3.1. Soil Type Effect on Crop Water Status and Evapotranspiration
Figure 1 shows for the six studied crops the evolution in time of the pre-dawn leaf water potential in clay and loam soil. All crops showed a similar trend of an increase after irrigation, followed by a decrease during the irrigation interval. According to the species, the pre-dawn leaf water potential decreased from a maximum value, between -0.1 to -0.3 MPa, to a minimum value, between -0.4 to -0.6 MPa.
Values of maximum stomatal conductance (Figure 2) observed immediately after irrigation depend on the species. The differences among species indicate the role of the stomatal density, pore length and relative (respect to the total) pore area (de Parcevaux, 1972). The decrease in the maximum stomatal conductance with time, observed especially in sunflower and, to a lesser extent, in sugar beet may be attributed to the increasing air saturation deficit during the growing period (Fereira and Katerji, 1992).
The soil type significantly affected the stomatal conductance values of potato, sunflower, and sugar beet. These values are statistically lower on clay than on loam. In contrast, the soil type did not affect the stomatal conductance in the case of maize and tomato. Soybean behaviour was intermediate: the soil type effect being significant only in the case of maximum values of stomatal conductance, measured just after an irrigation.
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