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Optimal crop load and positioning of fruit in cherimoya (Annona cherimola Mill.) trees
Scientia Horticulturae 115 (2008) 129–134 www.elsevier.com/locate/scihorti
Optimal crop load and positioning of fruit in cherimoya (Annona cherimola Mill.) trees M. Gonza´lez a, J. Cuevas b,* a
Estacio´n Experimental de la Fundacio´n Cajamar, Autovı´a del Mediterra´neo, km 416.7, 04710 El Ejido, Almerı´a, Spain b Dpto. Produccio´n Vegetal, Universidad de Almerı´a, La Can˜ada de San Urbano s/n, 04120 Almerı´a, Spain Received 12 November 2006; received in revised form 14 June 2007; accepted 14 August 2007
Abstract Dichogamy and the lack of efficient pollinators in most producing countries make hand pollination of cherimoya unavoidable. In contrast to its high-cost, this technique allows growers to fix crop load levels and fruit location within the tree canopy. Over three consecutive years we have studied the optimal fruit load for the cultivar ‘Fino de Jete’ and checked the effects of the vigor of the fruit-bearing shoots and of the fruit position along the shoot on the resulting fruit set and quality. Our results show that yield in well-managed orchards fitted a logarithmic curve with a maximum close to 30 t ha 1 achieved by setting about 400 fruits per adult tree. No significant effects of crop load were detected on fruit size during the first 2 years; in the third year, however, trees with a lower yield produced heavier fruits. An upper limit of 0.22 kg (fresh fruit weight) cm 2 of trunk cross-sectional area is proposed if fruit quality is to be preserved. High yield during three consecutive years did not diminish tree growth. We found no consistent effects of shoot vigor on fruit set and quality. However, a gradient for heavier fruits was observed from the base to the apex of the bearing shoots. # 2007 Elsevier B.V. All rights reserved. Keywords: Cherimoya yield potential; Fruit quality; Seed index; Sink–source relationships
1. Introduction The consecution of high yields of great quality in any crop depends on the resolution of the sink–source relationships within the plant (Ho, 1988). These sink–source relationships are poorly known in cherimoya and, for instance, yield potential of the species has not yet been established. Current recommendations in Spain are to limit yield to 14–15 t ha 1, because increasing crop load beyond supposedly reduces fruit size and quality (Guirado et al., 2001). The source capacity to enlarge the fruits is primarily determined by the size of the canopy and by the photosynthesis rate. Guirado et al. (2001) take into account tree size in cherimoya and propose to fix crop load according to orchard density. In standard plantations (204 trees ha 1), these authors suggest a crop load of about 150 fruits per tree, while in high-density orchards (416 trees ha 1) a maximum number of 70 fruits per tree is proposed. This
* Corresponding author. Tel.: +34 950 015 559; fax: +34 950 015 939. E-mail address: [email protected] (J. Cuevas). 0304-4238/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2007.08.002
adjustment can also be expressed in terms of canopy volume or trunk diameter. On the opposite side, the sink strength of a cherimoya fruit is known to be influenced by seed number, parameter strongly dependent on pollination efficiency (Schroeder, 1941). Natural pollination of cherimoya flowers is hindered by dichogamy and by the lack of efficient pollinators in most producing countries (Wester, 1910; Schroeder, 1941; Gazit et al., 1982). The deficit in natural pollination in these countries has to be solved by means of artificial hand pollination. Artificial pollination represents a major cost for this crop, but allows the levels of fruit load to be closely fixed by selecting the number of handpollinated flowers. Besides fixing crop load, artificial pollination permits producers to set fruits in selected places within the tree canopy. Cherimoya flowers develop on basal nodes of current season’s growth and at leaf scars in 1-year-old shoots of different length and vigor (Fig. 1). Cautı´n et al. (1999) and Gardiaza´bal and Rosenberg (1994) recommend choosing shoots of intermediate vigor for hand-pollination, since weak twigs are unable to enlarge fruits and vigorous shoots are not prone to produce flowers. Ahmed (1936) and Cabezas and Garcı´a-Tapia (1998) prefer, on the contrary, vigorous shoots
M. Gonza´lez, J. Cuevas / Scientia Horticulturae 115 (2008) 129–134
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Fig. 1. Flowering habit of cherimoya (first author’s drawing).
because they are supposed to produce heavier fruits. On the other hand, the effect of the node on which the flower develops has not yet been experimentally studied. Although Guirado et al. (2001) recommend flowers from basal nodes of 1-year-old shoots because they may form heavier fruits, they do not provide experimental data to justify their election. As the information from literature is incomplete and contradictory, and too often lacking experimental support, we have studied during three consecutive years the effects of increasing crop load on the productivity and fruit quality of ‘Fino de Jete’. Other two objectives of this research are to analyze the effects of the vigor of the bearing shoots and of the position of the fruit along the shoot on fruit set and quality in this cultivar.
