Diffusional conductance to CO2 is the key limitation to photosynthesis in salt-stressed leaves of rice (Oryza sativa)

Salinity significantly limits leaf photosynthesis but the photosynthetic limiting factors in salt- stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl (0 and 150 m M ) concentration to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate ( A ) in salt stressed leaves. Our results indicated that salinity led to a decrease in A , leaf osmotic potential, electron transport rate and CO 2 concentrations in the chloroplasts ( C c ) of rice leaves. Decreased A in salt-stressed leaves was mainly attributable to low C c , which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity-tolerant rice cultivars. Abbreviations A photosynthetic rate C c , CO 2 concentration at carboxylation sites CE apparent Rubisco activity Chl total chlorophyll content C i intercellular CO 2 concentration ETR electron transport rate F 0 initial fluorescence of photosystem II in darkness F m maximum fluorescence of photosystem II F v maximum variable fluorescence of photosystem II F v / F m maximum quantum efficiency of photosystem II g m mesophyll conduction g s stomatal conduction J max maximum electron transport rate K leaf K content LMA leaf mass per area N leaf N content P leaf P content OP osmotic potential Protein leaf total soluble protein content qN non-chemical quenching efficiency R d day respiration R dark dark respiration Rubisco Rubisco content V cmax maximum carboxylation rate α leaf light absorptance efficiency β the distribution of electrons between PSI and PSII Γ * CO 2 compensation point in the absence of respiration Φ PSII quantum efficiency of photosystem II.