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 CO₂ assimilation rate (A N), stomatal conductance (g s), and photosystem II (PS II) operating efficiency ( [graphic removed] ) in response to changing intercellular CO₂ partial pressure (C i) were analyzed under controlled conditions. Drought decreased A N and g s, whereas [graphic removed] remained unchanged. This resulted in a higher ratio between electron transport rate (ETR) and gross CO₂ 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 ( [graphic removed] ) 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 [graphic removed] from ETR/A G ratios measured under water stress. The response of A N to [graphic removed] suggests that the CO₂ diffusional restriction is the main factor that limits photosynthesis in orange leaves under moderate water deficit.