Investigations were carried out in July on leaves of lilac variously situated in the head on the crown. Net photosynthesis was registered from 6 hr to 20 hr. The insolated leaves in the top part of the crown showed maximum photosynthesis between 8 hr and 11 hr, afterwards the intensity of photosynthesis decreasing in spite of the high intensity of irradiation in the midday hours. The insolated leaves located in the lower part of the crown showed maximum photosynthesis at the time of the greatest incident irradiation of the leaf. Shaded leaves, inside the crown, demonstrated the highest net photosynthesis between 8 hr and 15 hr. This amounted to about 20% of the maximum photosynthesis of insolated leaves.
Starch was present in the leaves both during the day and night. Before sunrise starch represented 6% of the dry weight of insolated leaves and 3% of the shaded leaves. In the insolated leaves since sunrise its content quickly increased up to 13-14% and remained at a slightly decreasing level till evening hours. Sunset was followed by a rapid decrease in the starch content in the leaves. In the shaded leaves, throughout day, the starch content remained at a constant level (3%).
Intensive photosynthesis of the lilac leaves causes a great accumulation of starch in the insolated leaves during the day. It is the main cause of reduction in the intensity of photosynthesis.