Shade compromises the photosynthetic efficiency of NADP-ME less than PEP-CK and NAD-ME C4 grasses

The high energy cost and apparently low plasticity of C 4 photosynthesis compared with C 3 photosynthesis may limit the productivity and distribution of C 4 plants in low light (LL) environments. C 4 photosynthesis evolved numerous times, but it remains unclear how different biochemical subtypes perform under LL. We grew eight C 4 grasses belonging to three biochemical subtypes [NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PEP-CK)] under shade (16% sunlight) or control (full sunlight) conditions and measured their photosynthetic characteristics at both low and high light. We show for the first time that LL (during measurement or growth) compromised the CO 2 -concentrating mechanism (CCM) to a greater extent in NAD-ME than in PEP-CK or NADP-ME C 4 grasses by virtue of a greater increase in carbon isotope discrimination ("  P) and bundle sheath CO 2 leakiness ("¢), and a greater reduction in photosynthetic quantum yield (â–ª max). These responses were partly explained by changes in the ratios of phosphoenolpyruvate carboxylase (PEPC)/initial Rubisco activity and dark respiration/photosynthesis (R d /A). Shade induced a greater photosynthetic acclimation in NAD-ME than in NADP-ME and PEP-CK species due to a greater Rubisco deactivation. Shade also reduced plant dry mass to a greater extent in NAD-ME and PEP-CK relative to NADP-ME grasses. In conclusion, LL compromised the co-ordination of the C 4 and C 3 cycles and, hence, the efficiency of the CCM to a greater extent in NAD-ME than in PEP-CK species, while CCM efficiency was less impacted by LL in NADP-ME species. Consequently, NADP-ME species are more efficient at LL, which could explain their agronomic and ecological dominance relative to other C 4 grasses.