Stomatal function, density and pattern, and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1-1 mutants.

Stomata modulate the exchange of water and CO2 between plant and atmosphere. Although stomatal density is known to affect CO2 diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO2 assimilation is not fully understood. We used wild types Col-0 and C24 and stomatal mutants sdd1-1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO2 assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed. Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1-1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis. Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO2 transport.

Mesophyll conductance to CO(2) transport estimated by two independent methods: effect of variable CO(2) concentration and abscisic acid.

Mesophyll conductance (g(m)) and stomatal conductance (g(s)) are two crucial components of the diffusive limitation of photosynthesis. Variation of g(m) in response to CO(2) concentration was evaluated by using two independent methods based on measurements of variable electron transport rate (J) and instantaneous carbon isotope discrimination, respectively. Both methods of g(m) estimation showed a very similar shape of the g(m)/C(i) relationship, with an initial increase at low substomatal CO(2) concentrations (C(i)), a peak at 180-200 micromol mol(-1) C(i), and a subsequent decrease at higher C(i). A good correlation was observed between values of g(m) estimated from the two methods, except when C(i) <200 micromol mol(-1), suggesting that the initial increase of g(m) at low C(i) was probably due to unreliable estimates over that range of C(i). Plants were also treated with abscisic acid (ABA), which induced a reduction in g(s) without significantly affecting the rate of photosynthesis, g(m) or the photosynthetic capacity. The present results confirm, using two independent methods, that g(m) is strongly sensitive to C(i), and that the relationship between g(s) and g(m) is not conservative, differing between control and ABA-treated plants.