Photosynthetic limitation by CO2 diffusion in drought stressed orange leaves on three rootstocks.

Photosynthetic limitations under moderate water deficit were evaluated in 'Valência' orange trees grafted on three different rootstocks, in pots. Net CO(2) assimilation rate (A (N)), stomatal conductance (g (s)), and photosystem II (PS II) operating efficiency (F'(q)/F'(m)) in response to changing intercellular CO(2 ) partial pressure (C (i)) were analyzed under controlled conditions. Drought decreased A (N) and g (s), whereas F'(q)/F'(m) remained unchanged. This resulted in a higher ratio between electron transport rate (ETR) and gross CO(2) assimilation rate (A (G)). Since the comparison of A (N)-C (i) gas exchange curves can lead to incorrect conclusions, a normalization of C (i) values (C(i)(f)) of stressed leaves was applied. Then, the relationship established for irrigated trees between the ETR/A (G) ratio and C (i) was used to estimate the C(i)(f) from ETR/A (G) ratios measured under water stress. The response of A (N) to C(i)(f) suggests that the CO(2) diffusional restriction is the main factor that limits photosynthesis in orange leaves under moderate water deficit.