Contribution of diffusional and non-diffusional limitations to midday depression of photosynthesis in Arbutus unedo L.

It is still unknown whether the midday depression of photosynthesis under severe water stress, frequently observed in plants growing in a Mediterranean-type climate, is primarily a consequence of diffusional or non-diffusional limitations. We carried out combined measurements of gas exchanges and chlorophyll fluorescence in field-grown Arbutus unedo L. trees during late spring and mid summer, and a quantitative limitation analysis was performed to distinguish between the different limitations to photosynthesis, i.e., diffusional D _L = stomatal (S _L) + mesophyll (MC_L)] and non-diffusional (carboxylation capacity and electron transport, B _L) limitations. Light-saturated assimilation at ambient CO₂ (A _max), stomatal conductance to water vapour (g _sw) and maximum carboxylation rate (V _cmax C _i) showed a marked midday depression during both periods. The total limitations tended to increase during the day and were remarkably similar in June and July (50 and 48%, respectively); on a daily basis, D _L was similar to B _L (about 23%) in June; whereas, in July the former was predominant (38 and 4%, respectively). We concluded that the midday depression in photosynthesis was largely caused by diffusional limitations, with non-diffusional limitations playing a smaller role. Although stomatal closure was the main diffusional limitation, the decline in mesophyll conductance was not negligible during the hottest and driest period.