Leaf and crop water measurements were analysed dynamically throughout the cycle of two crops of durum wheat subjected to contrasting conditions of water supply. The objectives were to compare short- and long-term crop reactions to water deficit, based on some hypotheses from the literature. Water status measurements were of the first type and phenological or morphological measurements of the second type. The droughted crop under a rainout shelter received no water between emergence and harvest, while the open-air crop was irrigated. Dynamic plant morphological measurements consisted of leaf area index and specific leaf area, and those characterising plant water status dynamics included predawn leaf water potential, bulk osmotic potential and the ratio of crop water uptake to evaporative demand. Continuous measurement of the surface temperature on the two plots served to calculate a degree-days temperature scale, allowing comparison of the two crops. We found that the droughted crop developed mechanisms of resistance to water deficit: the leaf area index and specific leaf area were reduced (from 6 to 1.5 m(2) m(-2) and from 300 to 260 cm(2) g(-1) for the maximum values of, respectively, the LAI and the SLA), the bulk osmotic potential was lower (from -2 to -3 MPa) and the soil layer affected by root uptake was deeper (from 50 to 100 cm) than that of the irrigated crop. The amount of water evapo-transpired by both crops was similar when related to climatic demand. Yet no clear relationship appeared between the specific leaf area index and osmotic potential. Furthermore, there appeared to be a clear difference between the vegetative and reproductive phases of both crops in terms of water functioning. The osmotic potential decreased markedly after anthesis and a peak of water uptake was observed in both crops during the grain-filling phase, which may have been linked to the specific functioning of the flag leaf or the ear. While this study demonstrated the efficiency of drought adaptive features in durum wheat, it also showed that the difference in the water behaviours of two contrasted durum wheat crops could be of the same order of magnitude as the differences between the vegetative and reproductive phases.

Numerous papers have reported the effects of water stress on wheat grain yield and its components, but limited information is available for modelling its consequences on harvest index. This knowledge is essential for simulation models which predict crop yield as a result of biomass and harvest index (IR). The biomass and harvest index responses to water-stress treatments differing in their intensity, duration and timing were therefore studied on lysimeters over 3 years. Yield, biomass and grain number decreased with water satisfaction rate (ETR/ETM). The booting-milky grain period was the most sensitive to water stress. Moderate stress did not affect IR but the decrease was rapid below ETR/ETM values of 0.75. The minimum value of IR was 0.19. IR increased with the contribution of water consumption after anthesis by up to 30%. Water stress during tillering increased the harvest index. Using these results, a simple model is proposed to predict the phasic response of IR to wafer stress.

The EPIC model (Williams et al, 1989) is commonly used to simulate the wheal crop in a crop rotation, though it overestimates grain yield in dry conditions. A new version of the model (EPICPHASE) has been developed by INRA in Toulouse to improve the model performance in drought-prone environments. The refinements introduced were: 1) a better description of the soil water extraction pattern, 2) a response of harvest index to water and nitrogen stress as a function of phenological phase. This study describes this new Version and proposes the corresponding calibration for winter soft wheat Literature and various experiments (lysimeters, pluri-annual field testing) were used to determine the optimal set of parameters. Functions describing wheat growth without limiting factors under water constraint were calibrated tie, biomass accumulation and partitioning, leaf area and roof depth, water uptake). The model was tested in Auzeville (Haute-Garonne, SW France) during two successive dry years (1989 and 1990) differing in their drought pattern, and on a range of fields differing in water and nitrogen supply. Referring to the extreme variability of the test situations, the predictions of the model were satisfactory for water budget, biomass, yield and nitrogen uptake, with mean relative errors of 8-18%, depending on variable and year. The sowing-heading duration was predicted with a root mean squared error of 59 degree days. The EPICPHASE-Wheat model should be quite valuable for assessing the impact of different management schemes in rainfed or irrigated wheat crops and for defining optimum strategies.