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

Abstract

P. juliflora trees produce leaves during two growth periods. The first cohort of leaves is produced during spring in cool conditions, while the second cohort is produced during monsoon under warm conditions. I studied photosynthetic characteristics of young, mature, and old leaves of the previous season (monsoon) in the spring season. Maximum net photosynthetic rate of a young leaf was lower than that of the mature and old leaves. The total CO2 fixed 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 transpiration rate and water use efficiency (WUE) were similar in the mature and old leaves, while they were much lower in the young leaves. Dark respiration 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 young leaves. The diurnal fluorescence characteristics (ΔF/Fm, qp, and qN) of the young, mature, and old leaves showed that photochemical efficiency of photosystem 2 during midday decreased more in the young and old leaves than in the mature ones. However, the fluorescence characteristics showed that in all the three leaf types there was complete recovery of the photochemical efficiency at sunset from the midday depression. Fv/Fm in the young and mature leaves also confirmed this. Hence the young and old leaves were photosynthetically less efficient than mature leaves, but they were well adapted to withstand the harsh environmental conditions.