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Purple spot in loquat (Eriobotrya japonica Lindl.) is associated to changes in flesh-rind water relations during fruit development
Scientia Horticulturae 119 (2008) 55–58
Contents lists available at ScienceDirect
Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti
Purple spot in loquat (Eriobotrya japonica Lindl.) is associated to changes in flesh-rind water relations during fruit development N. Gariglio a, C. Reig b, A. Martinez-Fuentes b, C. Mesejo b, M. Agustı´ b,* a b
Facultad de Ciencias Agrarias, Universidad Nacional del Litoral, Kreder 2805 (3080), Esperanza, Santa Fe, Argentina Instituto Agroforestal Mediterra´neo, Universidad Polite´cnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 June 2008 Accepted 7 July 2008
Experiments were conducted to assess the link between purple spot in loquat fruit (Eriobotrya japonica Lindl.) and changes in the water relations of the flesh and the rind. Panicles were thinned to 1, 3 or 5 fruit or left unthinned (control), fruit wrapped in foil or exposed to the sun, or trees grown under plastic (night temperature >15 8C) or in the open (night temperature 5–3 8C) to induce different levels of the disorder. Typically, spotting increased with thinning (R2 = 0.95), and was higher in exposed fruit (26.3% of fruit affected) than in wrapped fruit (nil), and higher with cool nights (16.2%) than with warm nights (2.7%). Mean tissue water potential (Cw) was similar in the flesh and rind, whereas osmotic potential (p) was higher (less negative) in the flesh, and pressure potential (Cp) lower in the flesh. There were no consistent effects of thinning on Cw, whereas p of the rind decreased (more negative) with thinning during fruit color break. This response was associated with an increase in Cp (more positive) in the rind at the same time. The external rind of exposed fruit had lower p than the external rind of warped fruit, and higher Cp. Similarly, the fruit from trees grown under cool nights had lower rind p and higher rind Cp than fruit under warm nights. These results suggest that low rind p and high rind Cp are associated with purple spotting in loquat, and possibly reflect relatively high sugar concentrations in the flesh that increases the gradient of solute concentration between the flesh and the rind, making easy a dehydration process in the rind, which is responsible for purple spot. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Fruit growth Osmotic potential Pressure potential Physiological disorder Water potential
1. Introduction Loquat fruit (Eriobotrya japonica Lindl.) are highly sensitive to purple spot, a physiological disorder which affects the crop in Taiwan (Liu et al., 1993), Brazil (Ojima et al., 1976) and Spain (Tuset et al., 1989), some times decreasing its commercial returns by up to 50%. Histological studies revealed that purple spot is due to cellular dehydration which appears initially at the deepest cells of the rind and finally affects all rind tissue; nevertheless, neither the structure of the cuticle nor its permeability to water are affected, and therefore, the dehydration of the rind is not caused by excessive fruit transpiration. Besides, the cells of the flesh are not affected (Gariglio et al., 2002). The incidence of purple spot is influenced by environment and cultivation. Low temperatures at color break correlated with purple spot incidence over seven years in Alicante, Spain, and its incidence was reduced by increasings night temperatures in a greenhouse
* Corresponding author. Tel.: +34 963879330; fax: +34 963877331. E-mail address: [email protected] (M. Agustı´). 0304-4238/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2008.07.006
(Gariglio et al., 2003b). Further, no fruit were affected when they were covered (Gariglio et al., 2003b). On the other hand, fruit from non-thinned trees had little spotting, whereas close to 35% of fruit were affected when panicles were thinned to a single fruit (Gariglio et al., 2003a). Thinning reduces competition among developing fruit, thus increases flesh sugar concentration (Agustı´ et al., 2000), which correlates with the incidence of spotting (Gariglio et al., 2003a), and modifies flesh-rind sugar partitioning (Gariglio et al., 2007). These observations lead us to hypothesize that dehydration of the rind is due to changes in flesh-rind sugar concentration when the fruit are growing rapidly (Gariglio et al., 2003a, 2007). According to the epidermal-growth-control hypothesis of growing organs (Kutschera, 1989, 1992), the inner tissue provides the driving force for growth, whereas the peripheral cells limit it, thus determining the rate of elongation. As a consequence, physical flesh-rind stress altering tissue–water relationships has been reported (Peter and Tomos, 1996; Opara et al., 1997). The aim of this research was to examine the water relations of flesh and rind of loquat fruit growing under different conditions which vary the incidence of purple spot. We used fruit thinning, different night temperatures and exposure to sunlight to vary the incidence of the disorder in experiments conducted in Spain.
