YIELD AND WATER-USE OF WHEAT (TRITICUM-AESTIVUM) IN A MEDITERRANEAN ENVIRONMENT - CULTIVAR DIFFERENCES AND SOWING DENSITY EFFECTS

Yield of eight wheat cultivars was evaluated under rainfed and irrigat ed conditions in a Mediterranean environment. Variation in grain yield resulted from variation in both aboveground biomass production and in harvest index. Under rainfed compared to irrigated conditions, grain yield, biomass and days to heading were decreased, whereas harvest ind ex was increased. Grain yield of the different cultivars under rainfed conditions correlated with that under irrigated conditions in one of the two years. Among cultivars, harvest index under rainfed and irriga ted conditions were correlated in both years. Water was used more effi ciently for biomass production, and equally efficiently for grain prod uction, under irrigated compared to rainfed conditions. Under rainfed conditions, crop water use efficiency was higher for cultivars develop ed for rainfed environments than for those developed for high-rainfall or irrigated environments. Cultivars with low-rainfall target environ ments had the lowest evapotranspiration under rainfed conditions. Unde r rainfed conditions, differences between the cultivar groups in crop water use efficiency corresponded with trends in water use efficiency of individual plants and with the ratio of photosynthesis to transpira tion, measured on plants grown in a growth room.Early in the season, w ater was used more efficiently for biomass production at high sowing d ensities than at low sowing densities. Through faster biomass producti on and ground cover a smaller proportion of the evapotranspired water was lost in soil evaporation and a larger proportion was transpired. H owever, the net effect was a greater water use in the early phases of growth and consequently a lower water availability later in the season , leading to similar yields regardless of sowing density.