Fruit tree crop models: an update.

Functional structural plant models of tree crops are useful tools that were introduced more than two decades ago. They can represent the growth and development of a plant through the in silico simulation of the 3D architecture in connection with physiological processes. In tree crops, physiological processes such as photosynthesis, carbon allocation and growth are usually integrated into these models, although other functions such as water and nutrient uptake are often disregarded. The implementation of the 3D architecture involves different techniques such as L-system frameworks, pipe model concepts and Markovian models to simulate branching processes, bud fates and elongation of stems based on the production of metamers. The simulation of root architecture is still a challenge for researchers due to a limited amount of information and experimental issues in dealing with roots, because root development is not based on the production of metamers. This review aims to focus on functional-structural models of fruit tree crops, highlighting their physiological components. The potential and limits of these tools are reviewed to point out the topics that still need more attention.

Evaluation of the S-locus in Prunus domestica, characterization, phylogeny and 3D modelling.

Self-compatibility has become the primary objective of most prune (Prunus domestica) breeding programs in order to avoid the problems related to the gametophytic self-incompatibility (GSI) system present in this crop. GSI is typically under the control of a specific locus., known as the S-locus., which contains at least two genes. The first gene encodes glycoproteins with RNase activity in the pistils., and the second is an SFB gene expressed in the pollen. There is limited information on genetics of SI/SC in prune and in comparison., with other Prunus species, cloning., sequencing and discovery of different S-alleles is very scarce. Clear information about S-alleles can be used for molecular identification and characterization of the S-haplotypes. We determined the S-alleles of 36 cultivars and selections using primers that revealed 17 new alleles. In addition, our study describes for the first time the association and design of a molecular marker for self-compatibility in P. domestica. Our phylogenetic tree showed that the S-alleles are spread across the phylogeny, suggesting that like previous alleles detected in the Rosaceae., they were of trans-specific origin. We provide for the first time 3D models for the P. domestica SI RNase alleles as well as in other Prunus species, including P. salicina (Japanese plum), P. avium (cherry), P. armeniaca (apricot), P. cerasifera and P. spinosa.

Long proleptic and sylleptic shoots in peach (Prunus persica L. Batsch) trees have similar, predetermined, maximum numbers of nodes and bud fate patterns.

BACKGROUND AND AIMS: In peach (Prunus persica) trees, three types of shoots can be distinguished depending on the time of their appearance: sylleptic, proleptic and epicormic. On proleptic shoots, an average of ten phytomers are preformed in dormant buds prior to shoot growth after bud-break, whereas all phytomers are considered neoformed in sylleptic and epicormic shoots. However, casual observations indicated that proleptic and sylleptic shoots appear quite similar in number of phytomers and structure in spite of their different origins. The goal of this research was to test the hypothesis that both proleptic and sylleptic shoots exhibit similar growth characteristics by analysing their node numbers and bud fate patterns. If their growth characteristics are similar, it would indicate that the structure of both types of shoots is primarily under genetic rather than environmental control. METHODS: The number of phytomers and bud fate patterns of proleptic and sylleptic shoots of four peach cultivars grown in the same location (Winters, California) were analysed and characterized using hidden semi-Markov models. Field data were collected during winter 2016, just prior to floral bud-break. KEY RESULTS: Sylleptic shoots tended to have slightly fewer phytomers than proleptic shoots of the same cultivars. The bud fate patterns along proleptic and sylleptic shoots were remarkably similar for all the cultivars, although proleptic shoots started growing earlier (at least 1 month) in the spring than sylleptic shoots. CONCLUSIONS: This study provides strong evidence for the semi-deterministic nature of both proleptic and sylleptic shoots across four peach cultivars in terms of number of phytomers and bud fate patterns along shoots. It is apparent that the overall structure of shoots with similar numbers of phytomers was under similar genetic control for the two shoot types. Understanding shoot structural characteristics can aid in phenotypic characterization of vegetative growth of trees and in providing a foundation for vegetative management of fruit trees in horticultural settings.

The phyllochron of well-watered and water deficit mature peach trees varies with shoot type and vigour.

