three block replicates and in 2009 it had two. Each block housed four plots each designated to one of the following treatments: FI–T, FI–NT, DI–T, and DI–NT. Each plot had three rows of six trees with the four central trees of the middle row being considered as experimental and all the others as guard trees. 2.5. Measurements of applied water and tree water status The amount of water applied in each experimental plot was measured with digital water meters (CZ2000-3M, Contazara, Zaragoza, Spain). Midday stem water potential (SWP) was measured weekly during Stage II. This was done with a pressure chamber (Model 3005; Soil Moisture Equipment, Santa Barbara, CA, USA.). Measurements were taken at solar noon ± 30 min from leaves located near the bases of the two central trees in each plot (one leaf per tree). To ensure equilibrium between the leaf and the stem attached to it, selected leaves were bagged with aluminium foil 1 h before measurement (McCutchan and Shackel, 1992). 2.6. Determination of fruit quality All fruit from each experimental tree were harvested, counted and weighed. Mean fruit fresh weight was then calculated. In 2008, fruits were graded immediately after harvest using an electronic fruit grader (Model S2010; SAMMO s.r.l., Cesena, Italy). The freshmarket yield for each tree was considered to be the weight of all fruit with a maximum cheek diameter equal to or higher than 65 mm. In 2008, twenty fruit per plot were randomly selected for the following measurements: fruit firmness (FF), juice soluble solids concentration (SSC) and titratable acidity (TA). Firmness for each fruit was evaluated on two opposite peeled surfaces using a manual penetrometer with a tip of 0.5 cm2 and fixed in a drill stand (Penefel, Copa-Technology, CTIFL, France). SSC and acidity were measured on a mixture of juice from 10 pears (2 replicates) from each plot. SSC was determined using a digital refractometer (PR-32␣ Palette Series, Atago Co. Ltd., Tokyo, Japan). Acidity was determined by titrating the juice with 0.1 N NaOH to an end-point of pH 8.2. In 2009, six fruit per tree were used for measurement of firmness, SSC and acidity. These determinations were carried out three times before harvest, at harvest, and twice during cold storage. The cold storage was at 0 ◦ C and 90% relative humidity and lasted for either two months or five months after harvest depending on the experiment. In 2009, eighteen fruit per experimental tree were selected, their total fresh weight measured, and cold-stored. Their total weight was measured again after two and five months of cold storage to determine weight loss. Results were expressed as weight loss per gram of fruit.
66
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
Table 1 Effects of irrigation and fruit thinning treatments on crop load, fruit weight at harvest, total yield and fresh-market yield. Data were subjected to analysis of variance. Treatments
2008 FI–T FI–NT DI–T DI–NT 2009 FI–T FI–NT DI–T DI–NT
Crop load (fruits tree−1 )
Fruit weight (g)
Total yield (kg tree−1 )
Fresh-market yield (kg tree−1 )
104 cd 187 a 77 d 163 ab
189 a 165 bc 159 c 120 d
19.7 b 30.2 a 12.7 c 19.9 b
18.2 b 22.5 a 10.1 c 8.7 c
71 c 186 a 87 bc 198 a
185 a 156 c 168 bc 128 d
13.1 c 28.9 a 14.6 c 25.3 b
Means followed by different letters in the same column for each year are significantly different at 5% according to Duncans test. The fresh-market yield for each tree was considered to be the weight of all fruit with a maximum cheek diameter equal to or higher than 65 mm. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T).
2.7. Determination of fruit ethylene production In 2009, fruit ethylene production was determined at harvest and after cold storage. It was measured on a daily basis over a period of 8–12 days using a flow-through system located in a room at 20 ◦ C. The system consisted of 1.5-l jars continuously aerated with humidified air at a flow rate of approximately 1.5 l h−1 . Four replicates (2 fruits per replicate) from each treatment were placed into the jars. Gas samples were taken of the effluent air from the jars using a 1-ml syringe. The gas was injected into a gas chromatograph (Hewlett-Packard 5890 Series II, Barcelona, Spain) equipped with a FID detector and an alumina column (1.5 m × 3.0 mm). Gas analyses were conducted isothermally at 100 ◦ C with the injector and detector kept at 120 ◦ C and 180 ◦ C, respectively. The flow rates used for the N2 carrier gas, air and H2 , were, respectively, 45, 400 and 45 ml min−1 . Ethylene production was determined under atmospheric pressure at 20 ◦ C and expressed in l kg−1 h−1 .
onset of Stage II until harvest (full data not shown). On average, firmness values decreased from 92 to 64 N and acidity values from 4.8 to 2.1 g malic acid l−1 . The opposite was observed for SSC that increased from 8.8% to 13.8%. 3.2. Differences in crop load, fruit weight and yield at harvest The crop loads (fruit number tree−1 ) were light in trees that received fruit thinning (T) and at the commercial level in trees that received no thinning (NT) (Table 1). No significant differences in crop load were found between irrigation treatments.
