Photosynthesis Under Field Conditions. XI. Soil-Plant-Water Relations During Drought Stress in Corn

The variation of leaf water potential at different heights in a corn crop canopy (Zea mays) and with time during a period of increasing drought stress were examlned along with soil moisture tension and evapotranspiration. These measurements plus growth measurements permitted an evaluation of the soil-plant-water transport system and its effects on plant growth during drought. On clear summer days the upper leaves of the corn crop had higher water potentials with a bimodal tendency in contrast to the lower leaves which had lower and almost constant water potentials. On high stress days the magnitude of the maximum afternoon leaf water potential increased by an amount equal to the increase in soil moisture tension. On days of comparable evaporative demand, the gradient of water potential from the upper leaves to the soil increased regularly with an increase in soll moisture tension in the root zone. Since total transpiration was not bimodal even though upper leaves exhibited bimodal variations of water potential, it is concluded that if midday stomatal closure of the upper leaves occurred, it apparently was ineffectual or compensating transpiration must have been carried on by lower leaves. A 2-week drought period was examined in detail. Computation of the “effective conductance” of liquid water in the plant was compared with the moisture conductivity of the soil. This permitted a calculation of the “effective root length” where the gradient of water potential from plant to soil was assumed to represent the gradient from the root surface to the ambient untapped soil moisture. This calculated length was small by comparison to those calculated by Slatyer (1957), but the computational error is large. Transpiration was not detectably changed by the decrease in effective plant conductance and soil conductivity, so it is. concluded that the plant was still able to keep pace by maintaining sufficient potential gradient to transport the needed amount of water. The growth processes, however, were slowed down with increasing leaf water potential and decreasing turgor of the tissues. This slowdown was evident even at the relatively low soil moisture tension of less than one bar and a leaf water potential, of less than 3.5 bars.