Crop load and fruit position influence variability in nectarine quality

Three 6 year-old Rose Bright nectarine trees trained to open vase and of similar growth habit and leaf density were thinned to one of three crop loads, low, commercial, or high, at 20 days after full bloom (DAFB). Each tree was approximately 4.5 m tall and contained five scaffold limbs. Each tree crop load was achieved by thinning fruiting branches to one fruit per 5-8 cm (high crop load), one fruit 10-12 cm (commercial), and one fruit per 15-20 cm (low) (Figure 1). Fruit was grown under commercial agronomic practice.

Prior to harvest each of five scaffold limbs per tree was horizontally divided into four heights and each limb and height position considered an individual fruit compartment (yellow box in Figure 2). Two fruiting branches attached to the scaffold branch within each compartment were then tagged. At 94 DAFB all fruit on each tree were harvested and weighed with 6 to 8 fruit from tagged branches segregated for quality measurements. Fruit yield (kg) per fruit compartment, scaffold limb and individual tree was calculated, and leaf density within each compartment scored visually using a rating scale where 1=low, 2= average, 3=high and 4=very high. Fruit quality measurements after harvest included fruit mass and diameter, percentage skin blush (redness), and fruit ripeness using a portable vis-NIR DA meter to obtain an Index of Absorbance Difference (I_AD) value. Soluble solids concentration (SSC; % w/w) was measured using a digital refractometer. SSC and I_AD measurements were conducted on each fruit cheek and the average value calculated.

Figure 1. Number of fruit per length of fruiting shoot in trees with high, commercial and low crop loads.

Figure 2. Rose bright tree height sections and scaffold compartments used in crop load experiment.

Physiological maturity of all fruit at harvest was measured based on Index of Absorbance Difference (I_AD) values using a DA meter (See Stefanelli et al. Usefulness of the I_AD index in the field: characterization of the Tatura Trellis training system on fruit maturity and quality in nectarine. Australian Stonefruit Grower 2/14:26-29). I_AD values for Rose Bright nectarine were correlated with the rate of fruit ethylene production using 60 fruit sampled from trees over a three week period prior to harvest, and three maturity classes were established (Ripe, Pre-ripe, and Immature) (Figure 3). In orchard experiments fruit is generally harvested at Pre-ripe maturity to simulate commercial practice but in this work fruit was allowed to ‘tree-ripen’. The average I_AD value of harvested fruit was less than 0.3 indicating that at each crop load and tree position fruit were ‘tree-ripe’ (table within Figure 3). With all fruit at a similar maturity the effect on fruit quality of crop load and tree position could be more effectively determined.

Figure 3. Relationship between fruit I_AD value measured with a DA meter and fruit maturity as measured via ethylene production. Average fruit I_AD values at harvest within high, commercial and low crop load trees presented in table within figure.

Total fruit yield at harvest was 76 kg in the low crop load tree, and 89 kg in the commercial crop load tree, with yield at these two crop loads statistically significantly lower than in the high crop load tree (129 kg). Fruit yield along a scaffold limb varied considerably at each tree crop load (Figure 4). At all crop loads the bottom of trees produced the lowest fruit yield whilst yield was also lower in the ‘Top’ position in low and high crop load trees. The yield difference between the high crop load tree and other trees was mainly due to significantly higher fruit numbers in the ‘Mid’ and ‘Mid Top’ positions. Fruit thinning based on the number of fruit per length of fruiting branch resulted in a high variability in yield within each tree, and differences in leaf-to-fruit ratio between tree height sections.

Differences in the percentage of fruit that met pre-specified quality criteria were also found between trees at each crop load, and between height sections within each tree. The percentage of fruit harvested with both a mass greater than 110 grams, and SSC higher than 7%, was determined for each height section within a scaffold limb (Figure 5). Fruit quality was higher in the low crop load tree compared to commercial and high load trees within the ‘Bottom’, ‘Mid’ and ‘Mid Top’ positions.

Tree crop load had little effect on fruit quality at the top of trees indicating that the leaf-to-fruit ratio was relatively high in this position at all crop loads. A significant reduction in fruit quality was observed at the ‘Bottom’ and ‘Mid’ positions within commercial and high crop load trees compared to the low load tree. Less than 25% of fruit at the bottom of high and commercial load trees met the pre-specified quality criteria. A high number of poor quality fruit at the base of trees was probably due to a low leaf density (ie. leaf number) relative to number of fruit remaining in this position after thinning.

Figure 4. Effect of tree crop load and height section on average fruit yield within a scaffold limb in Rose Bright nectarine when trees were thinned by number of fruit per fruiting shoot.

Figure 5. Effect of tree crop load and height section on the percentage (%) of Rose Bright fruit harvested that were both larger than 110 grams, and had greater than 7% SSC.

Fruit mass increased with greater height along a scaffold limb within trees at each crop load (Figure 6). This effect was probably linked to increasing carbohydrate availability higher in scaffold limbs due to higher leaf numbers, and greater sunlight duration and interception by leaves. The most significant effect of crop load on fruit size occurred in the mid-section of trees where fruit size in the ‘Mid’ and ‘Mid Top’ position within the low crop load tree was significantly greater than fruit size at comparable heights within the high load tree. No significant differences in fruit size due to tree crop load were found at the top and bottom of scaffold limbs.

