RELATIVE GROWTH-RATE, BIOMASS ALLOCATION PATTERN AND WATER-USE EFFICIENCY OF 3 WHEAT CULTIVARS DURING EARLY ONTOGENY AS DEPENDENT ON WATER AVAILABILITY

We have investigated the water use efficiency of whole plants and sele cted leaves and allocation patterns of three wheat cultivars (Mexipak, Nesser and Katya) to explore how variation in these traits can contri bute to the ability to grow in dry environments. The cultivars exhibit ed considerable differences in biomass allocation and water use effici ency. Cultivars with higher growth rates of roots and higher proportio ns of biomass in roots (Nesser and Katya) also had higher leaf growth rates, higher proportions of their biomass as leaves and higher leaf a rea ratios. These same cultivars had lower rates of transpiration per unit leaf area or unit root weight and higher biomass production per u nit water use. They also had higher ratios of photosynthesis to transp iration, and lower ratios of intercellular to external CO2 partial pre ssure. The latter resulted from large differences in stomatal conducta nce associated with relatively small differences in rates of photosynt hesis. There was little variation between cultivars in response to dro ught, and differences in allocation pattern and plant water use effici ency between cultivars as found under well-watered conditions persiste d under dry conditions. At the end of the non-watered treatment, relat ive growth rates and transpiration rates decreased to similar values f or all cultivars. High ratios of photosynthesis to transpiration, and accordingly high biomass production per unit of transpiration, is rega rded as a favourable trait for dry environments, since more efficient use of water postpones the decrease in plant water status.