2.8. Analysis of data The effect of the different irrigation and fruit thinning treatment combinations on midday SWP was analyzed by repeated measure analysis of variance (ANOVA). The effect of the treatment combinations on fruit quality was analyzed by ANOVA. The effect of crop load on fruit quality and ethylene production was evaluated by regression analysis. Statistical analyses were performed using the SAS software package (SAS Institute, Cary, NC). Statistical significance was established for P < 0.05. Duncan’s test was applied for separation of the least square means (LSM) of treatment combinations that differed significantly. 3. Results 3.1. Applied irrigation, variations in stem water potential and quality during pre-harvest All trees received the same amount of irrigation from bud-break until the onset of Stage II (100 mm in 2008 and 130 mm in 2009). In 2008, from the onset of Stage II until harvest, FI received 315 mm while DI received 66 mm (20% of FI). The corresponding values for 2009 were 367 and 63 with DI receiving 17% of FI. From harvest until leaf fall, all trees received the same amount of irrigation, 96 mm in 2008 and 160 mm in 2009. FI trees had a higher SWP than DI trees in both years (Fig. 1). Under DI, SWP was affected by the extent of thinning in both years. Fruit thinning led to less negative SWP (Fig. 1). The seasonal patterns of fruit quality obtained in 2009 revealed that firmness and acidity values of the fruit decreased from the
Fig. 1. Seasonal variations in midday stem water potential in response to irrigation and fruit thinning treatments. Data were subjected to repeated measure analysis of variance. Series followed by different letters for each year are significantly different at 5% according to Duncan’s test. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T).
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
67
Irrespective of fruit thinning treatment, DI trees had lower fruit weight, total yield and fresh-market yield at harvest than FI trees, except for the total yield in 2009 (Table 1). Under FI conditions, T trees had lower total yield and fresh-market yield than NT trees but higher fruit weight (Table 1). Under DI conditions, T trees had lower total yield at harvest than NT trees but higher fruit weight and equal fresh-market yield. 3.3. Effects of irrigation and fruit thinning on quality parameters at harvest and after cold storage Lower firmness and acidity values were found with two and five months of cold storage compared to values at harvest (Table 2). SSC values increased during cold storage (Table 2). In 2009, DI–NT fruit had higher FF and TA than FI–NT fruit at harvest and with two and five months of cold storage (Table 2). DI–NT fruit had also higher SSC than FI–NT fruit at harvest. This was also observed in 2008. DI–NT fruit had a SSC of 17.4% while this was 15.5% for FI–NT fruit. With two and five months of cold storage, DI fruit had higher FF and TA than FI fruit irrespective of fruit thinning treatment, except for FF in FI–T and DI–T with five months of cold storage (Table 2). No clear effect of fruit thinning on fruit quality was found at harvest and with two months of cold storage. However, T trees had higher SSC than NT trees with five months of cold storage (Table 2). Fruit weight loss in storage increased with time (Table 2). For NT it was lower in DI fruit than in FI fruit (Table 2). With two months in cold storage, the percentage weight loss for DI–NT and FI–NT were, respectively, 1.7% and 2.2%. The corresponding values with five months in cold storage were 2.3% and 2.8%. In contrast, when fruit thinning was applied, no significant differences in weight loss during cold storage were observed between irrigation treatments (Table 2). 3.4. Effects of irrigation and fruit thinning on ethylene production At harvest fruit ethylene production was between 2 and 8 l kg−1 h−1 and no peak in production was observed after 8 days in jars for any of the treatments (results not shown). Ethylene production increased during the cold storage period (Fig. 2). In general, fruit from thinned trees showed a stronger trend of producing more ethylene at 20 ◦ C than those from un-thinned trees especially after having been in cold storage for two months (Fig. 2). Maximum differences were found from day 5 to day 8 in jars. Ethylene production by fruit from thinned trees then decreased to reach that observed for NT fruit. However, due to the inherent variability in ethylene production and crop loads among replicates, the differences between thinning treatments were not significant. The effect of crop load on ethylene production was clearer when performing a regression analysis. In fruit left either for two months or for five months in storage, there was a significantly negative linear correlation between crop load and ethylene production (Fig. 3). Higher crop load led to lower ethylene production. 4. Discussion The average crop load is approximately 200 fruit per tree in the area of this experiment. In this area commercial irrigation replaces 100% of ETc when no fruit thinning is performed. These local growing conditions were represented by our FI–NT treatment. However, under water shortage, FI is not allowed and trees will have to grow under deficit irrigation. If no contingency plan to reduce waterstress is available, deficit irrigated trees will not undergo fruit thinning. This situation was represented by the DI–NT treatment. Choosing fruit thinning as a mitigation technique was represented by the DI–T treatment. In the following discussion the effects of
Fig. 2. Ethylene production of Conference pear fruit after cold storage in response to irrigation and fruit thinning treatments in 2009. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T). Separate bars denote the LSD for the means at the peak of ethylene production.
