Soybean Transpiration Rate Under Nitrogen Stress and Recovery.

This study investigated the effects of nitrogen stress and restoration of nitrogen on transpiration rate in soybean plants and on possible adjustments in their response to increasing vapor pressure deficit (VPD). Two soybean genotypes [Glycine max (L.) Merr.] were studied - PI 471938 and PI 416937. Both are being used as key germplasm components in soybean breeding programs. Past experiments have indicated that PI 416937 has the ability to control transpiration at high VPD, while PI 471938 does not. Non-nodulated soybean plants were grown in sand culture in a controlled environment at a temperature of 30C for a 14 day establishment period and then for four weeks when treatments were imposed. A complete nutrient solution with 2.0 mM N was supplied during establishment and to control plants throughout. A set of plants was deprived of N for the initial 15 days of treatment and then resupplied with 2.0 mM N during a 10 day recovery. Plant transpiration was continuously measured using balances with 0.01-g precision. Periodically during the treatment period, plants were exposed to a range of VPD conditions to determine their ability to control transpiration as VPD increased. The results indicate that in nitrogen-stressed plants, leaf expansion and transpiration per unit leaf area were markedly inhibited in both genotypes. Under all conditions, PI 416937 maintained the ability to limit transpiration at high VPD, with a breakpoint for lower transpiration remaining at about 2-2.5 kPa. The PI 417938 also had lower transpiration at high VPD in + and –N treatments, but the breakpoint did not occur until about 3.0 to 3.5 kPa. Furthermore, during recovery, transpiration of PI 471938 responded linearly to increasing VPD without any indication of control. The two PI lines, display somewhat different traits that might contribute to drought tolerant varieties.