Photosynthesis evolved early in the history of life (Blackenship, 2010), and despite the ubiquity and importance of biological carbon fixation, the process is still far from optimal. The majority of the world’s plant species perform C3 photosynthesis, whereby CO2 is initially fixed by the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase). However, Rubisco can also react with O2, leading to photorespiration, a process which consumes energy and releases previously fixed CO2. The cost of photorespiration, an inhibition of up to 40% of photosynthesis in today’s atmosphere, is thought to have been the driving force behind the evolution of C4 photosynthesis (Sage, 2004; Gowik & Westhoff, 2011). Species that perform C4 photosynthesis concentrate CO2 around Rubisco, thereby greatly enhancing its carboxylation efficiency and largely eliminating photorespiration. This translates into high productivity, and C4 species constitute some of our most successful crops, including maize (Zea mays) and sugarcane (Saccharum officinarum), as well our most promising biofuel species, such as switchgrass (Panicum virgatum) and Miscanthus×giganteus. Because the C4 photosynthetic pathway has evolved over 60 times in at least 19 families (Sage et al., 2011), the multitude of closely related C3 and C4 species provide a powerful tool for understanding the repeated evolution of C4-associated traits. Many studies focus on comparing characteristics between C3 and C4 species in a single lineage, raising the issue of whether traits associated with C4 species are truly C4-related or are due to common evolutionary histories or habitat preferences (Edwards & Still, 2008). In this issue of New Phytologist, Taylor et al. (pp. 387–396) assess differences in stomatal characteristics in a suite of related C3 and C4 grasses. The authors show that stomatal traits vary predictably between C3 and C4 species, even when phylogeny and growth environment are accounted for in the analysis, thereby clearly attributing differences to functional convergence based on photosynthetic pathway. This work makes a novel contribution to our knowledge of C4 biology and provides a hitherto missing link between stomatal characteristics and photosynthetic physiology.
‘When do changes in stomatal traits occur as a lineage evolves from an ancestral C3 state towards full C4 physiology?’
In leaves, the uptake of CO2 is inextricably linked to the loss of water through stomata, with an average of c. 2.7 g of carbon fixed per kilogram of water transpired in C3 plants under nonstressful conditions. Because of this inherent trade-off, the regulation of stomatal conductance can be viewed as an optimization problem, whereby carbon gain per unit water loss is maximized (Cowan & Farquhar, 1977). Accordingly, since C4 species have high photosynthetic rates even at low intercellular CO2 concentrations, they should maintain lower stomatal conductance rates than C3 species to reduce their transpiration rate and further increase their water-use efficiency (the ratio of photosynthesis to transpiration). Indeed, stomatal conductance rates are reduced in C4 species (Taylor et al., 2010), and C4 plants have higher water-use efficiency than C3 species (Monson, 1989; Sage, 2004; Vogan & Sage, 2011). Consistent with this earlier work, Taylor et al. demonstrate that even in a phylogenetically-controlled analysis, C4 grasses have lower maximum stomatal conductance rates (gmax) than their C3 relatives. However, the lower gmax of C4 plants compared to C3 species might still be due to differences in habitat (Edwards & Still, 2008). Photorespiration is enhanced at high temperatures and low intercellular CO2 concentrations, and C4 species tend to grow in hot and arid environments where photorespiratory costs are high (Sage, 2004). Since species in dry environments should restrict water loss regardless of their photosynthetic pathway, Taylor et al. also looked at precipitation niche to determine if differences in gmax were explained by water availability. In both mesic and arid environments, the authors found that even when accounting for phylogeny, C4 species had lower gmax than C3 species, demonstrating intrinsic differences in stomatal traits between the two groups. Generally, low gmax was achieved in C4 plants by producing smaller stomata for a given stomatal density, although the authors found differences between lineages, such that some C4 lines reduced stomatal density, while others preferentially reduced stomatal aperture. Evolutionary convergence towards a common functional solution, but using various anatomical or biochemical means, is a repeated theme in C4 evolution (Sage, 2004), and this work shows that the same holds true for stomata. These results demonstrate the importance of incorporating both phylogenetic and environmental data into analyses of C4 trait evolution, as well as filling in a crucial gap in our knowledge of C4 stomatal characteristics.