Water Stress Effects on Nitrogen Assimilation and Growth of Trifolium subterraneum L. Using Dinitrogen or Ammonium Nitrate

The relative effects of water stress on growth parameters of subterranean clover (Trifolium subterraneum L. cv. Woogenellup) dependent on either N(2) or 8 millimolar NH(4)NO(3) for N were examined. Whole-plant carbon exchange rate (CER), acetylene reduction (AR), dry matter production, and Kjeldahl N accumulation were measured on uniform, intact swards of clover that were maintained under adequately watered conditions or were subjected to three cycles of water stress (leaf water potential </=-30 bar) over an 18-day period. In the absence or presence of water stress, growth rate, net N accumulation rate, and total N concentration of plants dependent on N(2) were 25 to 26, 45 to 50, and 20 to 21% less, respectively, than plants supplied with 8 millimolar NH(4)NO(3). The water stress treatment produced less than a 50% decrease in CER regardless of plant N source, a 90% inhibition of AR in plants dependent on N(2), and a 41% decline in dry matter production on both N sources. Water stress decreased reduced N accumulation 55% in N(2)-dependent plants and 50% in NH(4)NO(3)-dependent plants. Changes in growth and N accumulation caused a 10 to 11% decrease in total plant N concentration of water-stressed plants compared to adequately irrigated controls, but water stress decreased the N concentration of tissue synthesized during the 18-day treatment period in N(2)-grown plants more than in plants supplied 8 millimolar NH(4)NO(3). Thus, the relative effect of water stress on growth under the two N regimes was similar, but N accumulation by N(2)-dependent clover was inhibited to a slightly greater extent (P </= 0.001) than in NH(4)NO(3)-dependent plants.