The diurnal variation of photosynthesis in leaves of Rhododendron fortunei

Diurnal variations of photosynthesis and response to light in the leaves of Rhododendron fortunei were studied under natural conditions. The results indicated that in the typical summer sunny days, the curve of diurnal variation of net photosynthetic rate (Pn) in the sunnyleaves was dual-peaked with obvious midday depression of the photosynthetic efficiency. The diurnal variation of internal CO 2 concentration (C i) was just opposite to Pn. The increase of C i at midday indicated that the midday depression of the photosynthetic efficiency was mainly influenced by the non-stomatal limited elements. The respiration rate (R) was the highest at midday with single-peaked curves of diurnal variation. There was small variation of Pn in the shadeleaves at the most time in a day. The response to light at different time in a day showed that the saturated light intensity and apparent quantum yield (AQY) were higher in the morning and decreased in the afternoon while the light compensation points increased. No matter in the morning or in the afternoon, the variations of these photosynthetic parameters in the shadeleaves were slight. These results indicated that the main reason of the midday depression of the photosynthetic efficiency in the sunnyleaves of Rhododendron fortunei was the photoinhibition caused by strong light. The analysis of stepwise multi-regression and path coefficient showed that the photosynthetically active radiation (PAR) and ambient temperature (T a) were the main direct factors which affected the diurnal variation of Pn.