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

50 M. Génard and F. Lescourret
for crop management. These models integrate the effects of cultural practices such
as fertilization and irrigation. They are based on ecophysiological or empirical
knowledge, but in all the cases, cultural practices and their effects on the system
modelled must be included. Many crop modelling groups have used their models
to optimize planting date, row spacing, choice of cultivar, and fertilizer application
rates for different types of soils (Boote et al., 1996; Bouman et al., 1996).
But in most cases, crop simulation models concern annual crops and focus on
yield. In fact, biotechnical models focusing on fruits are almost absent (except the
models of Doyle et al. (1989) in kiwifruits, Dumas et al. (1995) in processing
tomatoes and Gary and Tchamitchian (2001) in fresh tomatoes), and none of them
concern essential features of fruit quality such as taste or firmness. Even the different
fruit sizes produced by an orchard are almost never considered in biotechnical
models, though a grower’s benefit is directly related to such classifications. This
raises the issue of the variation of fruit quality which is as large within cultivars
as between cultivars (Génard and Bruchou, 1992). For a given cultivar, this variation
is structured at different levels: within trees, between trees in an orchard, and
between orchards. Lescourret et al. (1998a, b, 1999) have recently proposed a
biotechnical model on kiwifruit which attempted to account for fruit size variability
by considering factors occurring at different levels of organisation (fruit,
branch, plant, plot) in response to cultural practices and environmental factors.
In ecology, much research has been done on population growth or disease
development and predator-prey relationships and has contributed to strong theoretical
bases to study the dynamics of diseases and pests and their effect on crops
(de Wit and Goudriaan, 1978; Rijsdijk, 1986; Rabbinge et al., 1989). However,
the interactions between disease or pests and crops have not been much studied.
Exceptions include the pioneer studies of Gutierrez on various crop-pest systems,
especially on cassava (i.e. Gutierrez et al., 1988a, b, c, 1999) and cotton systems
(i.e. Gutierrez and Curry, 1989; Gutierrez et al., 1991), as well as those of Rossing
(1991) on aphid damage in winter wheat, Blaise et al. (1996) on the effect of disease
on grapevine yield, and of Chevalier-Gérard et al. (1994) or Colbach et al. (1999)
on the effect of cropping systems on disease development. However, all these studies
focus on crop yield and, to our knowledge, there is no model to simulate the effect
of diseases or pests on fruit quality.
According to Blaise et al. (1996), optimizing curative strategies is only possible
if the consequences of a crop protection decision can be quantified in terms of
risks of quality losses through the occurrence of disease or pest. Therefore, combining
disease or pest and crop quality models seems to be the actual challenge
for the near future (Habib and Lescourret, 1999).
4. THE QUALITY FROM THE ECOPHYSIOLOGIST’S POINT OF VIEW
For an ecophysiologist, quality results from a set of interrelated physiological
processes which depend on environmental conditions. At the fruit level, biophysical
processes involved in water, carbon, calcium, etc. fluxes and metabolism are
the main factors determining fruit quality such as fruit size, sweetness or acidity.