water stress for commercial crop load (FI–NT vs. DI–NT) and fruit thinning under water stress conditions (DI–NT vs. DI–T) are discussed into different sections to facilitate the presentation. 4.1. Effect of deficit irrigation on fruit quality at harvest DI–NT fruit had a higher SSC at harvest than FI–NT fruit (Table 2). This is consistent with previous research on pear (Ramos et al., 1994), peach (Li et al., 1989; Mercier et al., 2009), apple (Mills et al., 1994; Mpelasoka et al., 2000) and plum (Intrigliolo and Castel, 2010). Significant linear relationships have been found between tree water status and fruit SSC in ‘Barlett’ pear (Ramos et al., 1994) and in ‘O’Henry’ peach (Lopez et al., 2010), with higher levels of water stress leading to higher SSC. The increased SSC values for DI–NT fruit may indicate partial dehydration of fruit under water stress. Accordingly, higher fruit dry matter concentration (fruit dry weight/fresh weight) was found in DI–NT (0.20) fruit than in FI–NT (0.18) fruit at harvest. Increased SSC under DI could have a positive impact on fruit taste. For pear, Jackson (2003) reported that fruit taste is mainly related to the sugar concentration in the juice if acidity were low. Since our ‘Conference’ pear had low levels of acidity (about 2–3 g malic acid l−1 ) and DI–NT increased SSC values, DI–NT fruit could be perceived positively by consumers. This could justify not using pear acidity as a quality indicator under deficit irrigation. Moreover acidity results were not consistent between years. No effect on acidity was observed in 2008 (results not shown) while in 2009 DI–NT fruit were more acidic than FI–NT fruit (Table 2). In numerous studies reviewed by Behboudian et al. (2011) the effects of deficit irrigation on pear fruit acidity were not conclusive either.
68
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
Table 2 Effects of irrigation and fruit thinning treatments on fruit firmness (FF), soluble solids concentration (SSC), titratable acidity (TA) and weight loss at harvest and after cold storage in 2009. Data were subjected to analysis of variance. Treatments Harvest FI–T FI–NT DI–T DI–NT 2 months of cold storage FI–T FI–NT DI–T DI–NT 5 months of cold storage FI–T FI–NT DI–T DI–NT
Weight loss (g g−1 fruit)
FF (N)
SSC (%)
TA (g malic acid l−1 )
62.4 b 61.7 b 64.7 ab 67.2 a
13.9 ab 13.3 b 14.2 a 14.2 a
2.0 ab 1.6 b 2.0 a 2.1 a
57.3 b 58.1 b 63.5 a 61.2 a
15.4 a 14.4 b 15.7 a 15.1 ab
1.5 b 1.5 b 1.9 a 2.1 a
0.018 b 0.022 a 0.017 b 0.017 b
54.2 a 51.3 b 55.9 a 56.4 a
15.7 a 14.8 b 16.1 a 14.8 b
1.5 b 1.3 b 1.8 a 1.8 a
0.026 ab 0.029 a 0.024 b 0.023 b
Means followed by different letters in the same column for each year are significantly different at 5% according to Duncans test. Abbreviations: full irrigation (FI); deficit irrigation (DI); no thinning (NT); hand-thinning (T).
In 2009, DI–NT fruit were firmer than FI–NT fruit at harvest. Higher firmness under DI has also been reported for apple (Kilili et al., 1996b; Leib et al., 2006). Higher FF under DI might be because of lower fruit size. Water stress reduces fruit size and smaller fruit tend to be firmer than larger fruit. However, Mpelasoka et al. (2000) found higher FF in apple fruit produced under DI irrespective of the size.
