Dept. of Ecology & Evolutionary Biology, University of Toronto Toronto, Ontario, Canada
Plants using the C4 photosynthetic pathway consist of two physiological subtypes: the NAD-malic enzyme (NAD-ME) subtype and NADP-malic enzyme (NADP-ME) subtype. NAD-ME subtype species tend to occur in drier regions than NADP-ME subtypes, indicating differences in drought responses. However, physiological mechanisms explaining the geographical discrepancies remain uncertain. This study examines gas exchange patterns that might explain different geographic distributions observed between the two major subtypes of C4 photosynthesis. We first hypothesized that NAD-ME species have higher intrinsic water use efficiency (measured as the ratio of net CO2 assimilation rate, A to stomatal conductance, gs) than NADP-ME species. To assess this, we measured the response of leaf gas exchange to vapor pressure deficit (VPD) in plants from five distinct C4 clades having closely related NAD-ME and NADP-ME species using a Li-Cor 6400 gas exchange system. This phylogenetically-informed comparison minimizes the possibility of complications due to different evolutionary histories, which compromised earlier assessments of subtype differences.
At a given value of stomatal conductance (gs), NADP-ME species exhibited higher A/gs than the NAD-ME species, which did not support the hypothesis. However, NADP-ME species exhibited lower reductions in gs with increases in VPD than NAD-ME species, indicating lower ability to respond to dynamic VPD conditions which might aggravate transpiration in dry environments. Based on these results, we hypothesize that the greater response of gs to increasing VPD may enable NAD-ME plants to outperform NADP-ME plants in hot, dry environments where VPD is normally high and can change rapidly with wind gusts. How this response might relate to the distinct biochemistry of the C4 subtypes remains to be determined.