However, two or three hand pollination operations were often needed in treatments with high crop load. These pollinations were completed during the same week and no obvious effects due to the different date of pollination were detected. Given the high success of hand pollination in this species, the desired crop load was achieved by pollinating the same number of flowers as fruits required. However, in 2004, about 20% more flowers per level were pollinated because a failure was detected early in the season. Yield as t ha 1 and yield efficiency as kg cm 2 of trunk cross-sectional area (TCSA) were calculated for each crop load level. Fruit weight at harvest was measured on a sample of 25 fruits chosen at random from the most representative pick of each treatment. The number of seeds per fruit was counted and a seed index calculated as the number of seeds per 100 g of fresh fruit weight (Richardson and Anderson, 1996). The pulp of each fruit was processed to determine the total amount of soluble solids (TSS) (Brix) with an electronic refractometer, and the titratable acidity as milligram of malic acid per gram of fresh pulp weight (Alique, 1995). The growth of each tree was estimated after harvest by measuring every year the increase in trunk diameter 20 cm above ground, and also by the weight of the pruning wood. Pruning was performed at the end of February. 2.2. Shoot vigor effects on fruit set and quality
2. Materials and methods The trials were carried out in seasons 2003–2005 on a solid block of ‘Fino de Jete’ cherimoya trees located at the Experimental Station of Foundation Cajamar (El Ejido, Almerı´a, Spain). The trees were planted in 1987 at a distance of 8 m 6 m (208 trees ha 1). The orchard is non-tillaged and is drip irrigated with 7160 m3 ha 1 year 1. The fertilization program includes 120–80–100 NPK kg ha 1 year 1. The average trunk diameter 20 cm above ground was 26.7 2.7 cm at the beginning of the experiments. Pollen used in hand pollination was collected from preanthesis ‘Campas’ flowers and kept in refrigerator (4 8C) until the next morning when it was applied to ‘Fino de Jete’ flowers in their female phase between 7:00 and 11:00 am using a pollen puffer (Kurita Tabaco Co., Tochigi, Japan). The pollinated flowers were tagged with a view to removing undesirable small fruits product of open pollination. 2.1. Crop load effects on yield and fruit quality The effects of crop load on yield and fruit quality were assessed using regression analyses. Crop load levels from 100 to 500 fruits per tree in intervals of 50 fruits were studied using a single tree per level. Crop load levels were assigned at random and trees forming part of the trial were chosen based on their uniform size. The experiment was repeated over three consecutive years using the same trees with the aim of checking long-term effects. Hand pollination was performed on the same day provided the number of flowers available for pollination was enough.
The effect of shoot vigor on fruit set and quality was studied in the years 2003 and 2005 using a split-plot design. The first factor, shoot vigor, included three levels (low, medium and high vigor; see characteristics in Table 1); the second factor, fruit load, included two levels: two and four fruits per shoot. Sixteen shoots of each type were marked in each tree; eight with two fruits, and eight with four. Four trees acted as replications. Fruit set and weight, diameter, seed number and seed index were determined as above. The TSS content and fruit acidity were measured in ripe fruits as previously described. 2.3. Effects of fruit position along the shoot on fruit set and quality The effect on fruit set and quality of the shoot node where the flower develops was studied in 2003 and 2005 following a randomized block design. Four trees acted as blocks and replications whereas the treatments were represented by distinct flower positions on 1-year-old shoot as follows: basal (arising from nodes 1–4), middle (on nodes 5–8) and apical (on the ninth node or beyond). Flowers were compared using Table 1 Shoot vigor characterization: basal diameter (mm), length (cm) and number of nodes per shoot Vigor
Basal diameter (mm)
Length (cm)
Nodes (number)
Weak Medium High
6 0.5 8 0.7 10 1.0
28 4 42 8 72 9
82 13 4 21 6
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medium vigor shoots with an average of 13 nodes. Ten shoots per treatment were tagged in each tree. Two flowers per shoot were hand pollinated in this experiment. The fruit set and quality were determined as described above. Depending on the experimental design, the statistical analyses were performed using Statgraphics Plus 4.1 (Manugistics Inc., Rockville, MD), SigmaPlot (Systat Software Inc. San Jose, CA) or Statixtic (Analytical Software, St. Paul, MN) software packages. 3. Results 3.1. Crop load effects on yield and fruit quality The number of fruits harvested increased linearly with hand pollination levels (Table 2). However, the number of fruits in low-yield trees was always higher than the number of handpollinated flowers highlighting that natural setting occurred to some extent despite our efforts to remove misshapen fruits resulting from open pollination. The fruit set was unaffected by pollination levels, but changed depending on the season. A percentage above 90% of hand-pollinated flowers set fruit in 2003 and 2005 with no differences among treatments. In 2004, the final fruit set was established at around 75%. Preharvest fruit drop rose exponentially with pollination level in 2003 and 2005 (r2 = 0.80, p < 0.001 and r2 = 0.56, p < 0.05, respectively). This relationship failed in 2004. Preharvest drop was more intense in 2005 when the yield was higher. Yield increased with pollination levels following a logarithmic curve (Fig. 2). Yield efficiency also augmented with pollination level, in this case, linearly (Fig. 3). Increasing the number of fruits per tree did not affect their size in the first 2 years (Table 2). However, in the last season a remarkable increase in fruit size was observed in trees with a low fruit load. In this year, a negative logarithmic relationship was detected between fruit weight and number (r2 = 0.87 and p < 0.001). The number of seeds per fruit was independent of the crop load ( p > 0.05). On the contrary, a noticeable effect of the season was observed. The number of seeds per fruit was the highest in the third year. The seed index ratio did not change
Fig. 2. Yield (t ha 1) depending on the number of fruits per tree: 2003 (*); 2004 (*); 2005 (^).
Fig. 3. Yield efficiency as g of fresh fruit weight per cm2 of trunk crosssectional area (TCSA) depending on the number of fruits per tree: 2003 (*); 2004 (*); 2005 (^).
among treatments in the first 2 years; but a sharp increase in seed ratio was observed in response to increasing fruit loads in 2005 (Table 2). A trend for sweeter fruits was observed with lower crop load, although the differences in TSS content were slight and usually non-significant (data not shown). No effects on fruit acidity were observed in any year (data not shown). High-yield did not condition tree growth. Neither trunk enlargement along the experiment (r2 = 0.06 and p = 0.53), nor
Table 2 Fruit number per tree, fruit weight (g) and seed index (seeds per 100 g of fresh fruit) depending on the number of flowers pollinated per tree Treatment
T100 T150 T200 T250 T300 T350 T400 T450 T500 r2 lineal r2 logarithmic
Fruit number per treea
Fruit weight (g)
Seed index
2003
2004
2005
2003
2004
2005
2003
2004
2005
108 183 215 284 303 334 369 420 464 0.98*** 0.95***
172 134 178 222 273 216 353 233 396 0.67** 0.55**
110 178 222 277 246 318 375 420 483 0.96*** 0.87***
435.8 474.2 392.5 403.4 437.8 526.8 489.7 405.6 454.5 ns ns
418.2 381.9 244.2 342.1 381.4 384.7 404.9 407.6 358.1 ns ns
713.2 705.1 616.5 491.9 571.0 500.0 492.4 493.2 411.4 0.83*** 0.87***
16 12 14 18 17 15 – 17 – ns ns
11 11 8 10 9 11 13 11 12 ns ns
11 14 14 18 15 19 18 19 22 0.83*** 0.82***
ns, non-significant; *p < 0.05; **p < 0.01; a Significance according to the p-value.
