The effect of CO2 and drought on stomatal conductance and stomata density in white clover (Trifolium repens L.)

Abstract

The increase of CO2 in the atmosphere decreases stomatal (leaf) conductance, followed by a reduction in evapotranspiration. This can improve both water-use efficiency and soil-water status, but may affect the local climate. Trifolium repens L., an element of most European grasslands, was chosen to study changes in stomatal conductance. A) At elevated CO2 levels (600 µmol.mol-1), a 34% reduction in stomatal conductance was found to be independent of the availability of nitrogen. Conversely, the decrease in stomata density on the upper and lower leaf surfaces reached only ¿8% and ¿7%, respectively. B) At natural CO2 springs (Slovenia), clover plants were exposed to extremely high [CO2], fluctuating up to 27 000 µmol.mol-1. Even though clover often responded by showing a strong reduction in leaf conductance, no complete stomatal closure was observed, whereas changes in [CO2] modified conductance within minutes. Experiments with potted plants also showed only partial stomata closure, with 29% on the abaxial and 38% on the adaxial side (1 hour, 25.000 µmol.mol-1 full sun light). C) Under controlled, stress-free conditions, stolon leaves of different age showed only a negligible variability in conductance. Conversely, when temporarily exposed to drought or water-logged soils, porometer data of the same stolon revealed that older leaves tolerated these stresses much better than younger ones. This was predominately expressed in speed of recovery.