Water Stress Effects on Nitrogen Assimilation and Growth of Trifolium subterraneum L. Using Dinitrogen or Ammonium Nitrate 1

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The relative effects of water stress on growth parameters of subterranean clover (Trifolium subterraneum L. cv. Woogenellup) dependent on either N2 or 8 millimolar NH4NO3 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 N2 were 25 to 26, 45 to 50, and 20 to 21% less, respectively, than plants supplied with 8 millimolar NH4NO3. 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 N2, and a 41% decline in dry matter production on both N sources. Water stress decreased reduced N accumulation 55% in N2-dependent plants and 50% in NH4NO3-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 N2-grown plants more than in plants supplied 8 millimolar NH4NO3. Thus, the relative effect of water stress on growth under the two N regimes was similar, but N accumulation by N2-dependent clover was inhibited to a slightly greater extent (P ≤ 0.001) than in NH4NO3-dependent plants.

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