***
p < 0.001.
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Table 3 Fruit set (%), fruit weight (g), number of seeds per fruit, and seed index (number of seeds per 100 g fresh fruit weight) depending on the shoot vigor and fruit load
Vigor High Medium Weak p Load 2 fruits 4 fruits p
Fruit set 20 dapa (%)
Weight (g)
2003
2005
2003
2005
2003
2005
2003
2005
85 78 84
82 83 90
429 a 303 b 403 a
419 441 345
51 a 35 b 46 a
57 54 46
12.0 11.7 11.8
13.5 12.8 14.3
ns
ns
**
ns
*
ns
ns
ns
86 80
86 84
402 351
422 382
45 43
52 52
11.2 12.5
13.0 14.0
ns
ns
ns
ns
ns
ns
**
ns
Seeds per fruit
Seed index
Means comparisons with Tukey’s test. Different letters denote significant differences according to the p-value; ns, non-significant; *p < 0.05; **p < 0.01; ***p < 0.001. Percentage data were previously arcsine transformed. a Days after pollination.
the weight of the pruning wood was affected by crop load ( p = 0.15, p = 0.06 and p = 0.48, for seasons 2003–2005, respectively). It is noteworthy that in the last year of the experiment an average of 74 kg of wood per tree had to be removed during pruning. 3.2. Shoot vigor effects on fruit set and quality No interaction was detected between the vigor of the shoot (weak, medium or high) and the number of fruits per shoot (two or four) in either year; therefore for simplicity the effects of both factors on fruit set and quality will be analyzed separately. The percentage of flowers setting fruit did not vary with the type of shoot and under all circumstances hand-pollinated flowers set in a high percentage (Table 3). Fruit weight was less in shoots of medium vigor in 2003. This response was not observed in 2005. Similarly, the number of seeds per fruit was lower in shoots of medium vigor in 2003, but not in 2005 (Table 3). The seed index was unaffected since the diminution in the number of seeds reduced the weight of the fruit proportionaly. Significant effects of shoot vigor were not observed on fruit sweetness or acidity (data not shown). A trend for a lower fruit set and weight was observed when four flowers per shoot were hand pollinated compared with two. An average fruit weight of around 410 g was reached in shoots Table 4 Flower weight (g), fruit weight (g) and number of seeds per fruit depending on the flower position in 1-year-old shoot Position
Flower weight (g)
Fruit weight (g)
Seeds per fruit
2003
2005
2003
2005
2003
2005
Basal Medium Apical
0.23 a 0.23 a 0.24 a
0.26 a 0.22 b 0.19 c
574.0 a 501.8 a 485.6 a
497.4 a 376.7 a 269.3 a
65 a 62 a 59 a
34 a 32 a 25 a
p
ns
**
ns
ns
ns
ns
Comparison of means with the test of Tukey. Different letters (a, b and c) denote statistically significant differences in accordance with the p-value indicated.
with two fruits, while fruits weighed 40–50 g less on average in shoots with four fruits. The differences were not significant in either year. The seed index tended to increase with higher fruit load as a consequence of the reduced fruit weight observed in shoots with four fruits. As happened with the vigor of the shoot, TSS and fruit acidity did not change in response to the different number of fruits per shoot (data not shown). 3.3. Effects of fruit position along the shoot on fruit set and quality The position of the flower did not have a significant influence on fruit set. On the contrary, a trend for heavier fruits at basal nodes was evident in both years (Table 4). Although the high variability did not allow statistical significance to be reached, the average fruit borne at basal position weighed 90 g (2003) and 225 g (2005) more than the average fruit formed at the shoot apex. Smaller and lighter flowers were formed at the shoot apex in 2005, but no differences in flower size occurred in 2003 (Table 4). The number of seeds per fruit and the seed index were the same for all fruit positions (Table 4). The position of the fruit did not affect TSS content, but a higher acidity was observed for the fruit formed at the shoot apex in 2003 (data not shown). 4. Discussion Current recommendations in Spain are to limit cherimoya yield in standard orchards to around 14–15 t ha 1, setting no more than 150 fruits per tree (Guirado et al., 2001). Our results, however, indicate that in well-managed orchards of the same density yield can be doubled without fruit quality losses. A logarithmic relationship between the number of fruits per tree and the yield was established in our plot, with levels of 30 t ha 1 obtained with around 400 fruits per tree (Fig. 2). Such high yields were obtained over three consecutive years in the same trees without negative effects, thus indicating the sustainability of increasing the productivity in cherimoya. Furthermore, these high yields were compatible with tree
M. Gonza´lez, J. Cuevas / Scientia Horticulturae 115 (2008) 129–134
growth. Neither the trunk enlargement nor the amount of pruning wood was reduced in trees allocating more resources to fruits. Different authors propose increasing the yield in cherimoya orchards up to 30–40 t ha 1 by augmenting the number of trees per hectare and the number of fruits per tree (Gardiaza´bal and Rosenberg, 1994; Razeto and Rı´os, 2002). As in other crops the number of fruits per tree and their size is inversely related in Annona (George et al., 1986; Vergara et al. cited by Gallardo, 2001). However, fruit number and size only counteract when the capacity of the tree canopy to enlarge the fruit is exceeded. The upper limit of the tree yield efficiency (kg cm 2 TCSA) which does not compromise fruit size is still a matter of discussion in Annona (George and Campbell, 1991). Our results recommend not producing more than 0.22 kg (fresh fruit weight) cm 2 TCSA. In the first 2 years of our experiment, setting 400 fruits per tree did not have any detrimental