4.2. Effect of deficit irrigation on fruit quality in cold storage DI had no effect on fruit maturity based on ethylene production (Fig. 2). Hence, any effect of DI on fruit quality may be associated with water stress and not mediated through ethylene production. After two and five months of cold storage, higher firmness and acidity values were found in DI–NT fruit compared to FI–NT (Table 2). This indicates that the significant differences in FF and TA values at harvest between DI–NT and FI–NT fruit were maintained during the subsequent cold storage (Intrigliolo and Castel, 2010; Behboudian et al., 2011). This was not true for the SSC. The differences in SSC between DI–NT and FI–NT were not observed after two and five months of cold storage (Table 2). Therefore, SSC of ‘Conference’ pear subjected to deficit irrigation seems to have a different behavior during cold storage than FF and TA. Weight loss in ‘Conference’ pear increases with time in storage (Nguyen et al., 2007). However, lower weight loss was observed in DI–NT fruit than in FI–NT fruit. This has not been reported for pear but our finding is consistent with those observed in apple (Kilili et al., 1996a; Mpelasoka et al., 2000), apricot (Perez-Pastor et al., 2007) and peach (Mercier et al., 2009). The difference in weight loss between DI–NT and FI–NT fruit could be due to differences in the structure and/or composition of the skin or the epicuticular waxes covering the skin. Cuticle modification by water stress has been reported by Crisosto et al. (1994) in peach fruit where the lower rate of water loss under DI was explained by a thicker cuticle and a higher density of trichomes on the skin surface. A possible reason here could be the lower water content of DI pear. Differences in weight loss in ‘Conference’ pear are function of differences in the original water content (Nguyen et al., 2006). Irrespective of the mechanism involved, DI fruit could have better quality after cold storage than FI fruit due to a reduction in weight loss. The reduction in weight loss in DI fruit could prolong the cold storage life, facilitating marketing for a longer period of time. 4.3. Effect of fruit thinning on yield and fruit quality at harvest
Fig. 3. Relationships between crop load and ethylene production in relation to day of assessment after removal from cold storage in 2009. Relationships were adjusted to a simple linear equation for each day of evaluation. Each point represents a replicate (2 fruits per replicate) from each treatment. Day represents the number of days in jars and asterisks (*) indicate that the slope of the relationship is not equal to zero for P < 0.05.
DI–NT improved important quality attributes at harvest and after cold storage, but it reduced fruit size and fresh-market yield compared to FI–NT (Table 1). This indicates that under DI conditions ‘Conference’ pear cannot produce their maximum yield. It would be therefore necessary to lower the crop load by fruit thinning. Although T trees had lower total yield and fresh-market yield than NT under FI, no detrimental effects on fresh-market yield
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
was observed under DI (Table 1). Therefore, if full irrigation is not allowed fruit thinning can maintain the fresh-market yield (Marsal et al., 2010). Moreover, fruit weight at harvest was always higher in T than in NT trees (Table 1). DI–T trees had similar fruit weights at harvest to FI–NT trees, confirming that fruit size can be improved by fruit thinning as exemplified for peach (Lopez et al., 2006; Marsal et al., 2006) and pear (Marsal et al., 2008). Fruit size in pear is one of the most important quality attributes and fruit thinning under deficit irrigation can improve it thereby enhancing marketability. Fruit thinning did not affect fruit firmness, acidity, and SSC at harvest. But there was a trend, especially in 2008, for higher values of these parameters with fruit thinning (results from regression analysis not shown). Factors such as the timing of fruit thinning may explain the lack of significant responses. Under our conditions, thinning was performed late in the season to coincide with the timing of water shortage. There might not have been enough time available to the expected higher amounts of photoassimilates to have a desirable influence on fruit quality changes in the remaining fruit. A similar conclusion was obtained in another study in peach (Lopez et al., 2010). However, early season fruit thinning has increased firmness, SSC and acidity in apple (Wünsche and Ferguson, 2005) and in peach SSC (Crisosto et al., 1994). 