Introduction

To characterize the biomass growth, Monteith (1972) proposed a simple method. It was obtained by analyzing the relationship between the dry matter increase and the amount of solar radiation intercepted by the crop. Generally, under non-limiting conditions of water and mineral nutrition (Monteith, 1978), this relationship is represented by a linear function. The slope of the function stands for the radiation use efficiency (RUE), characteristic for each species. RUE is supposed to be constant during the vegetative cycle (Gallagher and Biscoe 1978; Gosse et al., 1986; Kiniry et al., 1989). This simplified approach is largely used for simulating crop growth and yield. Effectively it is commonly adopted in crop production models, such as CERES (Ritchie et al., 1994). CROPSYST (Stockle and Nelson, 1997), EPIC (Sharpley and Williams, 1990), STICS (Brisson et al., 2002).

The analysis of RUE values published in literature (Sinclair and Muchow, 1999) shows that the RUE of each species responds to different sources of variability.

Genetics is the first source of variability. As an example, maize (Tollenaar and Aguilera, 1992) or sugar beet (Damay and Le Gouis, 1993) are characterized by RUE values changing with the genotype.

Plant density is a second source of variability (Giauffret et al., 1991; Begue et al., 1991). The RUE values increase with plant density till a threshold and decrease afterwards.

The third source of variability is caused by environmental factors, among which the ratio between diffuse and solar radiation (Sinclair et al., 1992; Hammer and Wright, 1994), the daily minimum or mean air temperature (Bell et al., 1992; Andrade et al., 1993) and the vapour pressure deficit (Stockle and Kiniry, 1990; Goyne et al., 1993; Kiniry et al., 1989; Kemanian et al., 2004).

Finally, the soil water and nitrogen deficit (Muchow, 1992; Muchow and Davids, 1998; Whitfield, 1993; Wright et al., 1993), or excess (Ceotto and Castelli, 2002) significantly modify the RUE values.

In analyzing the possible sources of RUE variation, soil texture has never been considered as an environmental factor susceptible of modifying the RUE (i.a., see the detailed revue by Sinclair and Muchow, 1999). However, this omission is not supported by experimental evidence. On the contrary, a number of results reported in literature emphasizes the role of soil properties in satisfying crop water requirements (Ozier-Lafontaine et al., 1998). The following considerations merit attention:

• Soil water movement and available water, which depend on the soil hydrodynamic properties and therefore on soil texture (Gardner, 1988);

• Root system capacity for water uptake, which depends on root density and distribution, affected in turn by physical soil properties (Tardieu and Pellerin, 1991).

Taking these considerations into account, it is clear that the plant water requirements cannot be satisfied to the same degree in every soil type, even under well-watered conditions, (Gardner, 1988). Both models (Bruckler et al., 1991; Tardieu et al., 1992) and field surveys of plant water status and biomass growth (Tardieu et al., 1992; Bethenod et al., 1996) confirm this point. RUE values should necessarily be affected by soil type.

To the best of our knowledge, the effect of soil type on the RUE has never been reported in literature. In this paper we compare RUE values determined on six species (potato, sugar beet, maize, sunflower, soy-bean, and tomato) grown under the same conditions of weather, water and mineral nutrition, and simultaneously on two soil types (loam and clay). The objective was to answer the following questions:

• Under well-watered conditions, does soil type significantly modify crop water use?

• If so, can the observed modifications in evapotranspiration change the biomass accumulation, and, in turn, the RUE?

In the conclusion, the practical consequences derived from this study will be discussed.

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