effect on fruit quality. In the third year, the trees with low crop load produced heavier fruits. However, the differences were not due to a small fruit size in the trees with high yield, but to the very large fruits formed in trees with low yield. In other words, the trees with 400 fruits did not show any change among years in their capacity to enlarge the fruits; the difference appeared because the trees with few fruits were able to make the fruits still larger taking advantage of a favorable year. Nonetheless, Razeto and Rı´os (2002) found that high production levels may cause a slight diminution of the average fruit size and reduce the percentage of extra large fruits (>700 g). The efficiency with which the photosynthates are transported from leaves to fruit also conditions fruit quality. Thus, besides the number of fruits per tree, the vigor of the bearing shoot may also have an effect on fruit size. Our results, however, show that even low vigor shoots are able to set and enlarge up to four fruits (Table 3). Vigorous shoots probably have the potential to produce heavier fruits, but no consistent differences were found at the levels of fruit load here assessed. Gardiaza´bal and Rosenberg (1994) propose adjusting the number of fruits to the vigor of the bearing shoot, setting two fruits in weak shoots and up to four or five fruits in high vigor shoots. Other authors have not found clear effects of the vigor of the bearing shoot on cherimoya fruit size (Cautı´n et al., 1999; Gallardo, 2001; Dias et al., 2004). Some unexpected results inform, on the contrary, of the importance of seed number for determining final fruit size in cherimoya. In 2003, the shoots of intermediate vigor produced the lightest fruits; these small fruits formed less seeds (Table 3), suggesting that their small size was a consequence of a pollination failure resulting in a debilitated fruit demand. This anomalous result was not confirmed in 2005. At present, we do not rule out intermediate vigor shoots for producing high-quality cherimoyas. As well as vigorous shoots, basal flowers also have the potential to develop heavier fruits. In this respect, although all flowers were found capable of forming high-quality fruits, a gradient in fruit size from the base to the apex of the bearing shoot was patent. This pattern could be related to the size of the channel for photoassimilates transport and/or to the small size of the apical flowers (Table 4). No references can be found in the literature in this regard for cherimoya. However, in custard
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apple it has been observed that the basal nodes of the current season’s growth set more and larger fruits than apical nodes (George and Nissen, 1988; George and Campbell, 1991). Crop load as well as shoot type and fruit position on the shoot have been found also to influence in some extent fruit size and quality in stone fruits and pomes (Tustin et al., 1988; Ge´nard and Bruchou, 1992; Marini and Sowers, 1994; Farina et al., 2006). Nevertheless, cherimoya situation resembles more that of kiwi, a multi-seeded fruit where the number of fruits per vine can also be fixed by hand pollination. In this species, seed number has too a strong influence on fruit size and shape while crop load, the type of shoot and fruit position along the shoot are known to affect assimilate movement and fruit growth (Pyke and Alspach, 1986; Lawes et al., 1990). Also in kiwi, the size of the flowers seems to determine final fruit size in some seasons but not in others (McPherson et al., 2001). The comparison of seed index ratios in different scenarios represents a useful tool to further explore sink–source relationships in cherimoya. The seed index encompasses the fertilization ability of the pollination (higher success resulting in greater fruit sink strength) and the capacity of the canopy to enlarge the fruits (testing the limits of the source). If no limitation of the source exists, the sink strength will predominate for determining cherimoya fruit growth; more seeds will make larger fruits, while the seed index would remain more or less constant for each cultivar despite changes in fruit size (the seed index in ‘Fino de Jete’ is established at around 12). This was the rule in our experiments, demonstrating that fruit demand rarely exceeded the capacity of the source. Only a combination of a high number of seeds per fruit and a high number of fruits per tree would exceed the capacity of the canopy to satisfy the fruit demand of photosynthates. Such combination occurred in 2005. The consequence was a noticeable increase in seed index ratio, with values around 18–20 in trees with high yield (Table 2). The increase in the number of seeds per fruit observed in 2005, for an otherwise identical experiment to those carried out in 2003 and 2004, suggests changes in the maternal and paternal contribution on pollination success. Such changes merit further analyses. In summary, our results show that cherimoya productivity can be greatly increased by pollinating between 400 and 500 flowers per tree in well-managed orchards without losses in fruit quality. Although the number and quality of the fruit formed in different positions of the tree canopy do not differ greatly, it seems advisable to favor the production of vigorous and semi-vigorous shoots by heavy pruning. It also seems preferable to pollinate basal flowers to produce heavier fruits. However, given the reduced differences among different kinds of shoots and flower positions, other factors related with the location of the fruits as sunscald, bruising and quickness in harvesting operations can also be taken into account. Acknowledgement Research partially funded by UAL-CAJAMAR project no. CR-UAL-0212.