The branch construction of trees is based on phytomers, repetitive subunits defined as node + leaf + axillary meristem + internode. The rate at which phytomers are added to a shoot is termed the phyllochron, which is determined by genetics, endogenous regulation and environmental conditions. The phyllochron is fundamental to understanding the growth of plants. Most phyllochron studies on woody species are of young plants under controlled conditions without consideration for different types of shoots that are present in mature trees. In this 2-year field study, we investigated seasonal patterns of phyllochron development on both proleptic and epicormic shoots of mature peach trees (Prunus persica) exposed to two irrigation treatments. One treatment was not irrigated until significant stress was detected via water potential by pressure bombing. In the second treatment trees were normally irrigated with ~5.84 cm of water each week to match evapotranspirational loss. Midday stem water potential readings were regularly collected to assess the level of water stress experienced by the trees. Measurements of individual leaves and their corresponding internodes were taken along tagged proleptic and epicormic shoots three times per week from the beginning to the end of two growing seasons. Leaf measurements were used to calculate the phyllochron. The phyllochron increased as the season progressed. This increase could not be explained by traditionally accepted effects of temperature or light. The more vigorous epicormic shoots added leaves significantly faster than proleptic shoots on trees in both water deficit and normally irrigated treatments. Additionally, epicormic shoots produced leaves with significantly greater leaf length and leaf area. Midday stem water potentials were more negative in trees in the water deficit treatments only after proleptic shoots stopped growing. The phyllochron did increase while leaf length and leaf area decreased on epicormic shoots of deficit irrigated trees in the 2010 growing season. The phyllochron of both shoot types gradually increased over the season, which was primarily due to an endogenous rank or shoot ageing effect. Differences between shoot types indicate that the phyllochron is variable among shoots on the same tree and is associated with shoot vigour. Water deficit increased the phyllochron and over all shoot growth rate.

How different pruning severities alter shoot structure: a modelling approach in young 'Nonpareil' almond trees.

Axillary meristem fate patterns along shoots, also referred to as shoot structure, appear to be fairly consistent among trees within a genotype growing under similar conditions. Less is known about shoot structural plasticity following external manipulations, such as pruning. The aim of this study on almond (Prunus dulcis (Mill.)) shoots was to investigate how pruning severity affects the structure of 1-year-old shoots that grew after pruning (regrowth shoots), the 2-year-old portion of shoots that remained from the previous year's growth after pruning (pruned shoots), and whether regrowth shoots reiterate the structure of the original 1-year-old shoots before pruning. Three pruning severities were imposed and the structures along the different shoots were assessed by building hidden semi-Markov models of axillary meristem fates. The structures of regrowth and pruned shoots depended on pruning severity, but maintained some of the original shoot characteristics. Regrowth shoots developed more complex structures with severe pruning, but had simpler structures than original shoots indicating progressive simplification with tree age. Pruned shoot structures were affected by the severity of pruning, by the structure when the shoots were 1 year old, and probably by local competition among buds. Changes in structure due to pruning can be modelled and be predictable.

Nut crop yield records show that budbreak-based chilling requirements may not reflect yield decline chill thresholds.

Warming winters due to climate change may critically affect temperate tree species. Insufficiently cold winters are thought to result in fewer viable flower buds and the subsequent development of fewer fruits or nuts, decreasing the yield of an orchard or fecundity of a species. The best existing approximation for a threshold of sufficient cold accumulation, the "chilling requirement" of a species or variety, has been quantified by manipulating or modeling the conditions that result in dormant bud breaking. However, the physiological processes that affect budbreak are not the same as those that determine yield. This study sought to test whether budbreak-based chilling thresholds can reasonably approximate the thresholds that affect yield, particularly regarding the potential impacts of climate change on temperate tree crop yields. County-wide yield records for almond (Prunus dulcis), pistachio (Pistacia vera), and walnut (Juglans regia) in the Central Valley of California were compared with 50 years of weather records. Bayesian nonparametric function estimation was used to model yield potentials at varying amounts of chill accumulation. In almonds, average yields occurred when chill accumulation was close to the budbreak-based chilling requirement. However, in the other two crops, pistachios and walnuts, the best previous estimate of the budbreak-based chilling requirements was 19-32 % higher than the chilling accumulations associated with average or above average yields. This research indicates that physiological processes beyond requirements for budbreak should be considered when estimating chill accumulation thresholds of yield decline and potential impacts of climate change.

Functional-structural plant models: a growing paradigm for plant studies.

