Dry Matter Production and Translocation in Maize Subjected to Drought during Grain Fill 1

Because dry conditions most frequently occur during the second half of the growing season, the yield loss to be expected and the physiological mechanisms that cause the loss need to be understood in grain crops. In order to study this problem, the objective of this research was to withhold water from field‐grown maize ( Zea mays L.) during the grain filling stage to determine the relative effects of drought on photosynthesis and translocation. Located in bordered field plots, plants were grown in large pots arranged in 75‐cm row widths at a density of 40,770 plants/ha. Drought imposed 10 days after pollination caused leaf water potentials to range between −15 and −19 bars during the day for the rest of the growing season. In control plants, daytime leaf water potential remained between −3 and −16 bars. Soon after drought was imposed, leaf area began to decline and grain yield was ultimately reduced to 42% of the control. The drought increased grain protein concentration but decreased oil concentration. Analysis of dry matter accumulation showed that, as grain development progressed, the rate of grain fill began to exceed the rate of dry matter accumulation, indicating a net redistribution of stored assimilates. In the desiccated plants, the redistribution continued after the cessation of net plant dry matter production and occurred earlier and to a larger extent than in controls. A 14 C translocation study confirmed that grain fill drew on stored photosynthate to a greater extent in desiccated plants than in controls. Because of this redistribution of photosynthate, grain yield was related to the total dry matter accumulated by the plants during the growing season. It should be emphasized that the experiment was designed to assure complete floral development and fertilization and, therefore, the sink size for photosynthate was the same in the desiccated and control plants. To measure translocation during the drought, the second leaf above the ear was exposed to 14 C 7 and 21 days after water was withheld. The amount of 14 C translocated was inhibited by the dry conditions. On the first exposure date, desiccated plants retained a higher percentage of the 14 C in the exposed leaf and translocated a lower percentage of the 14 C to the grain than on the second exposure date. On the second exposure date, the proportion of 14 C in the grain was not significantly different from that in the control plants. This continued translocation, despite a lack of plant dry matter accumulation, indicates that photosynthesis was more inhibited than translocation during the dry conditions. The differences in 14 C translocation at different times suggest that interpretations of long term drought effects on translocation should be based on extended studies of dry matter or 14 C movement rather than short term movement of 14 C alone.