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N. Gariglio et al. / Scientia Horticulturae 119 (2008) 55–58
2. Material and methods 2.1. Plant material and experimental procedure The experiments were carried out using 15-year-old ‘Algerie’ loquat trees (E. japonica Lindl.), grafted onto loquat seedlings and grown at Alicante, Spain (388390 N; 008070 W). The trees were planted at a 4 m 3 m spacing, on a loamy clay, with drip irrigation. The experiments were laid out in randomized complete blocks, with single-tree plots and six replicates per treatment. To determine the effect of thinning on purple spot, panicles were hand-thinned to 1, 2, 3 or 5 fruit when they were 10 mm in diameter (phenological stage 702 of the BBCH scale; Martı´nezCalvo et al., 1999). Not thinned panicles (9–12 fruit per panicle) were used as control. To study the effects of night temperature, trees were grown in a plastic house (6 m wide, 20 m long, 5 m high) and the temperature maintained above 15 8C at night with an electric heater. Trees growing in the open, where the average minimum night temperature was 8.3 8C, were used as controls. To analyze the influence of exposure, 20 fruit per tree were wrapped in aluminium foil for 30 days before harvest, using uncovered fruit nearby on the same panicle as controls. For the last two experiments all the panicles were thinned to 2 fruit at the 702 phenological growth stage. The percentage of fruit affected by purple spot was evaluated at harvest. 2.2. Water relations In all the experiments, four fruit per tree from each treatment (located in every tree quadrant, at a height of 1.5–2.0 m) were collected on each sampling date from fruit set to maturation, for measurement of water potential (Cw) and osmotic potential (p). Samples were taken at dawn when the soil, plant and atmospheric water potentials were in equilibrium (Milad and Shackel, 1992). The flesh and the rind were analyzed separately and only the external portion of the flesh used. To measure Cw and p of flesh and epidermal tissues, 5 mm disks from the equatorial area of the fruit were excised with a cork borer. Fresh tissue was used for Cw, whereas frozen tissue was used for p. Disks of 1–2 mm thick for epidermal tissue and of 3–4 mm thick for flesh tissue were sliced with a blade and placed in a sampler chamber (C-52, Wescor Inc., Logan, UT, USA) connected to a psychrometer switchbox (Ps-10) and to a dew point microvoltimeter (HT-33T). The dew point hygrometer was previously calibrated with NaCl solutions of known concentrations. To ensure initial water vapor equilibrium, Cw and p were measured at least 4 h after setting the sample in the chamber. Pressure potential (Cp) was calculated by the equation Cw = Cp p (Milad and Shackel, 1992). 2.3. Statistical analysis Data on the incidence of purple spot, Cw, p, and Cp were analysed byanalysis of variance, and comparisons of means made byNewmanKeuls’ multiple range test. Percentages were analyzed after arc-sine transformation of the data. Thinning intensity and percentage of spotting relationship was evaluated by regression. The data were analyzed with Statgraphics 4.1 software (Statistical Graphics Corp.). 3. Results 3.1. Thinning Thinning increased the incidence of purple spot (data not shown), with significant negative relationship between the
Fig. 1. The influence of fruit thinning on the time-course of water potential of flesh (A) and rind (B) of ‘Algerie’ loquat. Trees were thinned to 1 (1F), 3 (3F) or 5 fruit per panicle (5F) or not thinned (C). Arrows indicate time of fruit color break. Standard errors are smaller than the symbols. Means market with different letters differ significantly (P 0.05).
percentage of fruit affected and the number of fruit per panicle (R2 = 0.95; P < 0.01). In loquat, average water potential (Cw) of the flesh and rind did not differ significantly (Fig. 1A and B), but changed with sampling date (Fig. 1). Water potential of both tissues decreased from the end of March up to one week after color break, and then increased (Fig. 1). Date of sampling showed statistical significance for both tissues. Water potential also differed significantly for thinning intensity, showing a significant interaction between thinning intensity and date of sampling. However, these sources of variation of Cw (thinning and thinning-sampling date interaction) did not appear to be consistent posible due to different behavior of thinning on each sampling date (Fig. 1A and B). Mean osmotic potential (p) of the flesh (Fig. 2A) was significantly higher (less negative) than that of the rind (Fig. 2B), with the difference increasing with thinning intensity and the incidence of spotting. As for Cw, p changed significantly with thinning intensity and sampling date. In the flesh, p hardly changed during fruit growth (Fig. 2A), and no clear trend was observed around fruit color break (Fig. 2A). In contrast, rind p remained almost constant throughout fruit growth in control plants, but decreased in thinned trees one week before color break (31 March) (Fig. 2B). Differences between the treatments were the greatest at color break (>0.75 MPa between control and 1 fruit per panicle) and the higher the thinning intensity, the lower rind p (Fig. 2B). There was a strong correlation btween spotting and p values increased (R2 = 0.97; P < 0.01). After color break, rind p increased, with control fruit having higher values than fruits from thinned panicles (Fig. 2B). The pressure potential (Cp) of the flesh from control was below to 0.15 MPa during fruit growth (Fig. 3A). Fruit from thinned trees had higher Cp values than the controls on the first and last
N. Gariglio et al. / Scientia Horticulturae 119 (2008) 55–58
57
Table 1 The effect of exposure to the sun on water (Cw), osmotic(p) and pressure potential (Cp) of the rind at fruit color break in the external and internal surface of ‘Algerie’ loquat Exposed fruit
Covered fruit
External surface Internal surface External surface Internal surface
Cw (MPa) 1.02 p (MPa) 1.84 a Cp (MPa) 0.82 a
0.90 1.11b 0.21 b
0.91 1.29 b 0.38 b
0.87 1.19 b 0.32 b
Values are the mean of six replicates per treatment. Means in a row followed by different letters in the same line indicate significant differences at P 0.05. There were no significant differences in the water relations of the flesh.
