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

Modelling Fruit Quality 57
fructose, respectively; ( ph is the proportion of sucrose in the phloem sugar pool,
which results from plant metabolism; k 1(t), k 2(t), k 3(t) and k 4(t) represent the relative
rates of sugar transformation for net sucrose transformation into glucose and fructose,
net sorbitol transformation into glucose and fructose, and the synthesis of compounds
other than sugars from glucose and fructose.
These relative rates are calculated according to the following equations
k2(t) = k2 k3(t) = k3 1
k4(t) = k4 Wdry dW dry
dt
where k 1, 3 is a constant equal to 1 day –1 , k 1, 1, k 1, 2, k 2, k 3 and k 4 are parameters,
and W dry is the dry mass of the fruit flesh.
(dM ph/dt) and (dM re/dt) are the phloem and respiration flows of carbon into and
out of the fruit, respectively
dM ph
dt
dMdry dMre = σfl + (3)
dt dt
where σ fl is the carbon content of flesh.
Sugar concentrations are computed as
Csu = 100Msu , Cso =
σsuWfresh 100Mso , Cgl =
σsoWfresh 100Mgl σglWfresh , C fr = 100M fr
σ frW fresh
where σ su, σ so, σ gl and σ fr are the carbon content of 1 g of sucrose, sorbitol, glucose
and fructose, respectively. W fresh is the fresh mass of the flesh.
The sweetness of each sugar is computed using its sweetness rating according
to Kulp et al. (1991).
Time-step in the model is one day. Daily mean temperature and daily dry and
fresh flesh masses are the inputs of SUGAR. Previous studies (Génard and Souty,
1996; Génard and Huguet, 1999) have shown that the SUGAR model predicts the
sugar content of peaches with a fairly good accuracy over a wide range of fruit
growth rates.
4.1.2.2. Seasonal variation of sugar concentrations and sweetness shown by SUGAR
The simulations were performed during the main period of flesh development (two
months before maturity for the peach cultivar ‘Suncrest’). Figure 3 presents a typical
output of the model. Sucrose concentration presents a seasonal increase whereas
sorbitol is always low, and glucose and fructose concentrations present almost no
change during the season. Total sweetness increases during the season. The effect
of an increase in assimilate supply, due to fruit thinning for example which increases
leaf to fruit ratio, is mainly an increase in sucrose and sweetness.
(2)
, (4)