These results suggest that tree crop load effect on fruit size was most important in the mid-section of scaffold limbs where an increase in fruit number results in greater competition for carbohydrates. Although fruit number was lowest at the bottom of trees, fruit size was still limited by lower carbohydrate availability (i.e., lower leaf-to-fruit ratio) relative to the rest of the tree. Fruit mass at the bottom of the low crop load tree was 20 grams higher than fruit mass at the base of commercial and high load trees. But fruit from the bottom of the low load tree was still significantly smaller than fruit in higher positions along the same scaffold limb. No difference in fruit size due to crop load was found at the top of trees suggesting that in all cases carbohydrate availability was sufficient to maximize fruit growth.

The pattern of SSC increase with tree height in Rose Bright fruit was similar to that found for fruit mass but the effect of a lower crop load and increasing height was not as consistent (Figure 7). Crop load had little effect on SSC at the bottom of trees where average SSC was below 7%. Thinning to a low crop load significantly increased SSC in fruit within the ‘Mid’ position relative to ‘Mid’ fruit in the commercial and high load trees. Within commercial and high crop load trees there was a significant jump in SSC in fruit harvested from the top half of trees as compared to the bottom half. But in the low load tree fruit harvested from the ‘Mid’, ‘Mid Top’ and ‘Top’ positions had a similar average SSC. Greater carbohydrate availability with decreasing crop load and increasing height likely resulted in higher average SSC, that was closely correlated with increasing fruit mass. Thus for this early season cultivar a lower fruit mass suggests greater competition for carbohydrates resulting in a lower rate of sugar accumulation, and SSC at harvest.

Figure 6. Effect of tree crop load and fruit height on Rose Bright nectarine size. Bars with the same letters are statistically not significantly different.

Figure 7. Effect of tree crop load and fruit height on Rose Bright nectarine SSC. Bars with the same letters are statistically not significantly different.

Increasing fruit skin blush (i.e., percentage of red colour on the fruit surface) has been associated with increased fruit exposure to sunlight, resulting in potential effects on fruit temperature, respiration, ripening rate, and sugar accumulation during growth. In this trial there was a strong positive correlation between fruit SSC and percentage of blushed skin area when fruit were grouped by height section along a scaffold limb (correlation coefficient = 74%) (Figure 8). There was no significant effect of tree crop load on the relationship between SSC, skin blush and tree height. The relationship between SSC and skin blush was consistent within each height section and scaffold limb below the ‘Top’ position. Fruit at the top of trees had the highest SSC, and skin blush was consistently greater than 80% fruit surface area. But at this tree height there was little correlation between skin blush and SSC. This indicates that at the top of trees the majority of fruit were highly exposed and even marginal shading had little effect on sugar development in fruit. At the bottom of trees increasing fruit SSC was associated with increasing skin blush. But greater fruit exposure to sunlight at this height is unlikely to compensate for reduced availability of carbohydrates as indicated by the relatively low SSC in exposed fruit at the base of trees.

Figure 8. Relationship between Rose Bright nectarine SSC and percentage (%) of blushed skin area on fruit as influenced by height of fruit within trees.

How does crop load and tree height influence variation in fruit quality within trees?

Variation in fruit size and SSC at each height section within a tree was determined by calculating a standard indicator of variability known as the coefficient of variation (%). For Rose Bright nectarine thinning to a low crop load reduced variation in fruit mass at all heights below ‘Top’ relative to the variation found in both commercial and high load trees (Figure 9). This suggests that lower fruit numbers and thus less competition between fruit reduced differences in fruit size within the tree.

Variation in fruit SSC was generally more uniform when compared to fruit mass both among tree heights at each crop load, and among trees at different crop loads (Figure 10). There seemed to be very little effect of crop load or height along a scaffold limb on variation in SSC, other than in the bottom of the high crop load tree where the CV for SSC was highest. These results suggest that changes in fruit competition for carbohydrates as influenced by tree crop load has a greater effect on variation in fruit mass than in the spread of SSC, among fruit within a height section.

Figure 9. Effect of tree crop load and height section on variation in Rose Bright fruit size measured using the coefficient of variation (CV %). Higher CV values indicate greater variability in fruit size.

Figure 10. Effect of tree crop load and height section on variation in Rose Bright fruit SSC measured using the coefficient of variation (CV %). Higher CV values indicate greater variability in fruit SSC.

For early-season Rose Bright nectarine the percentage of high quality fruit at harvest generally increased with a lower tree crop load, and increasing height along scaffold limbs. A lower relative crop load resulted in significant increases in fruit size at most heights within a tree but thinning was marginally less effective in increasing average SSC particularly at the bottom of trees. A strong positive correlation was found between fruit SSC and percentage of skin blush on fruit at most tree heights, with this effect independent of crop load, confirming previous studies that have found a similar relationship.

Although these results should be considered as preliminary, they do suggest that growers may be able to vary fruit thinning technique, crop load and fruit position to maximise fruit quality and reduce variability in fruit size and SSC within trees. This work has highlighted the effect on fruit quality of tree height which in open vase trees is closely associated with carbohydrate availability. It also indicated that thinning to achieve a more uniform leaf-to-fruit ratio along scaffold limbs may improve overall fruit quality. In this trial the leaf-to-fruit ratio was very high at the top of trees at all crop loads and relatively low at the bottom of trees when thinning was based on number of fruit per length of fruiting branch. Although there were fewer fruiting branches at the base of trees, carbohydrate supply was still limiting for the remaining fruit, even within the low crop load tree. This suggests that increasing fruit quality at the base of trees is quite difficult in practice.

In conclusion, maximising fruit numbers higher in the tree and potentially reducing fruit numbers at the base of a tree may increase average fruit quality and reduce variability in fruit size and SSC. Of course the cost of thinning based on leaf-to-fruit ratio, and harvesting more fruit from the top of trees, must be considered and balanced against potentially fewer harvests due to increased fruit uniformity and/or higher returns for higher quality fruit.