DYNAMIC-RESPONSE OF STOMATA TO CHANGING IRRADIANCE IN LOBLOLLY-PINE (PINUS-TAEDA L)

Dynamic changes in stomatal conductance and the rate of photosynthesis were measured as periods of shading (decrease in irradiance from 800 to 200 mu mol m(-2) s(-1)) between 5 and 60 min were imposed on needle s of Pinus taeda L. trees under laboratory conditions. Shading induced a 39% decrease in stomatal conductance but the rate of change was slo w. Average time constants (+/- standard error) were shorter for the de crease in stomatal conductance when shading was imposed for 30 min (14 .8 +/- 1.3 min) than for the increase in stomatal conductance when sha ding was removed (25.5 +/- 3.4 min). The time constants for increasing stomatal conductance when shading was removed were linearly related t o the length of the previous dark period. The rate of photosynthesis f ell immediately by 58% when shading was imposed and increased more rap idly than the change in stomatal conductance when shading was removed. The increase in photosynthesis during the induction phase after shadi ng was removed was limited by both stomatal and biochemical effects. T he long time constants for stomatal response contributed to the poor c orrelations between stomatal conductance and instantaneous measurement s of irradiance from field data. However, the slow response of stomata l conductance to changes in irradiance had little effect on total dail y transpiration, carbon gain and water-use efficiency.