A number of research groups in various areas of plant biology as well as computer science and applied mathematics have addressed modelling the spatiotemporal dynamics of growth and development of plants. This has resulted in development of functional-structural plant models (FSPMs). In FSPMs, the plant structure is always explicitly represented in terms of a network of elementary units. In this respect, FSPMs are different from more abstract models in which a simplified representation of the plant structure is frequently used (e.g. spatial density of leaves, total biomass, etc.). This key feature makes it possible to build modular models and creates avenues for efficient exchange of model components and experimental data. They are being used to deal with the complex 3-D structure of plants and to simulate growth and development occurring at spatial scales from cells to forest areas, and temporal scales from seconds to decades and many plant generations. The plant types studied also cover a broad spectrum, from algae to trees. This special issue of Annals of Botany features selected papers on FSPM topics such as models of morphological development, models of physical and biological processes, integrated models predicting dynamics of plants and plant communities, modelling platforms, methods for acquiring the 3-D structures of plants using automated measurements, and practical applications for agronomic purposes.

Measuring and modelling seasonal patterns of carbohydrate storage and mobilization in the trunks and root crowns of peach trees.

BACKGROUND AND AIMS: Developing a conceptual and functional framework for simulating annual long-term carbohydrate storage and mobilization in trees has been a weak point for virtually all tree models. This paper provides a novel approach for solving this problem using empirical field data and details of structural components of simulated trees to estimate the total carbohydrate stored over a dormant season and available for mobilization during spring budbreak. METHODS: The seasonal patterns of mobilization and storage of non-structural carbohydrates in bark and wood of the scion and rootstock crowns of the trunks of peach (Prunus persica) trees were analysed subsequent to treatments designed to maximize differences in source-sink behaviour during the growing season. Mature peach trees received one of three treatments (defruited and no pruning, severe pruning to 1·0 m, and unthinned with no pruning) in late winter, just prior to budbreak. Selected trees of each treatment were harvested at four times (March, June, August and November) and slices of trunk and root crown tissue above and below the graft union were removed for carbohydrate analysis. Inner bark and xylem tissues from the first to fifth rings were separated and analysed for non-structural carbohydrates. Data from these experiments were then used to estimate the amount of non-structural carbohydrates available for mobilization and to parameterize a carbohydrate storage sub-model in the functional-structural L-PEACH model. KEY RESULTS: The mass fraction of carbohydrates in all sample tissues decreased from March to June, but the decrease was greatest in the severely pruned and unthinned treatments. November carbohydrate mass fractions in all tissues recovered to values similar to those in the previous March, except in the older xylem rings of the severely pruned and unthinned treatment. Carbohydrate storage sink capacity in trunks was empirically estimated from the mean maximum measured trunk non-structural carbohydrate mass fractions. The carbohydrate storage source available for mobilization was estimated from these maximum mass fractions and the early summer minimum mass fractions remaining in these tissues in the severe treatments that maximized mobilization of stored carbohydrates. The L-PEACH sink-source carbohydrate distribution framework was then used along with simulated tree structure to successfully simulate annual carbohydrate storage sink and source behaviour over years. CONCLUSIONS: The sink-source concept of carbohydrate distribution within a tree was extended to include winter carbohydrate storage and spring mobilization by considering the storage sink and source as a function of the collective capacity of active xylem and phloem tissue of the tree, and its annual behaviour was effectively simulated using the L-PEACH functional-structural plant model.

Differences in proleptic and epicormic shoot structures in relation to water deficit and growth rate in almond trees (Prunus dulcis).

BACKGROUND AND AIMS: Shoot characteristics differ depending on the meristem tissue that they originate from and environmental conditions during their development. This study focused on the effects of plant water status on axillary meristem fate and flowering patterns along proleptic and epicormic shoots, as well as on shoot growth rates on 'Nonpareil' almond trees (Prunus dulcis). The aims were (1) to characterize the structural differences between proleptic and epicormic shoots, (2) to determine whether water deficits modify shoot structures differently depending on shoot type, and (3) to determine whether shoot structures are related to shoot growth rates. METHODS: A hidden semi-Markov model of the axillary meristem fate and number of flower buds per node was built for two shoot types growing on trees exposed to three plant water status treatments. The models segmented observed shoots into successive homogeneous zones, which were compared between treatments. Shoot growth rates were calculated from shoot extension measurements made during the growing season. KEY RESULTS: Proleptic shoots had seven successive homogeneous zones while epicormic shoots had five zones. Shoot structures were associated with changes in growth rate over the season. Water deficit (1) affected the occurrence and lengths of the first zones of proleptic shoots, but only the occurrence of the third zone was reduced in epicormic shoots; (2) had a minor effect on zone flowering patterns and did not modify shoot or zone composition of axillary meristem fates; and (3) reduced growth rates, although patterns over the season were similar among treatments. CONCLUSIONS: Two meristem types, with different latency durations, produced shoots with different growth rates and distinct structures. Differences between shoot type structure responses to water deficit appeared to reflect their ontogenetic characteristics and/or resource availability for their development. Tree water deficit appeared to stimulate a more rapid progression through ontogenetic states.

