Leaf photosynthesis, dark respiration and fluorescence as influenced by leaf age in an evergreen tree, Prosopis juliflora

P. juliflora trees produce leaves during two growth periods. The first coho rt of leaves is produced during spring in cool conditions, while the second cohort is produced during monsoon under warm conditions. I studied photosy nthetic characteristics of young, mature, and old leaves of the previous se ason (monsoon) in the spring season. Maximum net photosynthetic rate of a y oung leaf was lower than that of the mature and old leaves. The total CO2 f ixed per day by the young leaves was just 36 % of that in the mature leaves while the old leaves fixed 76 % of that of the mature leaf. The total tran spiration rate and water use efficiency (WUE) were similar in the mature an d old leaves, while they were much lower in the young leaves. Dark respirat ion rate was maximal in the young leaves as compared to the mature and old leaves. About 92 % of the total CO2 fixed per day were respired by the youn g leaves. The diurnal fluorescence characteristics (DeltaF/F-m, q(p), and q (N)) of the young, mature, and old leaves showed that photochemical efficie ncy of photosystem 2 during midday decreased more in the young and old leav es than in the mature ones. However, the fluorescence characteristics showe d that in all the three leaf types there was complete recovery of the photo chemical efficiency at sunset from the midday depression. F-v/F-m in the yo ung and mature leaves also confirmed this. Hence the young and old leaves w ere photosynthetically less efficient than mature leaves, but they were wel l adapted to withstand the harsh environmental conditions.