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.

Malic acid production, degradation, and storage during fruit development have been modelled. The model assumes that malic acid content is determined essentially by the conditions of its storage in the mesocarp cells, and provides a simplified representation of the mechanisms involved in the accumulation of malate in the vacuole and their regulation by thermodynamic constraints. Solving the corresponding system of equations made it possible to predict the malic acid content of the fruit as a function of organic acids, potassium concentration, and temperature. The model was applied to peach fruit, and parameters were estimated from the data of fruit development monitored over 2 years. The predictions were in good agreement with experimental data. Simulations were performed to analyse the behaviour of the model in response to variations in composition and temperature.

Citrate production and degradation during the last stage of fruit development were modelled by representing the fluxes through the enzymes of the citrate cycle and the malic enzyme, the transport of metabolites between the cytosol and the mitochondria, and the stoichiometry equations that relate these reactions. After solving the corresponding system of equations, the rate of citrate synthesis (or degradation) was expressed as a simple function of temperature, mesocarp weight, and respiration. The model was applied to peach fruit, and its parameters were estimated from the data of a 2-year field experiment. The predictions of the model were in agreement with experimental data. Simulations were made to analyse the responses to variations of temperature and fruit growth. Increasing fruit growth before stone hardening stimulated citrate production, while increasing fruit growth after stone hardening reduced it. Delaying the date at which the maximum growth rate was reached enhanced citrate production during most of the period. In the last weeks before harvest, increasing temperature depressed citrate production, while, at the beginning of the period studied, it enhanced it.

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The fruit is a hierarchically organized organ composed of cells from different tissues. Its quality, defined by traits such as fruit size and composition, is the result of a complex chain of biological processes. These processes involve exchanges (transpiration, respiration, photosynthesis, phloem and xylem fluxes, and ethylene emission) between the fruit and its environment (atmosphere or plant), tissue differentiation, and cell functioning (division, endoreduplication, expansion, metabolic transformations, and vacuolar storage). In order to progress in our understanding of quality development, it is necessary to analyse the fruit as a system, in which processes interact. In this case, a process-based modelling approach is particularly powerful. Such a modelling approach is proposed to develop a future 'virtual fruit' model. The value of a virtual fruit for agronomists and geneticists is also discussed.

Shoot development was studied for two consecutive years in peach trees fertilized with N either in the previous fall or in the middle of the growing season. During the first year, two additional treatments were studied: no N supply and nitrate supplied in the irrigation water throughout the growing season. The number of shoots that developed depended on nitrogen availability in the period following bud break. During shoot development, leaf emergence occurred in one, two, or three stages, which ended at about 500 to 600 degree days, 1,000 to 1,200 degree days, and 1,500 to 2,000 degree days after bloom, respectively. The proportion of shoots exhibiting a second or third developmental stage depended on nitrogen availability at the beginning of that stage. Increasing nitrogen availability during a developmental stage prolonged the stage and increased the number of leaves produced.

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Based on the citrate model of Lobit and colleagues and measured data, a new model, which is able to reproduce the variation over time of citrate concentration in two peach cultivars, has been proposed. As in the original one, the new model calculates the rate of citrate synthesis or degradation as the product of a 'synthesis potential' and an 'efficiency level'. While in the old model the 'efficiency level' was a simple linear function of temperature and respiration, in the new one its relationship with respiration is accounted for by a coefficient that decreases throughout fruit development. The differences in model parameters between the two cultivars were investigated: late-maturing cv. Suncrest had significantly lower citrate synthesis potential than mid-maturing cv. Fidelia. The responses of citrate concentration to model parameters, temperature, fruit respiration, and growth curves were studied. The most important parameter in the new model, k(4,2), represented the date when the relationship between respiration and 'efficiency level' changed from positive to negative. Raising mean temperature increased the citrate concentration at the beginning and decreased it near maturity for cv. Suncrest, while citrate concentration increased throughout fruit development and more strongly for cv. Fidelia. An increase in the mesocarp dry weight increased both fruit respiration and citrate concentration at the beginning of fruit development, while near maturity it increased fruit respiration but decreased citrate concentration. The model was also able to reproduce the effect of assimilate supply (leaf:fruit ratio). Further potential uses of the model were discussed.