Production Practices and Quality Assessment of Food Crops. Vol. 1

Modelling Fruit Quality 49
the density of plantation, fruit thinning, irrigation, or the management of pests and
diseases influence the above-cited factors.
It is clear that all the processes involved in the quality of fruits cannot be integrated
in biological models. But some degree of complexity is needed to consider
quality and the effect of agricultural practices. As presented hereafter, we are just
beginning to investigate this complexity and a lot remains to be done in the future.
3. MODELS OF FRUIT QUALITY ACROSS FIELDS OF SCIENCE
In ecophysiology, the priority has been given to modelling growth in dry mass
with the development of what has been called ‘photosynthesis driven models’
(Gary et al., 1998; Marcelis et al., 1998). Such models have been developed for
apples (Baumgaertner et al., 1984), grapes (Gutierrez et al., 1985), kiwifruits
(Buwalda, 1991), olives (Abdel-Razik, 1989), peaches (Grossman and Dejong, 1994)
and consider the plant as made of a few big compartments (leaf, wood, fruit) in
which carbon is accumulated. More recently, heterogeneity within the plant has been
investigated in tomatoes and peaches (Heuvelink and Bertin, 1994; Heuvelink, 1996;
Bruchou and Génard, 1999; Génard et al., 1999a). Modelling fruit growth has also
been improved by considering meristematic and non-meristematic tissues as in apple
fruits (Austin et al., 1999).
But fruits cannot be restricted to their carbon content or dry mass just because
the water is their main component. Only a few models have been proposed to
simulate water accumulation in the fruit. Fruit growth has been calculated by
integrating numerically the equation for water balance, using water uptake and
transpiration per unit of fruit area as a constant (Lee, 1990) or a variable (Génard
and Huguet, 1996). In a more mechanistic work, the difference in water potentials
in the stem and the fruit has been considered as the driving force in a model of water
import rate in tomatoes which also takes into consideration the role of the tomato
anatomy (Bussières, 1994). More recently, a model of fruit growth integrating both
the dry matter and the water accumulation within the fruit has been developed
(Fishman and Génard, 1998) and opened the way to considering the edible quality.
Though taste is now the motto of any advertisement for fruit products, it is still
absent from most models (Gary et al., 1998; Van Meeteren, 1998). But there are
some exceptions. Génard and Souty (1996) designed a model to predict the sweetness
of fruits based on carbon partitioning into various forms of sugars (sucrose,
glucose, fructose and sorbitol). Lobit (1999) has designed two models predicting
fruit acidity. The first one modelled citric acid production and degradation during
fruit development by representing the fluxes through the citrate cycle. In the second
one, malic acid content was assumed to depend mainly on the capacity of the cell
to store this acid in its vacuole. Both models have been combined with a model
of pH calculation (Lobit, 1999) in a global model able to predict the titratable acidity
of the fruit (Habib, 2000). This work on acidity is to our knowledge, one of the most
complex and interesting in the field of fruit quality modelling.
In agronomy, priority has been given to developing biotechnical models useful