WATER AVAILABILITY - A PHYSIOLOGICAL CONSTRAINT ON NITRATE UTILIZATION IN PLANTS OF AUSTRALIAN SEMIARID MULGA WOODLANDS

Nitrate was found to be the predominant form of available nitrogen in mulga soils. Nitrate reductase activities on a fresh mass basis of a r ange of plants from eastern (Queensland) mulga ecosystems 2 weeks afte r partial relief from drought were uniformly low for both herbaceous s pecies (165 +/- 25 pkat g(-1)) and woody perennials (77 +/- 14 pkat g( -1)). Supply of nitrate for 24 h to cut transpiring shoots of woody sp ecies or application of nitrate solution to the rooting zone of herbac eous species promoted little further increase in mean shoot nitrate re ductase activities. Most species exhibited high tissue nitrate concent rations during water stress and soluble organic N profiles were in man y cases dominated by the osmoprotective compounds, proline or glycine betaine. Species with low levels of proline or glycine betaine showed high foliar concentrations of other compatible osmotica such as polyol s or sugars. Effects of relieving water stress on nitrate reductase ac tivity, proline, glycine betaine and nitrate levels were followed over 3 d of irrigation. Available soil nitrate rose 10-fold immediately an d, following rapid restoration of leaf water status of the eight study species, a 4-fold increase occurred in mean nitrate reductase activit y together with progressive decreases in mean tissue concentrations of nitrate, proline and glycine betaine over the 3 d period. Similar cha nges in soil nitrate, nitrate reductase activity, proline and tissue n itrate were observed in the same ecosystem following a natural rainfal l event and in western (S.W. Australia) mulga following irrigation. It is concluded that, although nitrate nitrogen is present at high conce ntrations and is the predominant inorganic nitrogen source in soils of the mulga biogeographic region, its assimilation by perennial and eph emeral vegetation is limited primarily by water availability. A scheme is presented depicting interrelated physiological and biochemical eve nts in typical mulga species following a rain event and subsequent dry ing out of the habitat.