sampling. There were not significant differences due to thinning intensity at any other sampling date. Pressure potential of the rind (Fig. 3B) was higher than that of the flesh (Fig. 3A). Mean values of rind Cp were significantly higher in the thinned trees than in the controls. Furthermore, from color break (7 April) to maturity (26 April) the higher thinning intensity, the higher pressure potential of the rind (Fig. 3B). At fruit maturity, values of rind Cp in thinned trees averaged 2.7 fold higher than that in controls (Fig. 3B). 3.2. Exposure of the fruit Fig. 2. The influence of fruit thinning on the time-course of osmotic potential of flesh (A) and rind (B) of ‘Algerie’ loquat. Trees were thinned to 1 (1F), 3 (3F) or 5 fruit per panicle (5F) or not thinned (C). Arrows indicate time of fruit color break. Standard errors are smaller than the symbols. Means market with different letters differ significantly (P 0.05).
Exposure of the fruit did not affect the water relations of the flesh at fruit color break (data not shown). Likewise, Cw of the rind was not affected by exposure, whereas its components p and Cp were significantly altered (Table 1). The epidermis of the external face of the exposed fruit had a lower p and a higher Cp to the internal face and to covered fruit (Table 1). In contrast, the internal face of the exposed fruit had similar p and Cp to covered fruit (Table 1). The percentage of fruit affected by purple spot was 26.3% for control and nil for wrapped fruit (P < 0.01), and with 88% of the spots appearing on the exposed face of the fruit. 3.3. Night temperature Night temperature did not result in significant changes in flesh
Cw, p and Cp (data not shown), or in rind Cw (Table 2). However, p of the rind was higher and Cp lower under warm nights compared with the trees in the open (Table 2). The percentage of fruit affected by purple spot was 16.2% in control fruit and 2.7% in heated fruit (P < 0.01). 4. Discussion From color break to maturity, the flesh and the rind of loquat fruit accumulated between 80 and 90% of all their sugars (Hirai, 1980; Gariglio et al., 2007). Despite this, Cw of the rind and flesh and flesh p did not change significantly around color break or respond to thinning. However, rind p decreased from one week
Table 2 The effect of night heating (15 8C) on water (Cw), osmotic(p) and pressure potential (Cp) of the rind at fruit color break in ‘Algerie’ loquat
Fig. 3. The influence of fruit thinning on the time-course of turgor pressure of flesh (A) and rind (B) of ‘Algerie’ loquat. Trees were thinned to 1 (1F), 3 (3F) or 5 fruit per panicle (5F) or not thinned (C). Arrows indicate time of fruit color break. Standard errors are smaller than the symbols. Means market with different letters differ significantly (P 0.05).
Open plots (cool nights) Under plastic (warm nights)
Cw (MPa)
p (MPa)
Cp (MPa)
1.02 0.85
1.84 1.20*
0.82 0.35*
Average minimum night temperature outdoor was 8.3 8C. Values are the mean of six replicates per treatment. Column means followed by an asterisk are significantly different (P 0.05). There were no significant differences in the water relations of the flesh.
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N. Gariglio et al. / Scientia Horticulturae 119 (2008) 55–58
before color break due to thinning, showing a peak response at fruit color break. The fact that flesh p did not decrease due to sugar accumulation indicates that sugar and water accumulation were in equilibrium (Gariglio, 2001). In grape, a decrease in p and a decrease in flesh Cw occur simultaneously during sugar accumulation, indicating that water uptake lags behind sugar accumulation (Matthews et al., 1987). Further, an increase in the flesh Cp was observed in grape (Matthews et al., 1987), whereas it decreased in loquat (see Fig. 3A). Rind p decreased at fruit color break in response to thinning, suggesting that water content of the rind is restricted when flesh sugar concentrations are high. The concentrations of sugar in the rind on a dry weight basis increased from color break to maturity, irrespective of thinning (Gariglio et al., 2007). Thus, rind p should be equal or lower at maturity than at color break, as occurs in nonthinned trees (see Fig. 2B). However, rind p decreased at color break in fruit from thinned trees, but not in fruit from control trees (see Fig. 2). These results suggest that water uptake lags behind sugar accumulation in the rind or that there is a loss of water from the rind to the flesh of fruit from thinned trees (those with high flesh sugar concentration) causing dehydration, a symptom that characterizes purple spot histologically (Gariglio et al., 2002). Accordingly, high fruit growth rate conditions affects water balance between external an internal tissues (Kutschera, 1989; Peter and Tomos, 1996) and, thus, increases the risk of purple spot occurrence. In thinned trees, the period of high sugar accumulation (70–80% total sugars) and high fruit growth rate occurred simultaneously at color break (Gariglio et al., 2007) and spots the fruit. While growth rate and sugar accumulation remain high, rind tissue shows a constant decrease in p and a constant increase in Cp, allowing the dehydration process to proceed. The fact that the application of different mineral compounds reduced purple spot incidence by major water retention in epidermal tissue (Gariglio et al., 2005), reinforces the hypothesis of an osmotic effect as the responsible for purple spot. The lower p in the exposed rind compared with values in the wrapped fruit also suggests dehydration, since the sugar concentration were more than 20% higher in the rind of covered fruit (Gariglio et al., 2007). Similarly, the osmotic potential of fruit growing in the open was lower than that of fruit growing in the greenhouse, despite the concentration of sugar being 35% lower in the former (Gariglio et al., 2007). In conclusion, the origin of purple spot of loquat fruit is related to an alteration of water relationships between the flesh and the rind caused by the simultaneous occurrence at fruit color break of a period of high sugar accumulation in the flesh in addition to a high fruit growth rate. The dehydration process is enhanced by
cultivation practices (thinning intensity), and environmental factors (low temperature and sunlight exposure) which affect sugar and mineral assimilation and partitioning in favor of the flesh, increasing the gradient of solute concentration between both tissues. Acknowledgements This research was supported by grants from Cooperativa de Callosa d’En Sarria´ (Alicante, Spain). References Agustı´, M., Juan, M., Almela, V., Gariglio, N., 2000. Loquat fruit size is increased though the thinning effect of naphthaleneacetic acid. Plant Growth Regul. 31, 167–171. Gariglio, N., 2001. Caracterizacio´n morfolo´gica, histolo´gica y fisiolo´gica de la ‘‘mancha pu´rpura’’ del nı´spero (Eriobotrya japonica Lindl.). Ph.D. Thesis. Universidad Polite´cnica de Valencia, Valencia, Spain, 272 pp. Gariglio, N., Juan, M., Castillo, A., Almela, V., Agustı´, M., 2002. Histological and physiological study of purple spot of loquat fruit. Sci. Hortic. 92, 255–263. Gariglio, N., Castillo, A., Juan, M., Almela, V., Agustı´, M., 2003a. Effects of fruit thinning on fruit growth, sugars and purple spot in loquat fruit (Eriobotrya japonica Lindl.). J. Hortic. Sci. Biotechnol. 78, 32–34. Gariglio, N., Castillo, A., Alos, E., Juan, M., Almela, V., Agustı´, M., 2003b. The influences of environmental factors on the development of purple spot of loquat fruit (Eriobotrya japonica Lindl.). Sci. Hortic. 98, 17–23. Gariglio, N., Martı´nez-Fuentes, A., Mesejo, C., Agustı´, M., 2005. Control of purple spot of loquat fruit (Eriobotrya japo´nica Lindl.) by means of mineral compounds. Ann. Appl. Biol. 146, 415–420. Gariglio, N., Reig, C., Agustı´, M., 2007. Assimilate partitioning between the flesh and the rind is responsible for purple spot in loquat fruit. J. Hortic. Sci. Biotechnol. 83, 37–42. Hirai, M., 1980. Sugar accumulation and development of loquat fruit. J. Jpn. Soc. Hortic. Sci. 49, 347–353. Kutschera, U., 1989. Tissue stresses in growing plant organs. Physiol. Plant 77, 157– 163. Kutschera, U., 1992. The role of the epidermis in the control of elongation growth in stems and coleoptiles. Bot. Acta 105, 246–252. Liu, T.T., Lin, J.T., Chang, L.R., 1993. Control and prevention of diseases and physiological disorder in loquat. Proc. Symp. Tech. Loquat Production, Taichung District Agric. Improv. Stat. 34, 189–195. Martı´nez-Calvo, J., Badenes, M.L., Lla´cer, G., Bleiholder, H., Hack, H., Meier, U., 1999. Phenological growth stages of loquat tree (Eriobotrya japonica Lindl.). Ann. Appl. Biol. 134, 353–357. Matthews, M.A., Cheng, G., Weinbaum, S.A., 1987. Changes in water potential and dermal extensibility during grape berry development. J. Am. Soc. Hortic. Sci. 112, 314–319. Milad, R.E., Shackel, K.A., 1992. Water relations of fruit end cracking in French Prune (Prunus dome´stica L. cv. French). J. Am. Soc. Hortic. Sci. 117, 824–828. Ojima, M., Rigitano, O., Simao, S., Ique, T., 1976. The effect of the type of fruit protection on the incidence of purple spot and fruit development in loquats. Bragantia 35, XI–XLIV. Opara, L.U., Studman, C.J., Banks, N.H., 1997. Fruit skin splitting and cracking. Hortic. Rev. 19, 217–261. Peter, W.S., Tomos, A.D., 1996. The history of tissue tension. Ann. Bot. 77, 657–665. Tuset, J.J., Rodriguez, A., Bononad, S., Garcia, J., Monteagudo, E., 1989. La mancha morada del Nı´spero. Generalitat Valenciana, Valencia.
