DETERMINATION OF THE WATER-USE OF 2 PAIRS OF SOYBEAN ISOLINES DIFFERING IN STOMATAL FREQUENCY USING A HEAT-BALANCE STEM-FLOW GAUGE

Using heat-balance stem flow gauges, we were able to measure directly and continuously the sap flow rates in two pairs of soybean [Glycine m ar (L.) Merr.] isolines differing in stomatal frequency. Plants with h igh stomatal frequency transpired significantly more water than the lo w stomatal frequency plants at high soil moisture levels. Under low so il moisture levels, the water use rate decreased greatly for the high stomatal frequency plants. Plants with low stomatal frequency were abl e to maintain greater sap flow rates than those with high stomatal fre quency. Higher leaf temperatures associated with the low stomatal freq uency plants were likely due to lower transpiration rates which reduce d evaporative cooling especially under well-watered conditions.