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Physiological Responses of Field-Grown Soybean Leaves to Increased Reproductive Load Induced by Elevated Night Temperatures¹

Most studies involving the effect of assimilate demand on carbon dioxide exchange rate (CER) have investigated the short-term effects by extreme experimental manipulation of plants. This study investigated the long-term effect of increased assimilate demand on CER of field-grown soybeans without abrupt alteration of plant structure. Soybeans [Glycine max (L.) Merr. ‘S09-90’] were grown in the field at Oregon State University in 1981 and 1982 to assess the effect of increased assimilate demand on CER during reproductive development. Increased assimilate demand was brought about by elevating the night temperature from 2 weeks after crop emergence until physiological maturity. Mean minimum night temperature treatments included check (uncontrolled, ca. 10°C), 16, and 24°C. Single leaf CER, stomatal conductance (g₅), and transpiration rates (Tr) were measured on three dates in each year. These dates corresponded to the time of flowering, pod formation, and seed filling. In 1981, leaf water potential (), leaf osmotic potential (_J), and leaf turgor potential (_p) were also measured on the corresponding dates. Leaf starch content was also measured on selected dates in 1981. The 16x00B0;C treatment increased the seed growth without much effect on the leaf area as compared to the check, whereas the 24°C treatment increased seed growth and also reduced the leaf area. The effect of the treatments on the source/sink ratio became more apparent as plants proceeded in reproductive development. The CER of the 16°C plants increased significantly only on the second date in 1982 when mean single leaf CER increased by 18% over the check plants. The g₅ and Tr of these plants increased over the check plants by 33 and 26%, respectively, on that date. The CER of the 24°C plants increased over the check plants by 15 and 20% in 1981 and 22 and 90% in 1982 on the second and third dates, respectively. The g₅ of the 24°C plants increased over the check plants on these respective dates by 19 and 34% in 1981 and 37 and 93% in 1982. Compared to the check, Tr of the 24°C plants also increased by 15 and 31% in 1981 and 21 and 43% in 1982 on the second and third dates, respectively. The data for 1981 indicated that leaf water potential () generally decreased as night temperature increased. Although the starch content in the leaves of plants with warmer nights was significantly lower than in the check plants during the morning hours of the last two dates in 1981, the starch content in the leaves of all plants was believed to be too low to account for differences in CER among the treatments. The data indicated that elevated night temperatures increased CER of soybean plants during the pod formation and seed filling periods. Increased CER for plants with warmer nights was, at least in part, due to higher assimilate demand in these plants compared to the check. Variations in g₅, Tr, , and leaf starch content among the treatments appeared to be the effect rather than the cause of differences in CER among the treatments.

Key Words: Source-sink ratios • Carbon dioxide exchange rates • Stomatal conductance • Mesophyll resistance • Transpiration rate • Leaf water status • Leaf starch content • Feedback inhibition • Thermoperiodicity • Glycine max (L.)

² Former graduate research assistant, currently post doctoral research associate, and associate professor of crop science, Dep. of Crop Sci., Oregon State Univ., Corvallis, OR 97331.

Received for publication July 5, 1983.