Changes in photosynthetic performance of Ceratonia siliqua in summer

In carob tree (Ceratonia siliqua) radiant energy saturated net photosynthet ic rate (P-N) during summer was about 10 % of the spring values. This was a ccompanied by a reduction in stomatal conductance (g(s)), which only partia lly explains the strong reduction in P-N. Photosynthetic capacity (P-max) a nd quantum yield (Phi), both measured under saturating CO2, had the maximum in spring (about 34 mu mol m(-2) s(-1) and 0.08 mol mol(-1), respectively) and both decreased in late summer to about 55 % of their spring values. De spite strong decreases in Phi, photoinhibition of photosystem 2 (PS2) was n egligible or easily reversible in carob leaves subjected to summer drought, since F-v/F-m, measured in the morning, did not show appreciable changes. The recovery of affected parameters was very rapid after the first rains in late October. The chlorophyll (Chl) a/b ratio in the end of the summer was 2.6, a value significantly lower than 3.6 obtained in the spring, suggesti ng that Chl a was preferentially reduced.