Theodore M. DeJong

Silencing leaf sorbitol synthesis alters long-distance partitioning and apple fruit quality.

Sorbitol and sucrose are major products of photosynthesis distributed in apple trees (Malus domestica Borkh. cv. “Greensleeves”) that affect quality in fruit. Transgenic apple plants were silenced or up-regulated for sorbitol-6-phosphate dehydrogenase by using the CaMV35S promoter to define the role of sorbitol distribution in fruit development. Transgenic plants with suppressed sorbitol-6-phosphate dehydrogenase compensated by accumulating sucrose and starch in leaves, and morning and midday net carbon assimilation rates were significantly lower. The sorbitol to sucrose ratio in leaves was reduced by ≈90% and in phloem exudates by ≈75%. The fruit accumulated more glucose and less fructose, starch, and malic acid, with no overall differences in weight and firmness. Sorbitol dehydrogenase activity was reduced in silenced fruit, but activities of neutral invertase, vacuolar invertase, cell wall-bound invertase, fructose kinase, and hexokinase were unaffected. Analyses of transcript levels and activity of enzymes involved in carbohydrate metabolism throughout fruit development revealed significant differences in pathways related to sorbitol transport and breakdown. Together, these results suggest that sorbitol distribution plays a key role in fruit carbon metabolism and affects quality attributes such as sugar–acid balance and starch accumulation.

Integrating simulation of architectural development and source–sink behaviour of peach trees by incorporating Markov chains and physiological organ function submodels into L-PEACH

L-PEACH is an L-system-based functional-structural model for simulating architectural growth and carbohydrate partitioning among individual organs in peach (Prunus persica (L.) Batsch) trees. The original model provided a prototype for how tree architecture and carbon economy could be integrated, but did not simulate peach tree architecture realistically. Moreover, evaluation of the functional characteristics of the individual organs and the whole tree remained a largely open issue. In the present study, we incorporated Markovian models into L-PEACH to improve the architecture of the simulated trees. The model was also calibrated to grams of carbohydrate, and tools for systematically displaying quantitative outputs and evaluating the behaviour of the model were developed. The use of the Markovian model concept to model tree architecture in L-PEACH reproduced tree behaviour and responses to management practices visually similar to trees in commercial orchards. The new architectural model along with several improvements in the carbohydrate-partitioning algorithms derived from the model evaluation significantly improved the results related to carbon allocation, such as organ growth, carbohydrate assimilation, reserve dynamics and maintenance respiration. The model results are now consistent within the modelled tree structure and are in general agreement with observations of peach trees growing under field conditions.

Partitioning of leaf nitrogen with respect to within canopy light exposure and nitrogen availability in peach (Prunus persica)

SummaryRelationships between leaf nitrogen content and within canopy light exposure were studied in mature nectarine peach trees (Prunus persica cv. Fantasia) that had received 0, 112, 196, 280 or 364 kg of fertilizer nitrogen per hectare per year for the previous 3 years. The relationships between light saturated leaf CO2 assimilation rates and leaf nitrogen concentration were also determined on trees in the highest and lowest nitrogen fertilization treatments. The slope of the linear relationship between leaf N content per unit leaf area and light exposure was similar for all nitrogen treatments but the y-intercept of the relationship increased with increasing N status. The slope of the relationship between leaf N content per unit leaf area and light saturated CO2 assimilation rates was greater for the high N trees than the low N trees, but maximum measured leaf CO2 assimilation rates were similar for both the high and low N treatments. A diagrammatic model of the partitioning of leaf photosynthetic capacity with respect to leaf light exposure for high and low nitrogen trees suggests that the major influence of increased N availability is an increase in the photosynthetic capacity of partially shaded leaves but not of the maximum capacity of highly exposed leaves.

Leaf potassium concentration, CO2 exchange and light interception in almond trees (Prunus dulcis (Mill) D.A. Webb)

Abstract The goal of this study was to determine the effect of tree potassium status on CO2 exchange and light interception of field-grown almond trees (Prunus dulcis (Mill) D.A. Webb). Treatments consisted of four levels of potassium fertilization applied for three consecutive seasons. Potassium deficiency negatively affected tree light interception. Defoliation after mechanical harvesting indicated that potassium deficiency accelerated premature leaf senescence and premature leaf abscission. CO2 exchange rates were curvilinearly related to leaf potassium concentration during mid-summer after most vegetative shoot growth was completed. Leaf potassium concentrations less than 0.5–0.6% appeared to limit leaf CO2 exchange rate. Large variations in leaf potassium concentration were found in the non-fertilized trees. Indeed, most of the response curve of leaf CO2 exchange rate to leaf potassium concentration could be described by analyzing the leaves of non-fertilized trees. Stomatal conductance was not affected significantly by leaf potassium concentration and leaves with low potassium concentration had the highest calculated internal CO2 concentrations. Thus it appears that potassium deficiency in almond affected the leaf photosynthetic capacity via biochemical limitations and not through an effect on stomatal conductance.

Seasonal patterns of vegetative growth and competition with reproductive sinks in peach (Prunus persica)

Summary Growth of leaves, wood, and stems were studied over an entire growing season on four year old peach (Prunus persica (L.) Batsch.) trees having no crop, commercial crop loads, or heavy crop loads. Leaf, wood, and stem growth were reduced on cropping trees relative to defruited trees. The presence of fruit reduced leaf biomass growth during Stage I and II of fruit growth but not thereafter. Wood biomass growth was reduced by the presence of fruit during all stages of fruit growth. Stem biomass growth was most strongly affected by fruit during Stage III of fruit growth. Carbon partitioning to stems appeared to be influenced by both fruit sink demands and set developmental capacity for secondary radial growth. Total above-ground biomass production was similar in all three treatments, despite significantly greater leaf area in defruited trees. The total carbohydrate cost of the above ground biomass (the sum of biomass costs, calculated growth and maintenance respiration costs) was estimated to be similar for all three treatments. Cropping reduced root starch content and flowering density but did not influence percent fruit set during the subsequent growing season relative to non-cropped trees.

