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- Author or Editor: T.M. DeJong x
For the last several years, research in my laboratory has been focused on studying the developmental and environmental control of dry matter partitioning in peach trees based on the concept that plants grow as collections of semi-autonomous, but interacting, organs. This concept assumes that plant genotype, triggered by developmental and environmental signals, determines current organ specific growth potentials and that environmental conditions dictate conditional growth capacity and respiration (both growth and maintenance) requirements of each organ at any specific time. Dry matter partitioning at any given time is then determined by the availability of resources to be partitioned, the conditional growth capacity and maintenance requirements of each organ, and the relative ability of each organ to compete for the resources. In this presentation, I will demonstrate how developmental patterns of various organs influence dry-matter partitioning within the tree over time, how organ number can influence the amount of dry-matter partitioned collectively to an organ type, and propose an hypothesis for how environmental conditions may influence partitioning on a diurnal basis.
Differences in the photosynthetic capacity of leaves of peach [Prunus persica (L.) Batsch cv. Golden Glory] were investigated in conjunction with their leaf nitrogen and phosphorus content. Photosynthetic CO2 assimilation expressed on a leaf area basis, mesophyll conductance, and leaf conductance to water vapor were all linearly related to leaf nitrogen content expressed on a leaf area basis (R2 = 0.908, 0.921, 0.685, respectively). Leaf intercellular CO2 concentrations tended to decrease slightly with increasing CO2 assimilation rates and leaf N contents, indicating that CO2 assimilation was not being restricted by low intercellular CO2 concentrations and leaf conductances in leaves with lower assimilation capacity. CO2 assimilation, mesophyll conductance, and leaf conductance to water vapor were also linearly related to leaf phosphorus content, but these relationships were not as clear as for leaf nitrogen content. (R2 = 0.601, 0.687, 0.324, respectively). The maximum CO2 assimilation rate per unit of leaf nitrogen for peach leaves in this experiment was between 6.0 and 7.0 nmol CO2 mg N−1 s−1.
Photosynthetic CO2 assimilation characteristics of 5 Prunus species were compared with respect to leaf nitrogen content and water use. The mean maximum CO2 assimilation rates of peach, plum, and cherry were all very similar when expressed on a leaf area, leaf nitrogen, and water use basis (1.33–1.37 nmol CO2 cm−2s−1, 6.17–6.75 nmol CO2 mgN−ls−1, 0.19–0.21 nmol CO2 µgH2O−1). In comparison, the mean maximum CO2 assimilation rate of almond was higher when expressed on a leaf area basis (1.80 nmol CO2 cm−2s−1), the same when expressed on a leaf N basis (6.47 nmol CO2 mgN−1s−1), and lower when expressed on a water use basis (0.16 nmol CO2 µgH2O−1). The mean maximum rate of CO2 assimilation in apricot leaves was lower than that of peach, plum, and cherry when expressed on a leaf area (0.67 nmol CO2 cm−2s −1) and leaf N basis (3.76 nmol CO2 mgN−1s−1) but the same when expressed on a water use basis (0.19 nmol CO2 µgH2O−1). The interspecific relationship between leaf N content and leaf CO2 assimilation rate was found to be similar to that described previously for leaves of a single cultivar of peach.
Seasonal patterns of fruit growth were measured on early and late-maturing peach (Prunus persica L. Batsch) cultivars (‘June Lady’ and ‘O’Henry’, respectively). Seasonal relative growth rates of fruit dry matter accumulation were calculated. The relationships between fruit relative growth rates and respiration were used to develop a quantitative carbon budget model of peach fruit growth and sink activity. The resulting model indicates that the double-sigmoid growth pattern of peach fruits involves only two physiologically distinct phases of sink activity instead of the three stages that are traditionally recognized. The traditional stage II of fruit growth is apparently a function of the timing of the shift between these two physiological phases of sink activity.
The relationships between shoot light exposure in one year and the flower and fruit production characteristics of those shoots the following year were indirectly investigated in summer pruned and nonsummer pruned peach [Prunus persica (L.) Batsch.] trees by evaluating leaf characteristics (leaf N and dry matter content per unit leaf area; Na and Wa, respectively) on tagged shoots during one season and the flowering and fruiting characteristics during the subsequent season. There were significant positive linear relationships between leaf Na and Wa on shoots in one year and flower and fruit production per unit shoot length during the subsequent year. Summer pruning had relatively little influence on these relationships. There was no apparent relationship between percent fruit set in the spring and light exposure of the shoots the previous summer. Following dormant pruning and commercial thinning, trees summer-pruned the previous year had higher yields than nonsummer pruned trees because of less shoot mortality and more fruit per tree.