Vigour reduction in girdled peach trees is related to lower midday stem water potentials.

Stem or trunk girdling is a technique used in physiological studies and in horticultural practice for interrupting carbon flow through the phloem to other parts of the plant without influencing water flow in the xylem. Trunk girdling in peaches is practiced primarily to stimulate fruit growth but it also tends to decrease shoot vigour for a period of time after girdling. Water flow through the trunk or branches of peach trees is thought to be primarily dependent on the most recently formed ring of xylem and vegetative growth is closely related to stem water potential and stem hydraulic conductance. The aim of the present work was to determine whether vigour reduction due to girdling was correlated with a reduction in midday stem water potential during the period of time between girdling and the subsequent healing of stem tissue. 'Springcrest' peach trees were girdled on two different dates. Fruit yield and size, water sprout growth, proleptic shoot growth and stem water potential were measured. Early and late girdled trees yielded larger fruits and fewer and shorter water sprouts in comparison with control trees. Midday stem water potential declined significantly after girdling and gradually recovered until the time of fruit harvest. These results suggest that the vigour reduction of girdled trees is related to a decrease of midday stem water potential caused by girdling. Early tree girdling increased the reduction in midday stem water potential and shoot growth compared with the later girdling treatment. These results point out that even though girdling only removes bark and phloem tissue it can apparently affect water flow in xylem.

Detecting nonlinear response of spring phenology to climate change by Bayesian analysis.

The impact of climate change on the advancement of plant phenological events has been heavily studied in the last decade. Although the majority of spring plant phenological events have been trending earlier, this is not universally true. Recent work has suggested that species that are not advancing in their spring phenological behavior are responding more to lack of winter chill than increased spring heat. One way to test this hypothesis is by evaluating the behavior of a species known to have a moderate to high chilling requirement and examining how it is responding to increased warming. This study used a 60-year data set for timing of leaf-out and male flowering of walnut (Juglans regia) cultivar 'Payne' to examine this issue. The spring phenological behavior of 'Payne' walnut differed depending on bud type. The vegetative buds, which have a higher chilling requirement, trended toward earlier leaf-out until about 1994, when they shifted to later leaf-out. The date of male bud pollen shedding advanced over the course of the whole record. Our findings suggest that many species which have exhibited earlier bud break are responding to warmer spring temperatures, but may shift into responding more to winter temperatures (lack of adequate chilling) as warming continues.

Almond Leaf Scorch Disease Development on Almond Branches High-Grafted on Peach Rootstock.

Development of almond leaf scorch (ALS) disease was monitored on young almond (Prunus dulcis 'NePlus' and 'Peerless') shoots produced from almond scion wood that was high-grafted on peach rootstocks (P. persica 'Queencrest/Nemaguard'), after the almond shoots were mechanically inoculated with Xylella fastidiosa. The objective of this study was to evaluate the potential movement of X. fastidiosa through almond-peach graft unions. ALS symptoms developed on both X. fastidiosa-inoculated and uninoculated almond shoots that were high-grafted on different peach limbs of the same tree in September following inoculations with X. fastidiosa made in June and July 2002, respectively, when the average distance in peach rootstock between the two almond-peach graft unions was 30.5 cm or shorter. No ALS symptoms were observed on uninoculated almond shoots late in the growing season of the second year. The incidence of ALS-affected leaves on shoots inoculated with X. fastidiosa decreased in the second year on the inoculated shoots of Peerless as determined by the number of inoculated shoots showing ALS symptoms in 2002 versus 2003. No visible ALS symptoms were observed in NePlus late in the growing season of the second year, suggesting that survival of X. fastidiosa in NePlus was less than in Peerless. These data demonstrate that movement of X. fastidiosa through two almond-peach graft unions was possible during the year of inoculation when the peach rootstock stem distance between the two almond-peach graft unions was minimal. However, X. fastidiosa may not survive the winter season or survived so poorly as not to be able to produce ALS symptoms in the second growing season on the uninoculated shoots that previously showed ALS symptoms in the fall of the year when an adjacent shoot was inoculated. The results suggest that high-grafting almond scion wood on multiple peach rootstock limbs may provide a means to limit movement of X. fastidiosa from one almond limb to another on the same tree.

