Michel Génard

Water deficits uncouple growth from photosynthesis, increase C content, and modify the relationships between C and growth in sink organs

In plants, carbon (C) molecules provide building blocks for biomass production, fuel for energy, and exert signalling roles to shape development and metabolism. Accordingly, plant growth is well correlated with light interception and energy conversion through photosynthesis. Because water deficits close stomata and thus reduce C entry, it has been hypothesised that droughted plants are under C starvation and their growth under C limitation. In this review, these points are questioned by combining literature review with experimental and modelling illustrations in various plant organs and species. First, converging evidence is gathered from the literature that water deficit generally increases C concentration in plant organs. The hypothesis is raised that this could be due to organ expansion (as a major C sink) being affected earlier and more intensively than photosynthesis (C source) and metabolism. How such an increase is likely to interact with C signalling is not known. Hence, the literature is reviewed for possible links between C and stress signalling that could take part in this interaction. Finally, the possible impact of water deficit-induced C accumulation on growth is questioned for various sink organs of several species by combining published as well as new experimental data or data generated using a modelling approach. To this aim, robust correlations between C availability and sink organ growth are reported in the absence of water deficit. Under water deficit, relationships weaken or are modified suggesting release of the influence of C availability on sink organ growth. These results are interpreted as the signature of a transition from source to sink growth limitation under water deficit.

How Does Tomato Quality (Sugar, Acid, and Nutritional Quality) Vary with Ripening Stage, Temperature, and Irradiance?

The objective of this study was to understand the respective impact of ripening stage, temperature, and irradiance on seasonal variations of tomato fruit quality. During ripening, concentrations in reducing sugars, carotenes, ascorbate, rutin, and caffeic acid derivates increased, whereas those in titratable acidity, chlorophylls, and chlorogenic acid content decreased. Fruit temperature and irradiance affected final fruit composition. Sugars and acids (linked to fruit gustative quality) were not considerably modified, but secondary metabolites with antioxidant properties were very sensitive to fruit environment. Increased fruit irradiance enhanced ascorbate, lycopene, beta-carotene, rutin, and caffeic acid derivate concentrations and the disappearance of oxidized ascorbate and chlorophylls. Increasing the temperature from 21 to 26 degrees C reduced total carotene content without affecting lycopene content. A further temperature increase from 27 to 32 degrees C reduced ascorbate, lycopene, and its precursors content, but enhanced rutin, caffeic acid derivates, and glucoside contents. The regulation by light and temperature of the biosynthesis pathways of secondary metabolites is discussed.

QTL analysis of quality traits in an advanced backcross between Prunus persica cultivars and the wild relative species P. davidiana

Genetic control of the different attributes involved in peach quality has been investigated in an advanced backcross population derived from a cross between Prunus davidiana clone P1908, a wild parent with poor agronomic performance, and a commercial variety, Summergrand. A total of 24 physical and biochemical traits were investigated. Quantitative trait loci (QTLs) were detected for all the traits studied. We identified alleles from P. davidiana with agronomically favorable effects regarding fruit and stone sizes, sugar and acid concentrations and red flesh coloration, in clear contrast to its phenotype. We identified three main regions of the genome where alleles from P. davidiana had negative effects on multiple traits. In other regions, co-locations of QTLs with opposite effects on quality traits were also detected. We discuss the nature of these co-locations in the light of the probable physiological mechanisms involved. Strategies to cope with negative correlations between favorable traits and co-locations of P. davidiana alleles with negative effects on quality traits and positive effects regarding resistance to powdery mildew are discussed from a breeding point of view.

What controls fleshy fruit acidity? A review of malate and citrate accumulation in fruit cells

Fleshy fruit acidity is an important component of fruit organoleptic quality and is mainly due to the presence of malic and citric acids, the main organic acids found in most ripe fruits. The accumulation of these two acids in fruit cells is the result of several interlinked processes that take place in different compartments of the cell and appear to be under the control of many factors. This review combines analyses of transcriptomic, metabolomic, and proteomic data, and fruit process-based simulation models of the accumulation of citric and malic acids, to further our understanding of the physiological mechanisms likely to control the accumulation of these two acids during fruit development. The effects of agro-environmental factors, such as the source:sink ratio, water supply, mineral nutrition, and temperature, on citric and malic acid accumulation in fruit cells have been reported in several agronomic studies. This review sheds light on the interactions between these factors and the metabolism and storage of organic acids in the cell.

