Gas exchange in senescing leaves of Olea europaea L.

From the beginning of olive leaf yellowing to leaf fall (1 divided by 3 mon ths), there was a general trend from anabolism to catabolism. Rates of net photosynthesis (P-N) and respiration, areal dry mass, and contents of pigme nts, particularly of chlorophyll (Chl) a, starch, and above all nitrogen (N ) decreased. The detachment force decreased dramatically only in completely chlorotic leaves. Chl a. b ratio only declined in the last 10-20 d of sene scence, when the total Chl contents diminished by about 70 %, after which t he N content, P-N, and efficiency of the photochemical energy conversion of the remaining Chl and N dramatically declined. Consequently, for most of t he natural course of senescence P-N remained relatively high. The reduction in P-N was associated with the decreases in transpiration rate (E) and sto matal conductance (g(s)), but these probably did not cause the decline of P -N The recycling of saccharide compounds was low, while 50 % of the total N on a leaf area basis was relocated back before leaf abscission, changing t he leaf from a carbon source to a mineral source. Therefore, considering th at senescing leaves in olive trees contribute to carbon gain and allow the recycling of resources, it is essential to prevent the premature leaf absci ssion by avoiding deficits of water and mineral nutrients and by using prun ing and training systems that allow good irradiation of all leaves in the c rown.