Contents lists available at ScienceDirect
Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti
Purple spot in loquat (Eriobotrya japonica Lindl.) is associated to changes in flesh-rind water relations during fruit development N. Gariglio a, C. Reig b, A. Martinez-Fuentes b, C. Mesejo b, M. Agustı´ b,* a b
Facultad de Ciencias Agrarias, Universidad Nacional del Litoral, Kreder 2805 (3080), Esperanza, Santa Fe, Argentina Instituto Agroforestal Mediterra´neo, Universidad Polite´cnica de Valencia, Camino de Vera, s/n, 46022 Valencia, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 June 2008 Accepted 7 July 2008
Experiments were conducted to assess the link between purple spot in loquat fruit (Eriobotrya japonica Lindl.) and changes in the water relations of the flesh and the rind. Panicles were thinned to 1, 3 or 5 fruit or left unthinned (control), fruit wrapped in foil or exposed to the sun, or trees grown under plastic (night temperature >15 8C) or in the open (night temperature 5–3 8C) to induce different levels of the disorder. Typically, spotting increased with thinning (R2 = 0.95), and was higher in exposed fruit (26.3% of fruit affected) than in wrapped fruit (nil), and higher with cool nights (16.2%) than with warm nights (2.7%). Mean tissue water potential (Cw) was similar in the flesh and rind, whereas osmotic potential (p) was higher (less negative) in the flesh, and pressure potential (Cp) lower in the flesh. There were no consistent effects of thinning on Cw, whereas p of the rind decreased (more negative) with thinning during fruit color break. This response was associated with an increase in Cp (more positive) in the rind at the same time. The external rind of exposed fruit had lower p than the external rind of warped fruit, and higher Cp. Similarly, the fruit from trees grown under cool nights had lower rind p and higher rind Cp than fruit under warm nights. These results suggest that low rind p and high rind Cp are associated with purple spotting in loquat, and possibly reflect relatively high sugar concentrations in the flesh that increases the gradient of solute concentration between the flesh and the rind, making easy a dehydration process in the rind, which is responsible for purple spot. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Fruit growth Osmotic potential Pressure potential Physiological disorder Water potential
1. Introduction Loquat fruit (Eriobotrya japonica Lindl.) are highly sensitive to purple spot, a physiological disorder which affects the crop in Taiwan (Liu et al., 1993), Brazil (Ojima et al., 1976) and Spain (Tuset et al., 1989), some times decreasing its commercial returns by up to 50%. Histological studies revealed that purple spot is due to cellular dehydration which appears initially at the deepest cells of the rind and finally affects all rind tissue; nevertheless, neither the structure of the cuticle nor its permeability to water are affected, and therefore, the dehydration of the rind is not caused by excessive fruit transpiration. Besides, the cells of the flesh are not affected (Gariglio et al., 2002). The incidence of purple spot is influenced by environment and cultivation. Low temperatures at color break correlated with purple spot incidence over seven years in Alicante, Spain, and its incidence was reduced by increasings night temperatures in a greenhouse
* Corresponding author. Tel.: +34 963879330; fax: +34 963877331. E-mail address: [email protected] (M. Agustı´). 0304-4238/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2008.07.006
(Gariglio et al., 2003b). Further, no fruit were affected when they were covered (Gariglio et al., 2003b). On the other hand, fruit from non-thinned trees had little spotting, whereas close to 35% of fruit were affected when panicles were thinned to a single fruit (Gariglio et al., 2003a). Thinning reduces competition among developing fruit, thus increases flesh sugar concentration (Agustı´ et al., 2000), which correlates with the incidence of spotting (Gariglio et al., 2003a), and modifies flesh-rind sugar partitioning (Gariglio et al., 2007). These observations lead us to hypothesize that dehydration of the rind is due to changes in flesh-rind sugar concentration when the fruit are growing rapidly (Gariglio et al., 2003a, 2007). According to the epidermal-growth-control hypothesis of growing organs (Kutschera, 1989, 1992), the inner tissue provides the driving force for growth, whereas the peripheral cells limit it, thus determining the rate of elongation. As a consequence, physical flesh-rind stress altering tissue–water relationships has been reported (Peter and Tomos, 1996; Opara et al., 1997). The aim of this research was to examine the water relations of flesh and rind of loquat fruit growing under different conditions which vary the incidence of purple spot. We used fruit thinning, different night temperatures and exposure to sunlight to vary the incidence of the disorder in experiments conducted in Spain.