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George, A.P., Nissen, R.J., 1988. The effects of temperature, vapour pressure deficit and soil moisture stress on growth, flowering and fruit set of custard apple (Annona cherimola x Annona squamosa) African Pride. Sci. Hortic. 34 (3/4), 183–191. George, A.P., Nissen, R.J., Ko, H.L., 1986. Productivity of custard apple (Annona atemoya Hort.): factors affecting yield and fruit size. Acta Hortic. 175, 37–41. Guirado, E., Hermoso, J.M., Pe´rez, M.A., Garcı´a-Tapia, J., Farre´, J.M., 2001. Polinizacio´n del chirimoyo. Caja Rural de Granada, Granada, Spain, 47 pp. Ho, L.C., 1988. Metabolism and compartimentation of imported sugar in sink organs in relation to sink strength. Annu. Rev. Plant Physiol. Plant Mol. Biol. 39, 355–358. Lawes, G.S., Woolley, D.J., Lai, R., 1990. Seeds and other factors affecting fruit size in kiwifruit. Acta Hortic. 282, 257–264. Marini, R.P., Sowers, D.L., 1994. Peach fruit weight is influenced by crops density and fruiting shoot length but not position on the shoot. J. Am. Soc. Hortic. Sci. 100 (2), 180–184. McPherson, H.G., Richardson, A.C., Snelgar, W.P., Patterson, K.J., Currie, M.B., 2001. Flower quality and fruit size in kiwifruit (Actinidia deliciosa). N.Z. J. Crop Hortic. Sci. 29, 93–101. Pyke, N.B., Alspach, P.A., 1986. Inter-relationships of fruit weight, seed number and seed weight in kiwifruit. N.Z. J. Agric. Sci. 20, 153–156. Razeto, B., Rı´os, M., 2002. Effect of fruit load on yield efficiency and fruit size in cherimoya tree (Annona cherimola Mill.). In: Proceedings of the Third International Congress on Annonaeae. Quillota, Chile, pp. 83–89. Richardson, A.C., Anderson, P.A., 1996. Hand pollination effects on the set and development of cherimoya (Annona cherimola) fruit in a humid climate. Sci. Hortic. 65, 273–281. Schroeder, C.A., 1941. Hand pollination effects in cherimoya (Annona cherimola). Calif. Avocado Soc. Yearbook 26, 94–98. Tustin, S.D., Hirst, P.M., Warrington, I.J., 1988. Influence of orientation and position of fruiting laterals on canopy light penetration, yield, and fruit quality of ‘Granny Smith’ apple. J. Am. Soc. Hortic. Sci. 113, 693–699. Wester, P.J., 1910. Pollination experiments with Annonas. Bull. Torrey Bot. Club 37, 529–539.
Optimal crop load and positioning of fruit in cherimoya (Annona cherimola Mill.) trees M. Gonza´lez a, J. Cuevas b,* a
Estacio´n Experimental de la Fundacio´n Cajamar, Autovı´a del Mediterra´neo, km 416.7, 04710 El Ejido, Almerı´a, Spain b Dpto. Produccio´n Vegetal, Universidad de Almerı´a, La Can˜ada de San Urbano s/n, 04120 Almerı´a, Spain Received 12 November 2006; received in revised form 14 June 2007; accepted 14 August 2007
Abstract Dichogamy and the lack of efficient pollinators in most producing countries make hand pollination of cherimoya unavoidable. In contrast to its high-cost, this technique allows growers to fix crop load levels and fruit location within the tree canopy. Over three consecutive years we have studied the optimal fruit load for the cultivar ‘Fino de Jete’ and checked the effects of the vigor of the fruit-bearing shoots and of the fruit position along the shoot on the resulting fruit set and quality. Our results show that yield in well-managed orchards fitted a logarithmic curve with a maximum close to 30 t ha 1 achieved by setting about 400 fruits per adult tree. No significant effects of crop load were detected on fruit size during the first 2 years; in the third year, however, trees with a lower yield produced heavier fruits. An upper limit of 0.22 kg (fresh fruit weight) cm 2 of trunk cross-sectional area is proposed if fruit quality is to be preserved. High yield during three consecutive years did not diminish tree growth. We found no consistent effects of shoot vigor on fruit set and quality. However, a gradient for heavier fruits was observed from the base to the apex of the bearing shoots. # 2007 Elsevier B.V. All rights reserved. Keywords: Cherimoya yield potential; Fruit quality; Seed index; Sink–source relationships
1. Introduction The consecution of high yields of great quality in any crop depends on the resolution of the sink–source relationships within the plant (Ho, 1988). These sink–source relationships are poorly known in cherimoya and, for instance, yield potential of the species has not yet been established. Current recommendations in Spain are to limit yield to 14–15 t ha 1, because increasing crop load beyond supposedly reduces fruit size and quality (Guirado et al., 2001). The source capacity to enlarge the fruits is primarily determined by the size of the canopy and by the photosynthesis rate. Guirado et al. (2001) take into account tree size in cherimoya and propose to fix crop load according to orchard density. In standard plantations (204 trees ha 1), these authors suggest a crop load of about 150 fruits per tree, while in high-density orchards (416 trees ha 1) a maximum number of 70 fruits per tree is proposed. This
* Corresponding author. Tel.: +34 950 015 559; fax: +34 950 015 939. E-mail address: [email protected] (J. Cuevas). 0304-4238/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2007.08.002
adjustment can also be expressed in terms of canopy volume or trunk diameter. On the opposite side, the sink strength of a cherimoya fruit is known to be influenced by seed number, parameter strongly dependent on pollination efficiency (Schroeder, 1941). Natural pollination of cherimoya flowers is hindered by dichogamy and by the lack of efficient pollinators in most producing countries (Wester, 1910; Schroeder, 1941; Gazit et al., 1982). The deficit in natural pollination in these countries has to be solved by means of artificial hand pollination. Artificial pollination represents a major cost for this crop, but allows the levels of fruit load to be closely fixed by selecting the number of handpollinated flowers. Besides fixing crop load, artificial pollination permits producers to set fruits in selected places within the tree canopy. Cherimoya flowers develop on basal nodes of current season’s growth and at leaf scars in 1-year-old shoots of different length and vigor (Fig. 1). Cautı´n et al. (1999) and Gardiaza´bal and Rosenberg (1994) recommend choosing shoots of intermediate vigor for hand-pollination, since weak twigs are unable to enlarge fruits and vigorous shoots are not prone to produce flowers. Ahmed (1936) and Cabezas and Garcı´a-Tapia (1998) prefer, on the contrary, vigorous shoots
M. Gonza´lez, J. Cuevas / Scientia Horticulturae 115 (2008) 129–134
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Fig. 1. Flowering habit of cherimoya (first author’s drawing).
because they are supposed to produce heavier fruits. On the other hand, the effect of the node on which the flower develops has not yet been experimentally studied. Although Guirado et al. (2001) recommend flowers from basal nodes of 1-year-old shoots because they may form heavier fruits, they do not provide experimental data to justify their election. As the information from literature is incomplete and contradictory, and too often lacking experimental support, we have studied during three consecutive years the effects of increasing crop load on the productivity and fruit quality of ‘Fino de Jete’. Other two objectives of this research are to analyze the effects of the vigor of the bearing shoots and of the position of the fruit along the shoot on fruit set and quality in this cultivar.
