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

To calculate the hydrostatic pressure, the following procedure is used: the relative
rate of volume (V) growth of the fruit compartment is presented by Lockhart’s
equation (Lockhart, 1965)
dV
dt = V f(P f – Y) if P f > Y
dV
dt = 0 if P f ≤ Y
Modelling Fruit Quality 53
where f is the coefficient describing the extensibility of the cell walls and Y is the
threshold value that the hydrostatic pressure of the fruit has to exceed before
irreversible expansion occurs. The change in fruit volume can also be calculated
from equation 1 (with D w the water density) as
dV
dt = U x + U p – T f
D w
Under the condition of steady irreversible growth, equations 5 and 6 must be equal.
Setting equations 5 and 6 equal, and inserting the flux from equation 2, the resulting
equations for P f can be solved.
The time-step in the model is one hour. Total fruit mass, volume, and water
content may be calculated using the state variables W water(t) and W dry(t).
4.1.1.2. Using SWAF to explain seasonal and diurnal patterns of peach fruit growth
A set of model computations was performed to simulate the combined effect of water
stress and crop load in peach trees (Figure 1).
Sugar content in the phloem directly influences the rate of its uptake by the
fruit and results in lower dry fruit mass and respiration under conditions of high
crop load. In this case, the osmotic pressure is lower than with a low crop load,
which leads to high fruit water potential and low water uptake. Turgor pressure is
lower with a high crop load, mainly in the first period of fruit development, which
results in a low growth. Although transpiration decreases with the increase in fruit
load, the decrease in water uptake is such with a high crop load that fresh fruit
mass is always lower than with a low crop load. Water stress causes a significant
decrease in fresh fruit mass but the change in dry fruit mass is negligible because
carbon uptake through mass flow is assumed to be low. The occurrence of water
stress increases the osmotic pressure which leads to a decrease in fruit water
potential and water influx into the fruit. The patterns of variation of carbon and water
uptake are quite different, with a maximal carbon uptake occurring one month before
the maximal water uptake. This important water uptake during the last month of
the fruit development is related to the high transpiration of the fruit during this
period.
The diurnal patterns of fruit growth show additional features of the growth process
(Figure 2). The model predicts that diurnal patterns of dry and fresh masses of
the fruit are not directly correlated. It can be noted that during daytime, when
transpiration reaches its maximum value, fresh mass does not increase and even
(5)
(6)