56
N. Gariglio et al. / Scientia Horticulturae 119 (2008) 55–58
2. Material and methods 2.1. Plant material and experimental procedure The experiments were carried out using 15-year-old ‘Algerie’ loquat trees (E. japonica Lindl.), grafted onto loquat seedlings and grown at Alicante, Spain (388390 N; 008070 W). The trees were planted at a 4 m 3 m spacing, on a loamy clay, with drip irrigation. The experiments were laid out in randomized complete blocks, with single-tree plots and six replicates per treatment. To determine the effect of thinning on purple spot, panicles were hand-thinned to 1, 2, 3 or 5 fruit when they were 10 mm in diameter (phenological stage 702 of the BBCH scale; Martı´nezCalvo et al., 1999). Not thinned panicles (9–12 fruit per panicle) were used as control. To study the effects of night temperature, trees were grown in a plastic house (6 m wide, 20 m long, 5 m high) and the temperature maintained above 15 8C at night with an electric heater. Trees growing in the open, where the average minimum night temperature was 8.3 8C, were used as controls. To analyze the influence of exposure, 20 fruit per tree were wrapped in aluminium foil for 30 days before harvest, using uncovered fruit nearby on the same panicle as controls. For the last two experiments all the panicles were thinned to 2 fruit at the 702 phenological growth stage. The percentage of fruit affected by purple spot was evaluated at harvest. 2.2. Water relations In all the experiments, four fruit per tree from each treatment (located in every tree quadrant, at a height of 1.5–2.0 m) were collected on each sampling date from fruit set to maturation, for measurement of water potential (Cw) and osmotic potential (p). Samples were taken at dawn when the soil, plant and atmospheric water potentials were in equilibrium (Milad and Shackel, 1992). The flesh and the rind were analyzed separately and only the external portion of the flesh used. To measure Cw and p of flesh and epidermal tissues, 5 mm disks from the equatorial area of the fruit were excised with a cork borer. Fresh tissue was used for Cw, whereas frozen tissue was used for p. Disks of 1–2 mm thick for epidermal tissue and of 3–4 mm thick for flesh tissue were sliced with a blade and placed in a sampler chamber (C-52, Wescor Inc., Logan, UT, USA) connected to a psychrometer switchbox (Ps-10) and to a dew point microvoltimeter (HT-33T). The dew point hygrometer was previously calibrated with NaCl solutions of known concentrations. To ensure initial water vapor equilibrium, Cw and p were measured at least 4 h after setting the sample in the chamber. Pressure potential (Cp) was calculated by the equation Cw = Cp p (Milad and Shackel, 1992). 2.3. Statistical analysis Data on the incidence of purple spot, Cw, p, and Cp were analysed byanalysis of variance, and comparisons of means made byNewmanKeuls’ multiple range test. Percentages were analyzed after arc-sine transformation of the data. Thinning intensity and percentage of spotting relationship was evaluated by regression. The data were analyzed with Statgraphics 4.1 software (Statistical Graphics Corp.). 3. Results 3.1. Thinning Thinning increased the incidence of purple spot (data not shown), with significant negative relationship between the
Fig. 1. The influence of fruit thinning on the time-course of water potential of flesh (A) and rind (B) of ‘Algerie’ loquat. Trees were thinned to 1 (1F), 3 (3F) or 5 fruit per panicle (5F) or not thinned (C). Arrows indicate time of fruit color break. Standard errors are smaller than the symbols. Means market with different letters differ significantly (P 0.05).
percentage of fruit affected and the number of fruit per panicle (R2 = 0.95; P < 0.01). In loquat, average water potential (Cw) of the flesh and rind did not differ significantly (Fig. 1A and B), but changed with sampling date (Fig. 1). Water potential of both tissues decreased from the end of March up to one week after color break, and then increased (Fig. 1). Date of sampling showed statistical significance for both tissues. Water potential also differed significantly for thinning intensity, showing a significant interaction between thinning intensity and date of sampling. However, these sources of variation of Cw (thinning and thinning-sampling date interaction) did not appear to be consistent posible due to different behavior of thinning on each sampling date (Fig. 1A and B). Mean osmotic potential (p) of the flesh (Fig. 2A) was significantly higher (less negative) than that of the rind (Fig. 2B), with the difference increasing with thinning intensity and the incidence of spotting. As for Cw, p changed significantly with thinning intensity and sampling date. In the flesh, p hardly changed during fruit growth (Fig. 2A), and no clear trend was observed around fruit color break (Fig. 2A). In contrast, rind p remained almost constant throughout fruit growth in control plants, but decreased in thinned trees one week before color break (31 March) (Fig. 2B). Differences between the treatments were the greatest at color break (>0.75 MPa between control and 1 fruit per panicle) and the higher the thinning intensity, the lower rind p (Fig. 2B). There was a strong correlation btween spotting and p values increased (R2 = 0.97; P < 0.01). After color break, rind p increased, with control fruit having higher values than fruits from thinned panicles (Fig. 2B). The pressure potential (Cp) of the flesh from control was below to 0.15 MPa during fruit growth (Fig. 3A). Fruit from thinned trees had higher Cp values than the controls on the first and last
N. Gariglio et al. / Scientia Horticulturae 119 (2008) 55–58
57
Table 1 The effect of exposure to the sun on water (Cw), osmotic(p) and pressure potential (Cp) of the rind at fruit color break in the external and internal surface of ‘Algerie’ loquat Exposed fruit
Covered fruit
External surface Internal surface External surface Internal surface
Cw (MPa) 1.02 p (MPa) 1.84 a Cp (MPa) 0.82 a
0.90 1.11b 0.21 b
0.91 1.29 b 0.38 b
0.87 1.19 b 0.32 b
Values are the mean of six replicates per treatment. Means in a row followed by different letters in the same line indicate significant differences at P 0.05. There were no significant differences in the water relations of the flesh.