4.4. Effect of fruit thinning on fruit quality in cold storage FI–T and DI–T fruit had higher values of SSC after five months of cold storage than FI–NT and DI–NT fruit (Table 2). The presumably higher photoassimilates arising from fruit thinning, that caused the trend for a higher SSC in the fruit at harvest, might have led to significantly higher SSC accumulated during cold storage. Ethylene production was used as an indicator of physiological maturity. It was measured in T and NT fruit at harvest and immediately after storage and during a 2-week period of shelf life at 20 ◦ C. After removal from cold storage, a significant negative linear relationship was found between crop load and the fruit capability to produce ethylene (Fig. 3). Fruit from trees with light crop load produced more ethylene than fruit from trees with a normal crop load. Low crop load therefore significantly enhanced the physiological maturity of the fruit. T fruit, even when picked at the same firmness value of NT fruit, were more mature. The more advanced physiological maturity with lower crop load has not been reported for pear. Our findings are in agreement with some experiments on apple reviewed by Wünsche and Ferguson (2005). 5. Conclusions This work aimed at introducing a contingency plan for growing pear under water shortage. We applied DI to trees with commercial load (∼180 fruits tree−1 ) which caused a moderate water stress. Under these circumstances, DI increased the fruit firmness, acidity and soluble solids concentrations at harvest while no changes were observed in fruit maturity. The increased firmness and acidity under DI were maintained during the subsequent cold storage. DI also reduced weight loss during storage but as expected also reduced fruit size. To counteract this expected effect we included another treatment, fruit thinning. This treatment, which reduced crop load by 45%, maintained the improved fruit composition associated to DI. Additionally, fruit thinning improved fruit size at harvest and the SSC values after five months of cold storage. Fruit thinning also advanced physiological fruit maturity. This may lead to earlier harvests which could be desirable in years with drought because fruit’s exposure to water stress will be reduced. If moderate water stress is imposed as a consequence of irrigation shortage, fruit quality
69
could improve in ‘Conference’ pear. Lowering crop load by applying fruit thinning produced more benefits compared to leaving a commercial crop load. We therefore recommend fruit thinning as a mitigation technique under pre-harvest water stress conditions. Acknowledgements We thank M. Mata, J. del Campo, B. Basile, C. Paris, and N. Bonastre for their help with the field work. We also thank D. Ubach for the assistance during ethylene determination. This study was funded by The Spanish Ministry of Research and Technology under project AGL2005-00538 and CONSOLIDER-INGENIO 2010 (CSD2006-00067). References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. Guidelines for computing crop water requirements. FAO irrigation and drainage paper n◦ . 56. Bain, J.M., 1961. Some morphological, anatomical, and physiological changes in pear fruit (Pyrus communis var. Williams Bon Chrétien) during development and following harvest. Aust. J. Bot. 9, 99–123. Behboudian, M.H., Marsal, J., Girona, J., Lopez, G., 2011. Quality and yield responses of deciduous fruits to reduced irrigation. Hortic. Rev. 38, 149–189. Crisosto, C.H., Johnson, R.S., Luza, J.G., Crisosto, G.M., 1994. Irrigation regimes affect fruit soluble solids concentration and rate of water loss of ‘O’Henry’ peaches. HortScience 29, 169–1171. Gelly, M., Recasens, I., Mata, M., Arbones, A., Rufat, J., Girona, J., Marsal, J., 2003. Effects of water deficit during stage II of peach fruit development and postharvest on fruit quality and ethylene production. J. Hortic. Sci. Biotechnol. 78, 324–330. Gelly, M., Recasens, I., Girona, J., Mata, M., Arbones, A., Rufat, J., Marsal, J., 2004. Effects of stage II and postharvest deficit irrigation on peach quality during maturation and after cold storage. J. Sci. Food Agric. 