However, two or three hand pollination operations were often needed in treatments with high crop load. These pollinations were completed during the same week and no obvious effects due to the different date of pollination were detected. Given the high success of hand pollination in this species, the desired crop load was achieved by pollinating the same number of flowers as fruits required. However, in 2004, about 20% more flowers per level were pollinated because a failure was detected early in the season. Yield as t ha 1 and yield efficiency as kg cm 2 of trunk cross-sectional area (TCSA) were calculated for each crop load level. Fruit weight at harvest was measured on a sample of 25 fruits chosen at random from the most representative pick of each treatment. The number of seeds per fruit was counted and a seed index calculated as the number of seeds per 100 g of fresh fruit weight (Richardson and Anderson, 1996). The pulp of each fruit was processed to determine the total amount of soluble solids (TSS) (Brix) with an electronic refractometer, and the titratable acidity as milligram of malic acid per gram of fresh pulp weight (Alique, 1995). The growth of each tree was estimated after harvest by measuring every year the increase in trunk diameter 20 cm above ground, and also by the weight of the pruning wood. Pruning was performed at the end of February. 2.2. Shoot vigor effects on fruit set and quality
2. Materials and methods The trials were carried out in seasons 2003–2005 on a solid block of ‘Fino de Jete’ cherimoya trees located at the Experimental Station of Foundation Cajamar (El Ejido, Almerı´a, Spain). The trees were planted in 1987 at a distance of 8 m 6 m (208 trees ha 1). The orchard is non-tillaged and is drip irrigated with 7160 m3 ha 1 year 1. The fertilization program includes 120–80–100 NPK kg ha 1 year 1. The average trunk diameter 20 cm above ground was 26.7 2.7 cm at the beginning of the experiments. Pollen used in hand pollination was collected from preanthesis ‘Campas’ flowers and kept in refrigerator (4 8C) until the next morning when it was applied to ‘Fino de Jete’ flowers in their female phase between 7:00 and 11:00 am using a pollen puffer (Kurita Tabaco Co., Tochigi, Japan). The pollinated flowers were tagged with a view to removing undesirable small fruits product of open pollination. 2.1. Crop load effects on yield and fruit quality The effects of crop load on yield and fruit quality were assessed using regression analyses. Crop load levels from 100 to 500 fruits per tree in intervals of 50 fruits were studied using a single tree per level. Crop load levels were assigned at random and trees forming part of the trial were chosen based on their uniform size. The experiment was repeated over three consecutive years using the same trees with the aim of checking long-term effects. Hand pollination was performed on the same day provided the number of flowers available for pollination was enough.
The effect of shoot vigor on fruit set and quality was studied in the years 2003 and 2005 using a split-plot design. The first factor, shoot vigor, included three levels (low, medium and high vigor; see characteristics in Table 1); the second factor, fruit load, included two levels: two and four fruits per shoot. Sixteen shoots of each type were marked in each tree; eight with two fruits, and eight with four. Four trees acted as replications. Fruit set and weight, diameter, seed number and seed index were determined as above. The TSS content and fruit acidity were measured in ripe fruits as previously described. 2.3. Effects of fruit position along the shoot on fruit set and quality The effect on fruit set and quality of the shoot node where the flower develops was studied in 2003 and 2005 following a randomized block design. Four trees acted as blocks and replications whereas the treatments were represented by distinct flower positions on 1-year-old shoot as follows: basal (arising from nodes 1–4), middle (on nodes 5–8) and apical (on the ninth node or beyond). Flowers were compared using Table 1 Shoot vigor characterization: basal diameter (mm), length (cm) and number of nodes per shoot Vigor
Basal diameter (mm)
Length (cm)
Nodes (number)
Weak Medium High
6 0.5 8 0.7 10 1.0
28 4 42 8 72 9
82 13 4 21 6
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medium vigor shoots with an average of 13 nodes. Ten shoots per treatment were tagged in each tree. Two flowers per shoot were hand pollinated in this experiment. The fruit set and quality were determined as described above. Depending on the experimental design, the statistical analyses were performed using Statgraphics Plus 4.1 (Manugistics Inc., Rockville, MD), SigmaPlot (Systat Software Inc. San Jose, CA) or Statixtic (Analytical Software, St. Paul, MN) software packages. 3. Results 3.1. Crop load effects on yield and fruit quality The number of fruits harvested increased linearly with hand pollination levels (Table 2). However, the number of fruits in low-yield trees was always higher than the number of handpollinated flowers highlighting that natural setting occurred to some extent despite our efforts to remove misshapen fruits resulting from open pollination. The fruit set was unaffected by pollination levels, but changed depending on the season. A percentage above 90% of hand-pollinated flowers set fruit in 2003 and 2005 with no differences among treatments. In 2004, the final fruit set was established at around 75%. Preharvest fruit drop rose exponentially with pollination level in 2003 and 2005 (r2 = 0.80, p < 0.001 and r2 = 0.56, p < 0.05, respectively). This relationship failed in 2004. Preharvest drop was more intense in 2005 when the yield was higher. Yield increased with pollination levels following a logarithmic curve (Fig. 2). Yield efficiency also augmented with pollination level, in this case, linearly (Fig. 3). Increasing the number of fruits per tree did not affect their size in the first 2 years (Table 2). However, in the last season a remarkable increase in fruit size was observed in trees with a low fruit load. In this year, a negative logarithmic relationship was detected between fruit weight and number (r2 = 0.87 and p < 0.001). The number of seeds per fruit was independent of the crop load ( p > 0.05). On the contrary, a noticeable effect of the season was observed. The number of seeds per fruit was the highest in the third year. The seed index ratio did not change
Fig. 2. Yield (t ha 1) depending on the number of fruits per tree: 2003 (*); 2004 (*); 2005 (^).