Relationships between spur- and orchard-level fruit bearing in almond (Prunus dulcis).

Almond is often considered to be a moderately alternate-bearing species but historical yield data typically do not exhibit clear patterns of alternate bearing at the orchard level, while research has indicated that spurs (the main fruit bearing unit in almond trees) rarely produce fruit in two subsequent years. The objective of the present work was to analyze the bearing behavior of almond trees at both the orchard level and the individual spur level over multiple years to explain this apparent paradox. The 10-year yield patterns of three almond cultivars grown at three different sites within California were analyzed for tendencies of alternate bearing at the orchard level. At the individual spur level, data on spur viability, and number of flowers and fruits per spur were collected on 2400 individually tagged spurs that were observed over 6 years to characterize bearing at that level. At the orchard level one cultivar (Nonpareil) did exhibit a tendency for alternate bearing at one site (Kern) but other cultivars and sites did not. The orchard and the individual trees in which the spur population study was conducted showed tendencies for alternate bearing but the spur population did not. Only a relatively small percentage of the total tagged spur population bore fruit in any given year and therefore while individual fruiting spurs exhibited a high level of non-bearing after fruiting the previous year the spurs that did produce fruit in any year generally did not constitute enough of the total spur population to exhibit alternate bearing at the whole population level. Our results suggest that annual bearing fluctuations in almond are probably mainly due to year-to-year variations of parameters affecting fruit set and that high rates of fruit set in a given year may involve a larger-than-normal percentage of a spur population in fruit bearing. This would limit the size of the spur population available for flowering in the subsequent year and could cause alternate year bearing. However, from historical records, this would appear to be the exception rather than a normal circumstance. Therefore, almond should not be considered to be a strictly alternate-bearing species.

Using functional­structural plant models to study, understand and integrate plant development and ecophysiology.

Functional­structural plant models (FSPMs) explore and integrate relationships between a plant's structure and processes that underlie its growth and development. In recent years, the range of topics being addressed by scientists interested in functional­structural plant modelling has expanded greatly. FSPM techniques are now being used to dynamically simulate growth and development occurring at the microscopic scale involving cell division in plant meristems to the macroscopic scales of whole plants and plant communities. The plant types studied also cover a broad spectrum from algae to trees. FSPM is highly interdisciplinary and involves scientists with backgrounds in plant physiology, plant anatomy, plant morphology, mathematics, computer science, cellular biology, ecology and agronomy. This special issue of Annals of Botany features selected papers that provide examples of comprehensive functional­structural models, models of key processes such as partitioning of resources, software for modelling plants and plant environments, data acquisition and processing techniques and applications of functional­structural plant models for agronomic purposes.

Spur behaviour in almond trees: relationships between previous year spur leaf area, fruit bearing and mortality.

In mature almond (Prunus dulcis) orchards, the majority of crop is borne on spurs (short, proleptic shoots) that can live for several years and can produce from one to five fruits. Previous research has led to the hypothesis that spur longevity is related to spur light exposure, cropping and age. However, limited quantitative data are available to substantiate these hypotheses. The objective of this study was to determine spur characteristics that were most highly correlated with spur productivity and longevity in mature, bearing almond trees. Previous year spur leaf area was strongly related to spur viability and flowering; the greater the leaf area in the previous year, the higher the probability of spur survival into the next year and the higher the probability for the spur to bear one or more flowers. Previous year bearing also appeared to influence viability and return bloom, especially in spurs with low leaf area. These results suggest that spur source-sink balance is basic to the life cycle of almond spurs. Furthermore, the results are consistent with the hypothesis that spurs are semi-autonomous organs with respect to carbohydrate balance for much of the growing season. Finally, this information provides general thresholds for maintaining spur viability and productivity that will be useful for developing and evaluating tree training systems and orchard management practices.

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.

Interactions between rootstock, inter-stem and scion xylem vessel characteristics of peach trees growing on rootstocks with contrasting size-controlling characteristics.

