This study aimed to follow the seasonal changes in the photosynthetic performance of two Ceratonia siliqua
cultivars – Mulata and Mulata do Espargal – growing in a non-irrigated orchard in Algarve (South Portugal).
Measurements and sampling (leaf water potential, leaf gas exchange, pigments and dark-adapted chlorophyll
fluorescence) were taken in days with similar photon flux density, during the dry season in June and July, and in
November after the first autumnal rainfalls (»91 mm) when minimum temperatures droped below 10°C.
Photosynthetic rates remained low for both cvs, and did not recover in November despite the observed increase in
stomatal conductance and y. Fv/Fm slowly decreased from June to November, specially at midday. This changes in
Fv/Fm were attributed mainly to increasing F0 which might reflect structural changes or damages at the chloroplast
membranes level. Dry season conditions seemed to have triggered some thermal photoprotective mechanisms, as
indicated through the increased ratio A+Z/VAZ. In November, however, the ratio A+Z/VAZ decreased. We suggest
that the lack of recovery of A associated with higher F0 measured in both cvs, either at pre-dawn or midday, could be
attributed to structural changes within the membranes mediated either by rehydration and/or by chilling
temperatures
