The physiological response of plants to water deficits are known to vary according
to the conditions of application of drought stress and the rate of development of leaf
water deficits. At the whole plant level the effect of the water shess is usually perceived
as a decrease in photosynthesis and growth, and is associated with alterations in C and
N metabolism (McDonald and Davies, 1996). The decrease in water potential affects
transpiration and hence xylem transport of nitrate or reduced N into growing regions.
The response of the photo-synthetic apparatus either to water stress or rehydration
seems to be dependent "on leaf age (O'Neill, 1983; Wolfe et al., 1988). Degradation
of both thylakoid and stromal N-containing compounds can occur in response to
water stress, recovery from which may pequire more than a week (Chaves, 1991)