BACKGROUND AND AIMS: The primary physiological mechanism influencing tree vigour in size-controlling rootstocks of peach has been related to the hydraulic conductance of the rootstock. Differences in rootstock hydraulic conductance are a function of rootstock xylem vessel characteristics. The present research examined whether the vigour and xylem vessel characteristics of the rootstock influence the xylem characteristics of the scion. We tested whether using a size-controlling rootstock genotype as an inter-stem influences the xylem vessel characteristics of either the rootstock below the inter-stem or the scion above it and vice versa. METHODOLOGY: Anatomical measurements (diameter and frequency) of xylem vessels were determined above and below the graft unions of the trunks of peach trees with differing scion/rootstock combinations. The three peach rootstocks were 'Nemaguard' (vigorous), 'P30-135' (intermediate vigour) and 'K146-43' (dwarfing). The vigorous scion cultivar was 'O'Henry'. The inter-stem experiment involved trees with 'Nemaguard' (vigorous) as the rootstock, 'K146-43' (dwarfing) as the inter-stem and 'O'Henry' as the scion. Based on anatomical measurements, we calculated the theoretical axial xylem conductance of each stem piece and rootstock genotype with the Hagen-Poiseuille law. PRINCIPAL RESULTS: Xylem vessel dimensions of rootstocks varied in conjunction with tree vigour. Scion xylem vessel dimensions of different scion/rootstock combinations were only marginally affected by rootstock genotype. The inter-stem sections from the dwarfing genotype ('K146-43') had narrower vessels and a lower calculated hydraulic conductance than the xylem from either the vigorous rootstock below ('Nemaguard') or the scion above ('O'Henry'). CONCLUSIONS: Rootstock genotype only marginally affected scion xylem vessel characteristics. Thus the xylem vessel characteristics of the dwarfing rootstock genotypes appear to influence tree growth directly rather than through an effect on the xylem characteristics of the scion. A dwarfing rootstock genotype used as an inter-stem appeared to work as a physical restriction to water movement, reducing potential xylem flow and conductance of the whole tree.

Relationships between xylem vessel characteristics, calculated axial hydraulic conductance and size-controlling capacity of peach rootstocks.

BACKGROUND AND AIMS: Previous studies indicate that the size-controlling capacity of peach rootstocks is associated with reductions of scion water potential during mid-day that are caused by the reduced hydraulic conductance of the rootstock. Thus, shoot growth appears to be reduced by decreases in stem water potential. The aim of this study was to investigate the mechanism of reduced hydraulic conductance in size-controlling peach rootstocks. METHODS: Anatomical measurements (diameter and frequency) of xylem vessels were determined in shoots, trunks and roots of three contrasting peach rootstocks grown as trees, each with different size-controlling characteristics: 'Nemaguard' (vigorous), 'P30-135' (intermediate vigour) and 'K146-43' (substantially dwarfing). Based on anatomical measurements, the theoretical axial xylem conductance of each tissue type and rootstock genotype was calculated via the Poiseuille-Hagen law. KEY RESULTS: Larger vessel dimensions were found in the vigorous rootstock ('Nemaguard') than in the most dwarfing one ('K146-43') whereas vessels of 'P30-135' had intermediate dimensions. The density of vessels per xylem area in 'Nemaguard' was also less than in 'P30-135'and 'K146-43'. These characteristics resulted in different estimated hydraulic conductance among rootstocks: 'Nemaguard' had higher theoretical values followed by 'P30-135' and 'K146-43'. CONCLUSIONS: These data indicate that phenotypic differences in xylem anatomical characteristics of rootstock genotypes appear to influence hydraulic conductance capacity directly, and therefore may be the main determinant of dwarfing in these peach rootstocks.

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.

Seasonal carbohydrate storage and mobilization in bearing and non-bearing pistachio (Pistacia vera) trees.

We analyzed annual carbohydrate storage and mobilization of bearing ("on") and non-bearing ("off") 'Kerman' pistachio (Pistacia vera L.) trees growing on three different rootstocks. On all rootstocks, carbohydrate storage in shoots and branches of "on" and "off" trees was lowest following the spring growth flush. In "off" trees, stored carbohydrates increased and remained high after the initial growth flush. In "on" trees, stem carbohydrates increased temporarily in early summer, but were mobilized in mid-season during kernel fill, and then increased again after nut harvest. During the dormant season, the only substantial differences in carbohydrate storage between previously "on" and "off" trees were found in the roots of the weakest rootstock. The annual carbohydrate storage and mobilization pattern in canopy branches of heavily cropped pistachio trees appeared to be driven by carbohydrate demands related to nut development and untempered by tree vigor. Mobilization of carbohydrates from current-season and 1- and 2-year-old stem wood of "on" trees during the primary period of kernel fill corresponded with the period of inflorescence bud abscission. Thus, the alternate bearing pattern associated with inflorescence bud abscission in 'Kerman' pistachio may be a function of mid-season mobilization of stored carbohydrates in current-season stems resulting in stimulation of inflorescence bud abscission.