© 1995 Heron Publishing—Victoria, Canada
Dynamic response of stomata to changing irradiance in loblolly pine (Pinus taeda L.)
David Whitehead (1) and Robert O. Teskey (2)
1. Manaaki Whenua Landcare Research, P.O. Box 31-011, Christchurch, New Zealand / 2. Warnell School of Forest Resources, University of Georgia, Athens, GA 30602, USA / Received June 23, 1994
Summary
Dynamic changes in stomatal conductance and the rate of photosynthesis were measured as periods of shading (decrease in irradiance
from 800 to 200 µmol m–2 s–1) between 5 and 60 min were imposed on needles of Pinus taeda L. trees under laboratory conditions. Shading induced a 39% decrease in stomatal conductance but the rate of change was slow.
Average time constants (± standard error) were shorter for the decrease in stomatal conductance when shading was imposed for
30 min (14.8 ± 1.3 min) than for the increase in stomatal conductance when shading was removed (25.5 ± 3.4 min). The time
constants for increasing stomatal conductance when shading was removed were linearly related to the length of the previous
dark period.
The rate of photosynthesis fell immediately by 58% when shading was imposed and increased more rapidly than the change in
stomatal conductance when shading was removed. The increase in photosynthesis during the induction phase after shading was
removed was limited by both stomatal and biochemical effects.
The long time constants for stomatal response contributed to the poor correlations between stomatal conductance and instantaneous
measurements of irradiance from field data. However, the slow response of stomatal conductance to changes in irradiance had
little effect on total daily transpiration, carbon gain and water-use efficiency.
Keywords:
photosynthesis, shading, stomatal conductance, time constant.
