Changes in the Photosynthetic Activity During Aging in Different Parts of Intact Tobacco Leaves

Abstract

The photosynthetic activity in the different portions of leaf blade and in both upper and lower epidermal surfaces of intact tobacco leaves (Nicotiana tabacum “Bright Yellow”) was measured at various stages of leaf growth, using an infrared carbon dioxide analyzer.
The photosynthetic activity and the growth rate of apical, middle and basal portions of the leaf varied markedly with the stage of leaf development. The activity increased with the proceedings of leaf age and reached a maximum after
the stage of full expansion of respective portions. Therefore, in early expanding stage of the leaf it was difficult to point out which portion of a leaf might have the highest activity. After the termination of the expansion of the basal portion, however, the photosynthetic activity of individual portions had a steady tendency, that the activity was high in the basal, low in the apical, and medium in the middle portion.
The photosynthetic activity on the upper epidermal surface was usually low in older leaves, but the activity rised temporarily soon after the turning up of light and then fell down rapidly.
On the lower epidermal surface, the saturation level of the photosynthesis-light intensity curve shifted to lower with the proceedings of leaf age, but no detectable change of the level was observed with the upper epidermal surface. Both the saturated light intensity and the amount of fixed CO2 per unit leaf area were nearly independent of the direction of illumination, from above or below.