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Michel Génard and Michel Souty

The edible quality of peaches (Prunus persica L. Batsch) to a great extent depends on their sweetness, which is related to sugar composition. Our objective was to develop a model to predict carbon partitioning within fruit flesh and to predict the sucrose, sorbitol, glucose, and fructose contents. The model is dynamic and deterministic and was designed to be driven by the flesh dry-weight growth curve, flesh water content, and temperature data. It uses differential equations where the state of the system is defined by variables that describe how much carbon is present as each form of sugar and as other compounds (acids and structural carbohydrates). The rates of change of these amounts of carbon depend on the current values of corresponding variables and on the transfer functions between them. These functions are defined by rate constants or by functions of degree-days after full bloom. The model was calibrated and tested using data sets from treatments that covered several leaf: fruit ratios. The predictions of the model were in fairly good agreement with experimental data. A sensitivity analysis was performed to identify the most influential transfer function parameters. Carbon flows between sugar forms were analyzed. Sucrose, which was the most abundant sugar, and fructose, which is the sweetest, contributed most to fruit sweetness. Simulations were performed to study the effects of changes in fruit growth-curve parameters on sugar contents and concentrations.

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Michel Génard and Frédéric Baret

Gap fractions measured with hemispherical photographs were used to describe spatial and temporal variations of diffuse and direct light fractions transmitted to shoots within peach trees. For both cultivars studied, spatial variability of daily diffuse and direct light transmitted to shoots was very high within the tree. Diffuse and daily direct light fractions transmitted to shoots increased with shoot height within the tree and for more erect shoots. Temporal variations of hourly direct light were also large among shoots. Hourly direct light fractions transmitted to shoots were analyzed using recent developments in multivariate exploratory analysis. A gradient was observed between shoots sunlit almost all day and other shoots almost never sunlit. Well sunlit shoots were mostly located at the top of the tree and were more erect. Shoots located in the outer parts of the tree crown were slightly but significantly more sunlit than others for one cultivar. Principal component analysis additionally discriminated shoots according to the time of the day they were sunlit. This classification was related to shoot compass position for one cultivar. Spatial location of the shoot in the tree explained only a small part of light climate variability. Consequences of modeling light climate within the tree are discussed.

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Michel Génard and Claude Bruchou

An approach to studying fruit growth is presented for peach fruit (Prunus persica L. Batsch). It combines a functional description of growth curves, multivariate exploratory data analysis, and graphical displays. This approach is useful for comparing growth curves fitted to a parametric model, and analysis is made easier by the choice of the model whose parameters have a meaning for the biologist. Growth curves were compared using principal component analysis (PCA) adapted to the table of estimated parameters. Growth curves of 120 fruits were fitted to a model that assumes two growth phases. The first one described the pit growth and the first part of the flesh growth. The second described the second part of the flesh growth. From PCA, firstly it was seen that fruit growth varied according to cumulated growth during both growth phases and to date of maximal absolute growth. Secondly, fruit growth varied according to cumulated growth and relative growth rates during each phase. Further examples are presented where growth curves were compared for varying fruit number per shoot and leaf: fruit ratio, and for different sources of variation (tree, shoot, and fruit). Growth of individual fruit was not related to fruit number per shoot or to leaf: fruit ratio. Growth variability was especially high between fruit within shoots.

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Jocelyne Kervella, Loïc Pagès and Michel Génard

Genotypic variations in the length-diameter relationship of branches among peach and nectarine [Prunus persica (L.) Batsch.] cultivars were investigated. The length and basal diameter of all undamaged first-order shoots from 1-year-old trees of 14 cultivars and one accession were measured. Statistical analysis of the allometric relationship between length and basal diameter of shoots provided evidence of genotypic differences for that relationship, although the diameter of very short shoots did not differ between genotypes. A gradient existed from `Armking' with thin shoots (9 mm in diameter for 85.5-cm-long shoots) to `Flavorcrest' with thick shoots (16.4 mm in diameter for 85.5-cm-long shoots). Early selection for shoot thickness should be possible in breeding programs. The likely consequences of observed shoot thickness variations on the mechanical and hydraulic properties of shoots are discussed.

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Jocelyne Kervella, Loïc Pagès and Michel Génard

Leaf emergence was studied on main and first-order shoots of peach and nectarine [Prunus persica (L.) Batsch.] trees belonging to nine standard cultivars, during their first growing season. The number of emerged leaves was recorded on main shoots (originating from the grafted buds) and on first-order shoots (inserted directly on main shoots). Similarly shaped leaf emergence curves were observed on main and first-order shoots for all the cultivars. Leaf emergence rate decreased gradually as the number of leaves increased. The number of emerged leaves could be modeled as a monomolecular function of accumulated thermal units. Significant differences were found between cultivars in a multiple analysis of variance of the model parameters, for main and first-order shoots. The ranking of the cultivars was similar for both types of shoots. Leaf emergence rate was lower on first-order shoots than on main shoots. Differentiating between shoot types is necessary for a reliable comparison of genotypes.

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Caroline Gibert, Joël Chadœuf, Gilles Vercambre, Michel Génard and Françoise Lescourret

Investigations on “natural” cuticular cracks were conducted on nectarine fruit [Prunus persica (L.) Batsch var. nucipersica (Suckow) C.K. Schneid.]. A method for quantifying the cuticular crack surface area on a whole fruit basis was proposed. By using a stratified sampling design, the spatial distribution of the cuticular cracks over three regions (stylar end, peduncle, and cheek), their morphology, and the estimation of the total proportion of cuticular cracks on the fruit were studied. These features were examined during fruit development and in response to several fruit growing conditions corresponding to various crop loads and irrigation regimes. Cuticular cracks on nectarine fruit occurred during the final rapid fruit growth stage. Larger fruit presented higher cuticular crack densities in the apical regions than in the cheek regions. Thin and larger cuticular cracks occurred continuously during fruit development. Cuticular cracks represented 10% to 12.5% of the fruit surface area for well irrigated or low crop load trees, whereas they covered less than 4.5% for the heavy crop load and water deficit treatments. Cheek regions largely contributed to the total cuticular crack surface area (>60%), regardless of the fruit growing conditions. After irrigation was restricted, cuticular crack development was limited. A positive relationship was established between the cuticular crack surface area per fruit surface area and the fruit fresh weight.

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Isabelle Grechi, Nadine Hilgert, Michel Génard and Françoise Lescourret

Whereas quality is an increasingly important aspect of peach fruit [Prunus persica (L.) Batsch] production at this time, it is still not adequately addressed in crop models. Our objective was to develop a model to assess an essential trait of peach fruit quality (the refractometric index at harvest) to include it in existing crop models and to address the issue of quality in programs dealing with the improvement of crop management. The model predicts the fruit refractometric index, an indicator of sugar content (the most decisive parameter in consumer satisfaction) commonly used by the fruit industry. The model was simple enough so that it could be easily linked to carbon-based crop models. It was calibrated and tested using several independent data sets representing many growing conditions. To account for the effect of uncertainty in input and model parameters, the output of the model was qualified by a prediction interval. Results indicated that the model accurately predicted refractometric indices under 12% (relative root mean squared error values of 0.09 and 0.12 for two data sets), which corresponds to the fruit industry's range of interest. Prediction intervals revealed that the uncertainty in model parameters has moderate effects, whereas the uncertainty of the model input has important effects.