Stomata on the abaxial and adaxial leaf surfaces contribute differently to leaf gas exchange and photosynthesis in wheat

Abstract

Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces.Using a combination of differential illuminations and CO 2 concentrations at each leaf surface, we found that mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than those associated with the abaxial leaf surface, which is supported by an increased stomatal conductance (driven by differences in stomatal density).When vertical gas flux at the abaxial leaf surface was blocked, no compensation by adaxial stomata was observed, suggesting each surface operates independently. Similar stomatal kinetics suggested some co-ordination between the two surfaces, but factors other than light intensity played a role in these responses.Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial stomatal conductance, which would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed c. 50% to leaf photosynthesis and therefore represents an important contributor to overall leaf gas exchange.

The impact of growth at elevated [CO2] on stomatal anatomy and behavior differs between wheat species and cultivars

“…Similar to the patterns described for SD, there was a tendency for reduction in g smax with e[CO 2 ] driven mostly by adaxial g smax . However, interestingly, not all of the changes in SD translated into changes in g smax , strongly indicating a role for changes in GCL with e[CO 2 ] to compensate for changes in density, maintaining a similar g smax ( Lawson and Morison, 2004 ; Harrison et al , 2020 ; Wall et al , 2022 ); for example, the diploid and tetraploid species have similar g smax to that of the hexaploid species even though SD is much higher in the diploid species.…”

Longer epidermal cells underlie a quantitative source of variation in wheat flag leaf size

“…Sinclair & Sheeny, 1999 ). Indeed, wheat has palisade mesophyll cells on both the upper (adaxial) and lower (abaxial) leaf surfaces, and has similar densities of stomata on both surfaces (Wall et al ., 2022 ). As erect flag leaves are more likely to receive direct light to either of the leaf surfaces, the cellular characteristics of wheat should help maintain high photosynthetic carbon assimilation irrespective of which leaf surface faces the sun at any given point in the day.…”

Phenotypic evolution of agricultural crops

“…Escaping colimitation has proven difficult and could constrain the evolution of higher CER in crops (Flexas & Carriquí, 2020). This hypothesis is supported by the fact that crops have not increased CER after domestication, despite a more even distribution of stomata between both leaf sides (Milla et al, 2013), which should boost stomatal and mesophyll conductance, and therefore CER (Wall et al, 2022;Xiong & Flexas, 2020). The fact that CER has not increased consistently after domestication supports either a colimitation scenario (Gago et al, 2019), or saturation of effective stomatal conductance (Mott et al, 1982).…”