sampling. There were not significant differences due to thinning intensity at any other sampling date. Pressure potential of the rind (Fig. 3B) was higher than that of the flesh (Fig. 3A). Mean values of rind Cp were significantly higher in the thinned trees than in the controls. Furthermore, from color break (7 April) to maturity (26 April) the higher thinning intensity, the higher pressure potential of the rind (Fig. 3B). At fruit maturity, values of rind Cp in thinned trees averaged 2.7 fold higher than that in controls (Fig. 3B). 3.2. Exposure of the fruit Fig. 2. The influence of fruit thinning on the time-course of osmotic potential of flesh (A) and rind (B) of ‘Algerie’ loquat. Trees were thinned to 1 (1F), 3 (3F) or 5 fruit per panicle (5F) or not thinned (C). Arrows indicate time of fruit color break. Standard errors are smaller than the symbols. Means market with different letters differ significantly (P 0.05).
Exposure of the fruit did not affect the water relations of the flesh at fruit color break (data not shown). Likewise, Cw of the rind was not affected by exposure, whereas its components p and Cp were significantly altered (Table 1). The epidermis of the external face of the exposed fruit had a lower p and a higher Cp to the internal face and to covered fruit (Table 1). In contrast, the internal face of the exposed fruit had similar p and Cp to covered fruit (Table 1). The percentage of fruit affected by purple spot was 26.3% for control and nil for wrapped fruit (P < 0.01), and with 88% of the spots appearing on the exposed face of the fruit. 3.3. Night temperature Night temperature did not result in significant changes in flesh
Cw, p and Cp (data not shown), or in rind Cw (Table 2). However, p of the rind was higher and Cp lower under warm nights compared with the trees in the open (Table 2). The percentage of fruit affected by purple spot was 16.2% in control fruit and 2.7% in heated fruit (P < 0.01). 4. Discussion From color break to maturity, the flesh and the rind of loquat fruit accumulated between 80 and 90% of all their sugars (Hirai, 1980; Gariglio et al., 2007). Despite this, Cw of the rind and flesh and flesh p did not change significantly around color break or respond to thinning. However, rind p decreased from one week
Table 2 The effect of night heating (15 8C) on water (Cw), osmotic(p) and pressure potential (Cp) of the rind at fruit color break in ‘Algerie’ loquat
Fig. 3. The influence of fruit thinning on the time-course of turgor pressure of flesh (A) and rind (B) of ‘Algerie’ loquat. Trees were thinned to 1 (1F), 3 (3F) or 5 fruit per panicle (5F) or not thinned (C). Arrows indicate time of fruit color break. Standard errors are smaller than the symbols. Means market with different letters differ significantly (P 0.05).
Open plots (cool nights) Under plastic (warm nights)
Cw (MPa)
p (MPa)
Cp (MPa)
1.02 0.85
1.84 1.20*
0.82 0.35*
Average minimum night temperature outdoor was 8.3 8C. Values are the mean of six replicates per treatment. Column means followed by an asterisk are significantly different (P 0.05). There were no significant differences in the water relations of the flesh.
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before color break due to thinning, showing a peak response at fruit color break. The fact that flesh p did not decrease due to sugar accumulation indicates that sugar and water accumulation were in equilibrium (Gariglio, 2001). In grape, a decrease in p and a decrease in flesh Cw occur simultaneously during sugar accumulation, indicating that water uptake lags behind sugar accumulation (Matthews et al., 1987). Further, an increase in the flesh Cp was observed in grape (Matthews et al., 1987), whereas it decreased in loquat (see Fig. 3A). Rind p decreased at fruit color break in response to thinning, suggesting that water content of the rind is restricted when flesh sugar concentrations are high. The concentrations of sugar in the rind on a dry weight basis increased from color break to maturity, irrespective of thinning (Gariglio et al., 2007). Thus, rind p should be equal or lower at maturity than at color break, as occurs in nonthinned trees (see Fig. 2B). However, rind p decreased at color break in fruit from thinned trees, but not in fruit from control trees (see Fig. 2). These results suggest that water uptake lags behind sugar accumulation in the rind or that there is a loss of water from the rind to the flesh of fruit from thinned trees (those with high flesh sugar concentration) causing dehydration, a symptom that characterizes purple spot histologically (Gariglio et al., 2002). Accordingly, high fruit growth rate conditions affects water balance between external an internal tissues (Kutschera, 1989; Peter and Tomos, 