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

Is it the only level to be considered? No, because fruit quality varies according to
different levels of organisation, as mentioned before. In fruit trees, the tree is the
key level, since most variations emerge at this level (Marini and Trout, 1984;
Habib et al., 1991; Audergon et al., 1993; De Silva et al., 2000), and also because
it is the target of most technical interventions. Thus, modelling fruit development
with an emphasis on variability within the tree is a crucial step towards improving
fruit quality through horticultural practices. The high variability of fruit quality
observed at the level of the tree mainly results from sink-source relationships
within the tree. We will now analyse these two levels.
4.1. Models of fruit physiology
Modelling Fruit Quality 51
Size is one of the main parameters of fruit quality. Indeed, the price paid to the
grower closely depends on fruit size. Many models have dealt with fruit growth.
Some authors have tried to handle the variability observed in fruit growth, either
by using a stochastic approach of growth rates (Hall and Gandar, 1996) or by
considering the factors influencing the sink strength of the fruit, e.g. the individual
number of seeds in the case of kiwifruit (Doyle et al., 1989; Lescourret et al., 1998b).
However, these studies usually neglect the effect of the tree on fruit development.
Therefore, we developed a model of fruit growth able to simulate dry matter and
water accumulation in the fruit according to the water and carbon status of the
plant (Fishman and Génard, 1998). Sweetness is also a significant feature of fruit
quality. It is more and more considered by producers and consumers and begins
to take part in fruit price determination. A model of sugar accumulation in peach
simulating the sweetness as a function of fruit development has been proposed
(Génard and Souty, 1996). These two models will be presented hereafter: the first
model is an example of what can be called a biophysical model, and the second a
metabolic model.
4.1.1. SWAF: a biophysical model of Sugar and Water Accumulation in the Fruit
The present model is based on the biophysical representation of water and dry matter
transport combined with the growth process stimulated by turgor pressure. It simulates
the period of fruit growth which does not involve cell division. Fruit flesh
is described as one compartment separated from the atmosphere and parent plant
by membranes. The parent plant supplies the fruit with water and sugars which
are brought through xylem vessels and phloem sieve tubes. The fruit consumes
carbon and water through the respiration and transpiration processes. The main state
variables of the system are the amount of water (W water) and of dry matter (W dry)
in the fruit. The hourly inputs of the model are temperature and relative humidity
of the ambient atmosphere, water potential in xylem vessels, and sugars concentration
in the phloem sap. Only the main equations of the model are presented
hereafter, the readers interested in a complete description of this model will find a
more accurate description in Fishman and Génard (1998).