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Water Stress Recovery in Soybeans as Affected by Photoperiod During Seed Development¹

Since long-day photoperiods during seed filling can reduce reproductive sink demand of soybeans [Glycine max (L.) Merr.], a study was conducted to determine whether long-day treatment during seed filling would also enhance the capacity of the plants to recover from a temporary drought stress. Plants of ‘Ransom’ and ‘D72-8126’, a high protein line, were grown under long or short-day photoperiods and subjected to a temporary drought stress during seed-filling stage. Carbon dioxide exchange rates and leaf water potentials were measured during and immediately after the stress treatment. Dry matter distribution and leaf area also were measured throughout the reproductive growth period. Water-stressed Ransom and D72-8126 plants grown under long days maintained 45 and 25% of their maximum leaf areas, respectively, vs. 16 and 0% under short-day conditions. Carbon exchange rates during the period of water deficit were comparable between plants exposed to long and short days, but recovery was 73 vs 38% for Ransom plants grown under long and short days, respectively, and 76 and 0% for D72-8126 plants grown under long and short days, respectively. In addition, dry matter assimilation efficiency (mg dry matter cm^-2 day^-1) showed no recovery for plants of either genotype under short days, while recovering approximately 77% for Ransom and 100% for D72-8126 under long days. For Ransom under short days, water stress shortened the seed-filling period by 9 days and reduced yield by 33%; but under long days, the duration of seed filling was unaffected by the stress and yield was reduced 19%. for D72-8126, the water deficit occurred late in the seed-filling period, and no photoperiod differences were observed in the effect of the stress on seed-filling duration (2 days abbreviation for each treatment) or on yield (a 19 and 17% reduction under short days and long days, respectively). These results demonstrated that drought sensitivity during reproductive growth of soybeans may be reduced, and yield losses may be minimized, through manipulation of photoperiodic responses.

Key Words: Glycine max (L.) Merr. • Source/sink relationships • Seed yield • Photosynthesis

² Former graduate research assistant, Dep. of Crop Science; professor, Dep. of Soil Science; professor, Dep. of Crop Science; professor, Dep. of Soil Science, North Carolina State University, Raleigh, NC, 27650.

Received for publication November 16, 1981.