Multivariate analysis of within-tree factors accounting for the variation of peach fruit quality

Abstract The colour, firmness and flavour of 101 peach fruit from the south side of the tree were related to the characteristics of fruit-bearing shoots using a multivariate approach. The variability of quality was high between fruit. Fruit from the upper parts of the tree were more purple, less orange-red, less firm, had a higher sucrose content, a lower citric acid content and a higher pH than fruit from the lower parts. Fruit well exposed during the day had a purple skin, whereas less exposed fruit were more orange-red and had higher luminance (L∗) and chrominance (a∗, b∗). Fruit exposed to light essentially in the afternoon were more purple, less yellow and orange and more firm, had a higher citric acid content, lower sucrose and malic acid contents than fruit exposed to light in the morning. Colour variables were weakly correlated to the total leaf area. Fruit borne by thick shoots with high total leaf area, high leaf area per fruit late in season and a high yield were sweet and had high malic acid content. Sour fruit were linked with high total leaf area and high leaf area per fruit early in season. A semantic model for quality elaboration is proposed.

Carbon allocation in fruit trees: from theory to modelling

Carbon allocation within a plant depends on complex rules linking source organs (mainly shoots) and sink organs (mainly roots and fruits). The complexity of these rules comes from both regulations and interactions between various plant processes involving carbon. This paper presents these regulations and interactions, and analyses how agricultural management can influence them. Ecophysiological models of carbon production and allocation are good tools for such analyses. The fundamental bases of these models are first presented, focusing on their underlying processes and concepts. Different approaches are used for modelling carbon economy. They are classified as empirical, teleonomic, driven by source–sink relationships, or based on transport and chemical/biochemical conversion concepts. These four approaches are presented with a particular emphasis on the regulations and interactions between organs and between processes. The role of plant architecture in carbon partitioning is also discussed and the interest of coupling plant architecture models with carbon allocation models is highlighted. As an illustration of carbon allocation models, a model developed for peach trees, describing carbon transfer within the plant, and based on source–sink and Münch transport theory is presented and used for analyzing the link between roots, shoots and reproductive compartments. On this basis, the consequences of fruit load or plant pruning on fruit and vegetative growth can be evaluated.

A simulation model of growth at the shoot-bearing fruit level: I. Description and parameterization for peach

Abstract A simulation model of daily C assimilation and allocation in an isolated shoot-bearing fruit is presented. The system is divided into three compartments: fruits, one-year-old stem, and leafy shoots. The pool of C assimilates available daily for distribution is the daily assimilation of C, plus that mobilized from reserves if the demand of sink organs exceeds the product of photosynthesis. Equations of leaf photosynthesis incorporate a feedback inhibition through the leaf storage reserves, and the effect of light extinction caused by foliage. Carbon assimilation of fruits is considered. The mobilization of reserves first concerns the leafy shoot, and then the one-year-old stem to a lesser extent. The model simulates carbon partitioning based on organ demands and priority rules. Maintenance respiration costs, which are calculated on the basis of the Q 10 concept, have first priority. Vegetative and reproductive growth are given second and third priority. Daily carbon demands for the vegetative and reproductive organs are based on an analytical formulation of the potential growth rate at any time. The accumulation or replenishing of reserves, first in the leafy shoot compartment and then, in the case of saturation, in the one-year-old stem compartment, is given last priority. A parameterization of the model is presented for peach, with several values for cultivar-dependent and location-dependent parameters (two cultivars and locations considered). The input data required by the model are described. Finally, some simulations concerning a sensitivity analysis are presented to illustrate the model behaviour.