The fruit growth of three peach (Prunus persica (L.) Batsch cvs. `Spring Lady', `Flamecrest', `Cal Red') and two apple cultivars (Malus domestica Borkh. cvs. `Cox Orange', `Golden Delicious') was measured weekly during the 1988 growing period. Seasonal patterns of fruit relative growth rate calculated on a dry weight basis were very similar for both species. Changes in nonstructural carbohydrate composition of peach mesocarp and apple pericarp were correlated with the two physiological phases of sink-activity of the relative growth rates Changes in sucrose concentrations seemed to coincide with increasing dry matter accumulation for both species, even though fructose was a dominant sugar in apples.
Dry weights of whole fruit and of different fruit tissues, such as the mesocarp (with exocarp) and the endocarp (with seed), were accumulated on early (`Spring Lady'), midseason (`Flamecrest'), and late-maturing (`Cal Red') peach [Prunus persica (L.) Batsch] cultivars during the 1988 growing season. Seasonal relative growth rate (RGR) patterns of whole fruit showed two distinct phases for `Flamecrest' and `Cal Red'; however, `Spring Lady' did not exhibit two distinct RGR phases. The shift from phase I to phase II of the whole fruit RGR curve was related to an intersection of mesocarp and endocarp RGR curves, indicating a change of physiological sink activities in those fruit tissues in the later-maturing cultivars, but not in the early cultivar. Nonstructural carbohydrate compositional changes in concentration or content were similar in the three peach cultivars. Sucrose accounted for most of the seasonal increase in mesocarp nonstructural carbohydrate concentration. A sudden rise of sucrose was associated with the phase shift of the fruit RGR curves of the midseason and late-maturing cultivars, but not of the early maturing cultivar; however, in the early maturing cultivar, mesocarp compositional carbohydrate changes and, particularly, the sucrose increase, indicate that the physiological processes normally associated with the two phases exist in very early maturing fruit but are not associated distinctly with two separate RGR phases.
Peach [Prunus persica (L.) Batsch] fruit thinning was used to reduce the competition for assimilates among peach fruits and to identify periods of source- and sink-limited growth during development. Individual fruit size, based on diameter or calculated dry matter accumulation, increased in trees with lower crop loads compared to fruits of unthinned trees in three peach cultivars. Relative growth rate analysis indicated that peach fruit growth was apparently limited by the assimilate supply (source-limited) or by its genetic growth potential (sink-limited) during specific growth periods. In stage I and at the beginning of stage III of the double-sigmoid growth curve, periods of source-limited growth occurred in the later-maturing cultivars Flamecrest and Cal Red. Peach fruit growth was apparently sink-limited during stage II of the growth curve when fruit relative growth rates were similar for the thinning treatments. Fruit growth in `Spring Lady', an early maturing cultivar, appeared to be primarily source-limited during the season. Although total fruit dry matter production was reduced by thinning, individual fruit dry weight on thinned trees was higher than that on trees with a heavy crop load. This typical thinning response was apparently caused by the differences in the amount of time that fruits grew under sink-vs. source-limited conditions with different crop loads. Final crop yield depended on fruit count per tree and on the available assimilate supply, and was affected by the individual fruit growth potential.
Plant dry matter production is proportional to light interception, but fruit production does not always increase with increased light interception. Vegetative growth potential, the effect of cropping on vegetative growth, light interception and cropping efficiency of a clingstone peach [Prunus persica (L.) Batsch `Ross' on `Nemaguard' rootstock] were assessed in four production systems differing in tree density and training system. The four systems were a perpendicular V (KAC-V) system, a high-density perpendicular V (HiD KAC-V) system, a cordon system, and an open vase system. Vegetative growth potential, assessed on defruited trees, was higher in the cordon system and lower in the open vase system compared to the V systems. Cropping reduced leaf growth on the V and cordon systems and stem growth on the KAC-V and cordon systems. On a ground area basis, the HiD KAC-V system had the highest crop yields and the open vase system had the lowest. The cordon and HiD KAC-V systems intercepted more light and produced more fruit, stem, and leaf biomass than the open vase system. However, the modified harvest increment, the ratio of fruit dry mass to the sum of fruit, leaf, and stem dry mass, was lower in the cordon system than in the other systems. Thus, on this basis, the cordon system was the least efficient. On a trunk cross-sectional area basis, there were no significant differences in fruit production among any of the four training systems. For current year production, crop production per unit ground area is the best measure of economic efficiency. However, when planning the spacing, training and pruning of orchard trees, the most appropriate goal seems to be a system that increases light interception without increasing vegetative growth potential, such as the HiD KAC-V system.