Fig. 3. Yield efficiency as g of fresh fruit weight per cm2 of trunk crosssectional area (TCSA) depending on the number of fruits per tree: 2003 (*); 2004 (*); 2005 (^).
among treatments in the first 2 years; but a sharp increase in seed ratio was observed in response to increasing fruit loads in 2005 (Table 2). A trend for sweeter fruits was observed with lower crop load, although the differences in TSS content were slight and usually non-significant (data not shown). No effects on fruit acidity were observed in any year (data not shown). High-yield did not condition tree growth. Neither trunk enlargement along the experiment (r2 = 0.06 and p = 0.53), nor
Table 2 Fruit number per tree, fruit weight (g) and seed index (seeds per 100 g of fresh fruit) depending on the number of flowers pollinated per tree Treatment
T100 T150 T200 T250 T300 T350 T400 T450 T500 r2 lineal r2 logarithmic
Fruit number per treea
Fruit weight (g)
Seed index
2003
2004
2005
2003
2004
2005
2003
2004
2005
108 183 215 284 303 334 369 420 464 0.98*** 0.95***
172 134 178 222 273 216 353 233 396 0.67** 0.55**
110 178 222 277 246 318 375 420 483 0.96*** 0.87***
435.8 474.2 392.5 403.4 437.8 526.8 489.7 405.6 454.5 ns ns
418.2 381.9 244.2 342.1 381.4 384.7 404.9 407.6 358.1 ns ns
713.2 705.1 616.5 491.9 571.0 500.0 492.4 493.2 411.4 0.83*** 0.87***
16 12 14 18 17 15 – 17 – ns ns
11 11 8 10 9 11 13 11 12 ns ns
11 14 14 18 15 19 18 19 22 0.83*** 0.82***
ns, non-significant; *p < 0.05; **p < 0.01; a Significance according to the p-value.
***
p < 0.001.
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Table 3 Fruit set (%), fruit weight (g), number of seeds per fruit, and seed index (number of seeds per 100 g fresh fruit weight) depending on the shoot vigor and fruit load
Vigor High Medium Weak p Load 2 fruits 4 fruits p
Fruit set 20 dapa (%)
Weight (g)
2003
2005
2003
2005
2003
2005
2003
2005
85 78 84
82 83 90
429 a 303 b 403 a
419 441 345
51 a 35 b 46 a
57 54 46
12.0 11.7 11.8
13.5 12.8 14.3
ns
ns
**
ns
*
ns
ns
ns
86 80
86 84
402 351
422 382
45 43
52 52
11.2 12.5
13.0 14.0
ns
ns
ns
ns
ns
ns
**
ns
Seeds per fruit
Seed index
Means comparisons with Tukey’s test. Different letters denote significant differences according to the p-value; ns, non-significant; *p < 0.05; **p < 0.01; ***p < 0.001. Percentage data were previously arcsine transformed. a Days after pollination.
the weight of the pruning wood was affected by crop load ( p = 0.15, p = 0.06 and p = 0.48, for seasons 2003–2005, respectively). It is noteworthy that in the last year of the experiment an average of 74 kg of wood per tree had to be removed during pruning. 3.2. Shoot vigor effects on fruit set and quality No interaction was detected between the vigor of the shoot (weak, medium or high) and the number of fruits per shoot (two or four) in either year; therefore for simplicity the effects of both factors on fruit set and quality will be analyzed separately. The percentage of flowers setting fruit did not vary with the type of shoot and under all circumstances hand-pollinated flowers set in a high percentage (Table 3). Fruit weight was less in shoots of medium vigor in 2003. This response was not observed in 2005. Similarly, the number of seeds per fruit was lower in shoots of medium vigor in 2003, but not in 2005 (Table 3). The seed index was unaffected since the diminution in the number of seeds reduced the weight of the fruit proportionaly. Significant effects of shoot vigor were not observed on fruit sweetness or acidity (data not shown). A trend for a lower fruit set and weight was observed when four flowers per shoot were hand pollinated compared with two. An average fruit weight of around 410 g was reached in shoots Table 4 Flower weight (g), fruit weight (g) and number of seeds per fruit depending on the flower position in 1-year-old shoot Position
Flower weight (g)
Fruit weight (g)
Seeds per fruit
2003
2005
2003
2005
2003
2005
Basal Medium Apical
0.23 a 0.23 a 0.24 a
0.26 a 0.22 b 0.19 c
574.0 a 501.8 a 485.6 a
497.4 a 376.7 a 269.3 a
65 a 62 a 59 a
34 a 32 a 25 a
p
ns
**
ns
ns
ns
ns
Comparison of means with the test of Tukey. Different letters (a, b and c) denote statistically significant differences in accordance with the p-value indicated.