Genetic and physiological analysis of tomato fruit weight and composition: influence of carbon availability on QTL detection

Throughout tomato domestication, a large increase in fruit size was associated with a loss of dry matter and sugar contents. This study aims to dissect the contributions of genetic variation and the physiological processes underlying the relationships between fruit growth and the accumulation of dry matter and sugars. Fruit quality traits and physiological parameters were measured on 20 introgression lines derived from the introgression of Solanum chmielewskii into S. lycopersicum, under high (HL, unpruned trusses) and low (LL, trusses pruned to one fruit) fruit load conditions. Inter- and intra-genotypic correlations among traits were estimated and quantitative trait loci (QTL) for size, composition, and physiological traits were mapped. LL increased almost all traits, but the response of sugar content was genotype-dependent, involving either dilution effects or differences in carbon allocation to sugars. Genotype×fruit load interactions were significant for most traits and only 30% of the QTL were stable under both fruit loads. Many QTL for fresh weight and cell or seed numbers co-localized. Eleven clusters of QTL for fresh weight and dry matter or sugar content were detected, eight with opposite allele effects and three with negative effects. Two genotypic antagonistic relationships, between fresh weight and dry matter content and between cell number and cell size, were significant only under HL; the second could be interpreted as a competition for carbohydrates among cells. The role of cuticular conductance, fruit transpiration or cracking in the relationship between fruit fresh weight and composition was also emphasized at the genetic and physiological levels.

Under what circumstances can process-based simulation models link genotype to phenotype for complex traits? Case-study of fruit and grain quality traits

Detailed information has arisen from research at gene and cell levels, but it is still incomplete in the context of a quantitative understanding of whole plant physiology. Because of their integrative nature, process-based simulation models can help to bridge the gap between genotype and phenotype and assist in deconvoluting genotype-by-environment (GxE) interactions for complex traits. Indeed, GxE interactions are emergent properties of simulation models, i.e. unexpected properties generated by complex interconnections between subsystem components and biological processes. They co-occur in the system with synergistic or antagonistic effects. In this work, different kinds of GxE interactions are illustrated. Approaches to link model parameters to genes or quantitative trait loci (QTL) are briefly reviewed. Then the analysis of GxE interactions through simulation models is illustrated with an integrated model simulation of peach (Prunus persica (L.) Batsch) fruit mass and sweetness, and with a model of wheat (Triticum aestivum L.) grain yield and protein concentration. This paper suggests that the management of complex traits such as fruit and grain quality may become possible, thanks to the increasing knowledge concerning the genetic and environmental regulation of organ size and composition and to the development of models simulating the complex aspects of metabolism and biophysical behaviours at the plant and organ levels.

Regulation of tomato fruit ascorbate content is more highly dependent on fruit irradiance than leaf irradiance.

BACKGROUND AND AIMS The mechanisms involving light control of vitamin C content in fruits are not yet fully understood. The present study aimed to evaluate the impact of fruit and leaf shading on ascorbate (AsA) accumulation in tomato fruit and to determine how fruit sugar content (as an AsA precursor) affected AsA content. METHODS Cherry tomato plants were grown in a glasshouse. The control treatment (normally irradiated fruits and irradiated leaves) was compared with the whole-plant shading treatment and with leaf or fruit shading treatments in fruits harvested at breaker stage. In a second experiment, the correlation between sugars and AsA was studied during ripening. KEY RESULTS Fruit shading was the most effective treatment in reducing fruit AsA content. Under normal conditions, AsA and sugar content were correlated and increased with the ripening stage. Reducing fruit irradiance strongly decreased the reduced AsA content (-74 %), without affecting sugars, so that sugar and reduced AsA were no longer correlated. Leaf shading delayed fruit ripening: it increased the accumulation of oxidized AsA in green fruits (+98 %), whereas it decreased the reduced AsA content in orange fruits (-19 %), suggesting that fruit AsA metabolism also depends on leaf irradiance. CONCLUSIONS Under fruit shading only, the absence of a correlation between sugars and reduced AsA content indicated that fruit AsA content was not limited by leaf photosynthesis or sugar substrate, but strongly depended on fruit irradiance. Leaf shading most probably affected fruit AsA content by delaying fruit ripening, and suggested a complex regulation of AsA metabolism which depends on both fruit and leaf irradiance and fruit ripening stage.