Comparative photosynthetic and photochemical responses to cold in young and mature leaves of evergreen species Q. ilex, P. angustifolia and L. nobilis.

During winter, plants that retain green leaves are often exposed to a combination of low temperatures and high irradiance which can increase the sensitivity of photosystems to photoinhibition, restricting the light energy utilization. In the present work the photosynthetic and photochemical responses to cold were evaluated in young and mature leaves of evergreen species Q. ilex, P. angustifolia and L. nobilis. For this purpose, simultaneous measurements of gas exchange and chlorophyll a fluorescence were performed on attached leaves of three species in winter season at saturating irradiance. Compared with the young leaves, the mature leaves of all species showed an higher photosynthetic activity and apparent carboxylation efficiency as well as a greater thermal energy dissipation and maximum PSII photochemical efficiency. Conversely, the young leaves displayed a significant increase of photochemical dissipation processes other than CO2 fixation. No difference between two leaf populations in quantum yield of electron transport rate was detected. These results indicate that under saturating irradiance and chilling conditions mature leaves of all species, through the higher CO2 assimilation rate and thermal dissipation increase, cope more efficiently with the excess of absorbed light resulting in less sensitivity to photoinhibition. Among species both young and mature leaves of Q. ilex exhibited the highest photosynthetic performance indicating a better resistance to low temperatures.