Katerji et al., 1998b
2.3. Crop Water Status
Three parameters were used to characterize the crop water status:
• The pre-dawn leaf water potential. This parameter is measured when the equilibrium between soil and plant is approached (Katerji and Hallaire, 1984). It is determined at dawn, before sunrise, on leaves of the upper part of the canopy. At each determination, 5 leaves per treatment, equally distributed over the 5 lysimeters, were taken and the potential was immediately measured in a pressure chamber (Sholander type) at the experimental set-up.
• The stomatal conductance was always determined at midday, when the hourly transpiration attains its maximum value (Katerji et al., 1988). At each determination, 10 leaves per treatment, from the top of the vegetation, and equally distributed over the 5 lysimeters, were measured by means of a diffusive porometer (Licor 1600).
• The actual evapotranspiration was measured for each lysimeter as the difference between the amounts of irrigation and drainage water. Soil moisture sampling during the first experimental year showed almost the same moisture content, after each irrigation, corresponding to the field capacity. No infiltration or water logging problems were observed.
The leaf area and the above ground dry matter were determined at the successive main phenological stages of each species, on 5 plants (one per each lysimeter) chosen from the central part of each lysimeter, in order to avoid the edge effect. The leaf surface was determined by means of "LAI-Licor 1300". Dry matter was measured, on the same plant samples, after oven drying for 48 hours at 80°C. In the cases of potato and sugar-beet DM included the main yielding organs.
2.5. Determination of Radiation Use Efficiency (Monteith's Model)
The radiation use efficiency (RUE) is derived by the following equation (Monteith, 1993):
Where DM is the dry matter and XPARa equals the total of the photosynthetically active radiation (between 400 and 700|) intercepted during the crop cycle.
The daily estimate of PARa was made according to the following model (Varlet-Grancher, 1982):
Where PARi is incoming PAR estimated as: PARi = 0.48 • Rg and Rg is daily total incoming solar radiation (MJ m-2 d-1) measured with an Epply pyranometer, and reduced by 10 % for taking into account the plastic sheeting; si, the proportion of light absorbed, declines roughly exponentially with the leaf area index (LAI). Following Varlet-Grancher et al. (1982), si could be calculated as:
Where emaxis the maximum value of interception efficiency; it is quite close to 0.95 for most of the agricultural conditions; K is PAR extinction coefficient.
The K value is specific for each species. As for the studied crops, Varlet-Grancher et al. (1989). K values have been retained: 0.65 sugar beet; 0.42 potato; 0.70 maize, 0.97 sunflower, 0.50 tomato, 0.88 soy-bean.
For each year, DM was plotted versus accumulated PARa. Linear regressions forced through origin, were not significantly different from zero. The slope of these regressions (a) is an estimate of the Radiation Use Efficiency (RUE).
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