Contribution of diffusional and non-diffusional limitations to midday depression of photosynthesis in Arbutus unedo L.
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- Grassi, G., Ripullone, F., Borghetti, M. et al. Trees (2009) 23: 1149. doi:10.1007/s00468-009-0355-7
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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 [DL = stomatal (SL) + mesophyll (MCL)] and non-diffusional (carboxylation capacity and electron transport, BL) limitations. Light-saturated assimilation at ambient CO2 (Amax), stomatal conductance to water vapour (gsw) and maximum carboxylation rate (VcmaxCi) 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, DL was similar to BL (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.