84, 561–568. Girona, J., del Campo, J., Mata, M., Lopez, G., Marsal, J., 2011. A comparative study of apple and pear tree water consumption measured with two weighing lysimeters. Irrig. Sci. 29, 55–63. Intrigliolo, D.S., Castel, J.R., 2010. Response of plum trees to deficit irrigation under two crop levels: tree growth, yield and fruit quality. Irrig. Sci. 28, 525–534. Jackson, J.E. (Ed.), 2003. Biology of Apple and Pears. University of Cambridge, United Kingdom. Kilili, A.W., Behboudian, M.H., Mills, T.M., 1996a. Composition and quality of ‘Braeburn’ apples under reduced irrigation. Sci. Hortic. 67, 1–11. Kilili, A.W., Behboudian, M.H., Mills, T.M., 1996b. Postharvest performance of ‘Braeburn’ apples in relation to withholding of irrigation at different stages of the growing season. J. Hortic. Sci. 71, 693–701. Leib, B.G., Caspari, H.W., Redulla, C.A., Andrews, P.K., Jabro, J.J., 2006. Partial rootzone drying and deficit irrigation of ‘Fugiˇı apples in a semi-arid climate. Irrig. Sci. 24, 85–99. Li, S.H., Huguet, J.G., Schoch, P.G., Orlando, P., 1989. Response of peach-tree growth and cropping to soil water deficit at various phenological stages of fruit development. J. Hortic. Sci. 64, 541–552. Lopez, G., Mata, M., Arbones, A., Solans, J.R., Girona, J., Marsal, J., 2006. Mitigation of effects of extreme drought during stage III of peach fruit development by summer pruning and fruit thinning. Tree Physiol. 26, 469–477. Lopez, G., Behboudian, M.H., Vallverdu, X., Mata, M., Girona, J., Marsal, J., 2010. Mitigation of severe water stress by fruit thinning in ‘O’Henry’ peach: implications for fruit quality. Sci. Hortic. 125, 294–300, doi:10.1016/j.scienta.2010.04.003. Marsal, J., Lopez, G., Mata, M., Girona, J., 2006. Branch removal and defruiting for the amelioration of water stress effects on fruit growth during stage III of peach fruit development. Sci. Hortic. 108, 55–60. Marsal, J., Mata, M., Arbones, A., Del Campo, J., Girona, J., Lopez, G., 2008. Factors involved in alleviating water stress by partial crop removal in pear trees. Tree Physiol. 28, 1375–1382. Marsal, J., Behboudian, M.H., Mata, M., Basile, B., del Campo, J., Girona, J., Lopez, G., 2010. Fruit thinning in ‘Conference’ pear grown under deficit irrigation to optimise yield and to improve tree water status. J. Hortic. Sci. Biotechnol. 85, 125–130. McCutchan, H., Shackel, K.A., 1992. Stem water potential as a sensitive indicator of water stress in prune trees (Prunus Domestica L. cv French). J. Am. Soc. Hortic. Sci. 117, 607–611. Mercier, V., Bussi, C., Lescourret, F., Genard, M., 2009. Effects of different irrigation regimes applied during the final stage of rapid fruit growth on an early maturing peach cultivar. Irrig. Sci. 27, 297–306. Mills, T.M., Behboudian, M.H., Tan, P.Y., Clothier, B.E., 1994. Plant water status and fruit quality in ‘Braeburn’ apples. HortScience 29, 1274–1278. Mpelasoka, B.S., Behboudian, M.H., Dixon, J., Neal, S.M., Caspari, H.W., 2000. Improvement of fruit quality and storage potential of ‘Braeburn’ apple through deficit irrigation. J. Hortic. Sci. Biotechnol. 75, 615–621.
70
G. Lopez et al. / Scientia Horticulturae 129 (2011) 64–70
Mpelasoka, B.S., Behboudian, M.H., Mills, T.M., 2001. Water relations, photosynthesis, growth, yield and fruit size of ‘Braeburn’ apple: responses to deficit irrigation and to crop load. J. Hortic. Sci. Biotechnol. 76, 150–156. Nguyen, T.A., Dresselaers, T., Verboven, P., D’hallewin, G., Culeddu, N., Van Hecke, P., Nicolai, B.M., 2006. Finite element modelling and MRI validation of 3D transient water profiles in pears during postharvest storage. J. Sci. Food Agric. 86, 745–756. Nguyen, T.A., Verboven, P., Schenk, A., Nicolaï, B.M., 2007. Prediction of water loss from pears (Pyrus communis cv. Conference) during controlled atmosphere storage as affected by relative humidity. J. Food Eng. 83, 149–155.
Perez-Pastor, A., Ruiz-Sanchez, M.C., Martinez, J.A., Nortes, P.A., Artes, F., Domingo, R., 2007. Effect of deficit irrigation on apricot fruit quality at harvest and during storage. J. Sci. Food Agric. 87, 2409–2415. Ramos, D.E., Weinbaum, S.A., Shackel, K.A., Shcwankl, L.J., Mitcham, E.J., Mitchell, F.G., Snyder, R.G., Mayer, G., McGourty, G., 1994. Influence of tree water status and canopy position on fruit size and quality of Bartlett pears. Acta Hortic. 367, 192–200. Wünsche, J.N., Ferguson, I.B., 2005. Crop load interactions in apple. Hortic. Rev. 31, 231–290.