It has been found that under solar irradiation the temperature of the insolated side of stems may be up to 12°C higher than that of the air. The temperature of the shaded side of the stems is 2 - 3 °C lower than that of the insolated side. The temperature of the leaves exposed to the sun may be up to 8°C higher than the open air temperature, while that of the shaded leaves is up to 5°C lower.
Investigations on the effect of temperature on the photochemical activity of chloroplasts have shown that in April and June the optimal temperature for photochemical activity of chloroplasts isolated from bark falls within the range 25-35 °C, and in November - in the range 25-30°C. The optimal temperature for the activity of chloroplasts isolated from leaves is 35°C.
Solar irradiation, when warming the stems and leaves also increases the activity of chloroplasts. Only on hot summer days at noon is the temperature of the insolated bark higher than the optimal one, resulting in a decrease in chloroplast activity.