Loss of stomatal function in ageing hybrid poplar leaves

Under a variety of conditions old, non-senescent hybrid poplar (Populus sp.) leaves exhibited less stomatal control than young, mature leaves. Stomata of older leaves displayed oscillatory behaviour more frequently, and oscillations were more random, than in younger leaves. Also, diffusive conductance of older leaves changed less following sudden shifts from either dark to light, or vice versa, than in younger leaves, and temporal patterns of diffusive conductance in older leaves appeared to be relatively independent of microenvironmental conditions. Levels of conductance of older leaves were higher both in the dark and following excision than in younger leaves, while in the light the situation was reversed. Total range of response and stability of diffusive conductance were also lower in older rather than in younger leaves. All of the observed age-related differences suggest a loss of stomatal control with increasing leaf age.