Linking water stress effects on carbon partitioning by introducing a xylem circuit into L-PEACH

BACKGROUND AND AIMS Many physiological processes such as photosynthesis, respiration and transpiration can be strongly influenced by the diurnal patterns of within-tree water potential. Despite numerous experiments showing the effect of water potential on fruit-tree development and growth, there are very few models combining carbohydrate allocation with water transport. The aim of this work was to include a xylem circuit into the functional-structural L-PEACH model. METHODS The xylem modelling was based on an electrical circuit analogy and the Hagen-Poisseuille law for hydraulic conductance. Sub-models for leaf transpiration, soil water potential and the soil-plant interface were also incorporated to provide the driving force and pathway for water flow. The model was assessed by comparing model outputs to field measurements and published knowledge. KEY RESULTS The model was able to simulate both the water uptake over a season and the effect of different irrigation treatments on tree development, growth and fruit yield. CONCLUSIONS This work opens the way to a new field of modelling where complex interactions between water transport, carbohydrate allocation and physiological functions can be simulated at the organ level and describe functioning and behaviour at the tree scale.

Spring temperatures have a major effect on early stages of peach fruit growth

Summary Previous research has shown that Spring temperatures within 30 d after bloom (expressed as accumulated growing degree hours, GDH) are useful for predicting the harvest date of specific peach cultivars. The goal of the present research was to explore the relationship between GDH and additional environmental parameters on peach fruit development and growth during the period from the full bloom date (FBD) to the reference date (RD). Since heat accumulation during the first 30 d after bloom is a primary driver of fruit phenology, we hypothesised that years with high early Spring temperatures would result in smaller RD fruit size (RDFS) because trees cannot supply resources rapidly enough to support the potential growth associated with high rates of phenological development. Data on FBD, RD, and RDFS were collected at different locations in California between 1988 – 2004 and were analysed in conjunction with seasonal environmental data including accumulated GDH, rainfall, soil temperature, and solar radiation, from FBD to RD. Early Spring air temperatures appeared to be a primary environmental factor influencing RDFS. GDH accumulation during the first 30 d after bloom (GDH30) caused a decrease in the number of days between FBD and RD. RDFS increased with increases in the number of days between FBD and RD, and was negatively affected during years with high Spring temperatures. High GDH30 accumulations increased the rates of fruit growth d–1 but not enough to compensate for the shorter growth period from FBD to RD that occurred when GDH30 accumulation was high. The data supported the hypothesis that, with excessively high Spring temperatures, trees could not supply resources rapidly enough to support their maximum potential fruit growth rates.

A comparison of the combined effect of water stress and crop load on fruit growth during different phenological stages in young peach trees

Summary The combined effect of fruit load and water stress on fruit water content and dry-matter accumulation was analysed for three phenological stages of fruit growth. Irrigation treatments were no irrigation during Stage I (NI-SI), Stage II (NI-SII), or Stage III (NI-SIII) compared with a fully irrigated control. Three thinning treatments were imposed within each irrigation treatment resulting in fruit loads ranging from low to high. Fruit harvests at the end of Stage I, II and III were used to determine total tree fruit fresh and dry matter after each stage of fruit development. Fruit water accumulation was highly sensitive to the effect of water stress at high fruit loads in all fruit developmental phases, but reductions in fruit water content were more apparent during Stages II and III than during Stage I. On the other hand, fruit dry-matter accumulation was relatively insensitive to water stress at any fruit load level and developmental stage. However, reductions in dry-matter accumulation were obtained during Stage III from those trees that were not irrigated during Stage I (NISI). Since these reductions occurred only for mid-to-high fruit load conditions, the decreases in fruit growth during Stage III appeared to be related to a carbon source limitation. The possible reasons for this source limitation are discussed.

Effects of reproductive and vegetative sink activity on leaf conductance and water potential in Prunus persica L. Batsch

Abstract Reproductive and vegetative sink activities on mature trees of nectarine (Prunus persica L. Batsch. cultivar ‘Fantasia’) were controlled by removal of fruit buds and treatment with the chemical plant growth regulator paclobutrazol (PP-333), respectively. During the period corresponding to Stage III of fruit growth in fruiting trees, de-fruited trees had higher mean leaf water potential (ψ1) and lower leaf conductances (g1) than fruiting trees in a given paclobutrazol treatment. However, paclobutrazol-treated trees generally had both higher g1 and ψ1 than untreated trees within a given fruit treatment. Therefore, across both levels of vegetative growth activity, fruiting trees maintained higher g1 for a given ψl than non-fruiting trees.

Use of labeled nitrogen to monitor transition in nitrogen dependence from storage to current-year uptake in mature walnut trees

SummaryNitrogen (N) derived from both the soil during current-year uptake, and the withintree pool of storage N was distinguished in two groups of “Serr” walnut trees using labeled fertilizer (15N-depleted ammonium sulfate) applied in different years. Mass spectrometric analysis of N in xylem sap collected periodically in spring allowed quantification of the relative contributions of N from storage and current season uptake and the transition in N usage from previously assimilated (storage) N to the onset of current season uptake of soil N. N derived from storage accounted for > 50% of the xylem sap N during the staminate and pistillate bloom periods and throughout the period of spur leaf expansion.