Discussion

The small pre-dawn leaf-water potential variations, observed on the six species grown on clay or loam, indicate that in the present study the crop water status corresponded to well-watered conditions. Indeed, before senescence, the pre-dawn leaf water potential for tomato, soybean, maize and corn varied in a range which was 5 to 6 times smaller than the range measured on the same species grown in soils whose available water varied between field capacity and wilting point (Katerji et al., 1988; Mastrorilli et al., 1993; Rana et al., 2004).

With regard to leaf surface, dry matter growth, yield and RUE, potato, sunflower and sugar beet systematically show their sensitivity to soil type by lower values on clay than on loam. The yield sensitivity of potato and the indifference of tomato to soil type agree with results reported in literature (Chapman and Carter, 1976; Fisher and Neil, 1990).

The observations of this experiment do not validate the hypothesis that soil type does not affect RUE under well-watered conditions. Since half of the species examined show an effect of soil type on RUE, the hypothesis commonly reported in literature has no a general validity. In the present study, RUE was determined in small plots. Monteith (1978) found that several reports of high maximum growth rate were associated with significant amounts of lateral radiation interception on small plots. Consequently the values of RUE were overestimated. Such hypothesis does not seem verified in this study. Table 5 shows that the RUE values determined in this experimental set up are consistent with those reported in literature and supported by field experiments. Sunflower showed lower values, both on clay and on loam. This could be attributed to the high air saturation deficit (Stockle and Kinery, 1990), already mentioned with regard to the behaviour of the maximum stomatal conductance of sunflower.

The sensitivity is closely linked to plant water relationships, which are less favourable on clay. The three sensitive species show a soil type effect on the punctual measurements of the stomatal conductance and on the global measurements of the accumulated evapotranspiration. The convergent observations clearly underline that the water status of plants growing on clay is less favourable than the water status of plants growing on loam. The physiologic mechanisms regulating gas exchanges and growth on clay probably originated by root signals (Davies and Zhang, 1991; Tardieu et al., 1991) transmitted to the aerial plant portion for reducing leaf growth and stomatal conductance. These signals, indeed, represent the reaction to mechanical (Masle and Passioura, 1987) or water (Tardieu et al., 1992) constraints exerted by soil on the roots. However, in reaction to the same environmental constraint, root signal concentration in sap flow varies according to the species (Tardieu and Dreyer, 1997). This different reaction to a certain extent could explain why some species demonstrate higher sensitivity than others to soil constraints.

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