Photosynthetic life span of attached poplar leaves under favorable controlled environmental conditions.

Net photosynthetic rate declined to negligible levels by 65 days after full leaf expansion, whether leaves were green, green with yellow spots, or undergoing general yellowing. Stomatal resistance to CO₂ increased after full expansion to only moderate levels over the same period. Mesophyll resistance to CO₂ changed oppositely and proportionately to net photosynthetic rate (Ps) to levels extremely limiting to Ps. Internal leaf CO₂ concentration increased near the end of the monitoring period. Stomatal control, chlorophyll degradation, and loss of photosynthetic capacity may be independent processes in aging poplar leaves. Green poplar leaves are not necessarily capable of CO₂ fixation late in their life span. Caution is advisable when using yellowing as a criterion of leaf senescence.-from Author