ASSOCIATION OF WATER-USE EFFICIENCY AND NITROGEN USE EFFICIENCY WITH PHOTOSYNTHETIC CHARACTERISTICS OF 2 WHEAT CULTIVARS

We examined if higher plant water use efficiency is related to a higher investment of nitrogen in components of the photosynthetic apparatus and if this is associated with a less efficient use of nitrogen for photosynthesis or for growth, We also assessed the influence of a reduced supply of water and nitrogen on these relationships, For this purpose we used two Triticum aestivum cultivars, differing in whole plant water use efficiency, Differences in leaf water use efficiency (A/E) between cultivars or treatments were associated with differences in Rubisco activity or rate of electron transport and, in general, with differences in plant water use efficiency, They were not invariably related to a low photosynthetic nitrogen use efficiency (PNUE), A high A/E was associated with a low PNUE, e.g, when we compared cultivars at high irradiance. In this case, a high A/E and a high leaf nitrogen concentration were associated with a low stomatal conductance and an equal rate of photosynthesis, A high A/E was not associated with a low PNUE when differences in A/E were mainly caused by higher rates of photosynthesis associated with higher nitrogen concentrations, e,g, when we compared cultivars at low irradiance and high water and nitrogen supply. An increase in the proportion of total leaf nitrogen associated with the photosynthetic apparatus at low soil-nitrogen increased PNUE, without affecting A/E in the opposite way, We conclude that whole plant nitrogen use efficiency was low for the cultivars with a high plant water use efficiency.