with two fruits, while fruits weighed 40–50 g less on average in shoots with four fruits. The differences were not significant in either year. The seed index tended to increase with higher fruit load as a consequence of the reduced fruit weight observed in shoots with four fruits. As happened with the vigor of the shoot, TSS and fruit acidity did not change in response to the different number of fruits per shoot (data not shown). 3.3. Effects of fruit position along the shoot on fruit set and quality The position of the flower did not have a significant influence on fruit set. On the contrary, a trend for heavier fruits at basal nodes was evident in both years (Table 4). Although the high variability did not allow statistical significance to be reached, the average fruit borne at basal position weighed 90 g (2003) and 225 g (2005) more than the average fruit formed at the shoot apex. Smaller and lighter flowers were formed at the shoot apex in 2005, but no differences in flower size occurred in 2003 (Table 4). The number of seeds per fruit and the seed index were the same for all fruit positions (Table 4). The position of the fruit did not affect TSS content, but a higher acidity was observed for the fruit formed at the shoot apex in 2003 (data not shown). 4. Discussion Current recommendations in Spain are to limit cherimoya yield in standard orchards to around 14–15 t ha 1, setting no more than 150 fruits per tree (Guirado et al., 2001). Our results, however, indicate that in well-managed orchards of the same density yield can be doubled without fruit quality losses. A logarithmic relationship between the number of fruits per tree and the yield was established in our plot, with levels of 30 t ha 1 obtained with around 400 fruits per tree (Fig. 2). Such high yields were obtained over three consecutive years in the same trees without negative effects, thus indicating the sustainability of increasing the productivity in cherimoya. Furthermore, these high yields were compatible with tree
M. Gonza´lez, J. Cuevas / Scientia Horticulturae 115 (2008) 129–134
growth. Neither the trunk enlargement nor the amount of pruning wood was reduced in trees allocating more resources to fruits. Different authors propose increasing the yield in cherimoya orchards up to 30–40 t ha 1 by augmenting the number of trees per hectare and the number of fruits per tree (Gardiaza´bal and Rosenberg, 1994; Razeto and Rı´os, 2002). As in other crops the number of fruits per tree and their size is inversely related in Annona (George et al., 1986; Vergara et al. cited by Gallardo, 2001). However, fruit number and size only counteract when the capacity of the tree canopy to enlarge the fruit is exceeded. The upper limit of the tree yield efficiency (kg cm 2 TCSA) which does not compromise fruit size is still a matter of discussion in Annona (George and Campbell, 1991). Our results recommend not producing more than 0.22 kg (fresh fruit weight) cm 2 TCSA. In the first 2 years of our experiment, setting 400 fruits per tree did not have any detrimental effect on fruit quality. In the third year, the trees with low crop load produced heavier fruits. However, the differences were not due to a small fruit size in the trees with high yield, but to the very large fruits formed in trees with low yield. In other words, the trees with 400 fruits did not show any change among years in their capacity to enlarge the fruits; the difference appeared because the trees with few fruits were able to make the fruits still larger taking advantage of a favorable year. Nonetheless, Razeto and Rı´os (2002) found that high production levels may cause a slight diminution of the average fruit size and reduce the percentage of extra large fruits (>700 g). The efficiency with which the photosynthates are transported from leaves to fruit also conditions fruit quality. Thus, besides the number of fruits per tree, the vigor of the bearing shoot may also have an effect on fruit size. Our results, however, show that even low vigor shoots are able to set and enlarge up to four fruits (Table 3). Vigorous shoots probably have the potential to produce heavier fruits, but no consistent differences were found at the levels of fruit load here assessed. Gardiaza´bal and Rosenberg (1994) propose adjusting the number of fruits to the vigor of the bearing shoot, setting two fruits in weak shoots and up to four or five fruits in high vigor shoots. Other authors have not found clear effects of the vigor of the bearing shoot on cherimoya fruit size (Cautı´n et al., 1999; Gallardo, 2001; Dias et al., 2004). Some unexpected results inform, on the contrary, of the importance of seed number for determining final fruit size in cherimoya. In 2003, the shoots of intermediate vigor produced the lightest fruits; these small fruits formed less seeds (Table 3), suggesting that their small size was a consequence of a pollination failure resulting in a debilitated fruit demand. This anomalous result was not confirmed in 2005. At present, we do not rule out intermediate vigor shoots for producing high-quality cherimoyas. As well as vigorous shoots, basal flowers also have the potential to develop heavier fruits. In this respect, although all flowers were found capable of forming high-quality fruits, a gradient in fruit size from the base to the apex of the bearing shoot was patent. This pattern could be related to the size of the channel for photoassimilates transport and/or to the small size of the apical flowers (Table 4). No references can be found in the literature in this regard for cherimoya. However, in custard
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apple it has been observed that the basal nodes of the current season’s growth set more and larger fruits than apical nodes (George and Nissen, 1988; George and Campbell, 1991). Crop load as well as shoot type and fruit position on the shoot have been found also to influence in some extent fruit size and quality in stone fruits and pomes (Tustin et al., 1988; Ge´nard and Bruchou, 1992; Marini and Sowers, 1994; Farina et al., 2006). Nevertheless, cherimoya situation resembles more that of kiwi, a multi-seeded fruit where the number of fruits per vine can also be fixed by hand pollination. In this species, seed number has too a strong influence on fruit size and shape while crop load, the type of shoot and fruit position along the shoot are known to affect assimilate movement and fruit growth (Pyke and Alspach, 1986; Lawes et al., 1990). Also in kiwi, the size of the flowers seems to determine final fruit size in some seasons but not in others (McPherson et al., 2001). The comparison of seed index ratios in different scenarios represents a useful tool to further explore sink–source relationships in cherimoya. The seed index encompasses the fertilization ability of the pollination (higher success resulting in greater fruit sink strength) and the capacity of the canopy to enlarge the fruits (testing the limits of the source). If no limitation of the source exists, the sink strength will predominate for determining cherimoya fruit growth; more seeds will make larger fruits, while the seed index would remain more or less constant for each cultivar despite changes in fruit size (the seed index in ‘Fino de Jete’ is established at around 12). This was the rule in our experiments, demonstrating that fruit demand rarely exceeded the capacity of the source. Only a combination of a high number of seeds per fruit and a high number of fruits per tree would exceed the capacity of the canopy to satisfy the fruit demand of photosynthates. Such combination occurred in 2005. The consequence was a noticeable increase in seed index ratio, with values around 18–20 in trees with high yield (Table 2). The increase in the number of seeds per fruit observed in 2005, for an otherwise identical experiment to those carried out in 2003 and 2004, suggests changes in the maternal and paternal contribution on pollination success. Such changes merit further analyses. In summary, our results show that cherimoya productivity can be greatly increased by pollinating between 400 and 500 flowers per tree in well-managed orchards without losses in fruit quality. Although the number and quality of the fruit formed in different positions of the tree canopy do not differ greatly, it seems advisable to favor the production of vigorous and semi-vigorous shoots by heavy pruning. It also seems preferable to pollinate basal flowers to produce heavier fruits. However, given the reduced differences among different kinds of shoots and flower positions, other factors related with the location of the fruits as sunscald, bruising and quickness in harvesting operations can also be taken into account. Acknowledgement Research partially funded by UAL-CAJAMAR project no. CR-UAL-0212.
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