1996) and, thus, increases the risk of purple spot occurrence. In thinned trees, the period of high sugar accumulation (70–80% total sugars) and high fruit growth rate occurred simultaneously at color break (Gariglio et al., 2007) and spots the fruit. While growth rate and sugar accumulation remain high, rind tissue shows a constant decrease in p and a constant increase in Cp, allowing the dehydration process to proceed. The fact that the application of different mineral compounds reduced purple spot incidence by major water retention in epidermal tissue (Gariglio et al., 2005), reinforces the hypothesis of an osmotic effect as the responsible for purple spot. The lower p in the exposed rind compared with values in the wrapped fruit also suggests dehydration, since the sugar concentration were more than 20% higher in the rind of covered fruit (Gariglio et al., 2007). Similarly, the osmotic potential of fruit growing in the open was lower than that of fruit growing in the greenhouse, despite the concentration of sugar being 35% lower in the former (Gariglio et al., 2007). In conclusion, the origin of purple spot of loquat fruit is related to an alteration of water relationships between the flesh and the rind caused by the simultaneous occurrence at fruit color break of a period of high sugar accumulation in the flesh in addition to a high fruit growth rate. The dehydration process is enhanced by
cultivation practices (thinning intensity), and environmental factors (low temperature and sunlight exposure) which affect sugar and mineral assimilation and partitioning in favor of the flesh, increasing the gradient of solute concentration between both tissues. Acknowledgements This research was supported by grants from Cooperativa de Callosa d’En Sarria´ (Alicante, Spain). References Agustı´, M., Juan, M., Almela, V., Gariglio, N., 2000. Loquat fruit size is increased though the thinning effect of naphthaleneacetic acid. Plant Growth Regul. 31, 167–171. Gariglio, N., 2001. Caracterizacio´n morfolo´gica, histolo´gica y fisiolo´gica de la ‘‘mancha pu´rpura’’ del nı´spero (Eriobotrya japonica Lindl.). Ph.D. Thesis. Universidad Polite´cnica de Valencia, Valencia, Spain, 272 pp. Gariglio, N., Juan, M., Castillo, A., Almela, V., Agustı´, M., 2002. Histological and physiological study of purple spot of loquat fruit. Sci. Hortic. 92, 255–263. Gariglio, N., Castillo, A., Juan, M., Almela, V., Agustı´, M., 2003a. Effects of fruit thinning on fruit growth, sugars and purple spot in loquat fruit (Eriobotrya japonica Lindl.). J. Hortic. Sci. Biotechnol. 78, 32–34. Gariglio, N., Castillo, A., Alos, E., Juan, M., Almela, V., Agustı´, M., 2003b. The influences of environmental factors on the development of purple spot of loquat fruit (Eriobotrya japonica Lindl.). Sci. Hortic. 98, 17–23. Gariglio, N., Martı´nez-Fuentes, A., Mesejo, C., Agustı´, M., 2005. Control of purple spot of loquat fruit (Eriobotrya japo´nica Lindl.) by means of mineral compounds. Ann. Appl. Biol. 146, 415–420. Gariglio, N., Reig, C., Agustı´, M., 2007. Assimilate partitioning between the flesh and the rind is responsible for purple spot in loquat fruit. J. Hortic. Sci. Biotechnol. 83, 37–42. Hirai, M., 1980. Sugar accumulation and development of loquat fruit. J. Jpn. Soc. Hortic. Sci. 49, 347–353. Kutschera, U., 1989. Tissue stresses in growing plant organs. Physiol. Plant 77, 157– 163. Kutschera, U., 1992. The role of the epidermis in the control of elongation growth in stems and coleoptiles. Bot. Acta 105, 246–252. Liu, T.T., Lin, J.T., Chang, L.R., 1993. Control and prevention of diseases and physiological disorder in loquat. Proc. Symp. Tech. Loquat Production, Taichung District Agric. Improv. Stat. 34, 189–195. Martı´nez-Calvo, J., Badenes, M.L., Lla´cer, G., Bleiholder, H., Hack, H., Meier, U., 1999. Phenological growth stages of loquat tree (Eriobotrya japonica Lindl.). Ann. Appl. Biol. 134, 353–357. Matthews, M.A., Cheng, G., Weinbaum, S.A., 1987. Changes in water potential and dermal extensibility during grape berry development. J. Am. Soc. Hortic. Sci. 112, 314–319. Milad, R.E., Shackel, K.A., 1992. Water relations of fruit end cracking in French Prune (Prunus dome´stica L. cv. French). J. Am. Soc. Hortic. Sci. 117, 824–828. Ojima, M., Rigitano, O., Simao, S., Ique, T., 1976. The effect of the type of fruit protection on the incidence of purple spot and fruit development in loquats. Bragantia 35, XI–XLIV. Opara, L.U., Studman, C.J., Banks, N.H., 1997. Fruit skin splitting and cracking. Hortic. Rev. 19, 217–261. Peter, W.S., Tomos, A.D., 1996. The history of tissue tension. Ann. Bot. 77, 657–665. Tuset, J.J., Rodriguez, A., Bononad, S., Garcia, J., Monteagudo, E., 1989. La mancha morada del Nı´spero. Generalitat Valenciana, Valencia.