The negative impact of shade on photosynthetic efficiency in sugarcane may reflect a metabolic bottleneck

Abstract

Plants with C4 metabolism normally have higher photosynthetic rates than C3 ones. As a result, several of the most productive species known are NADP-ME C4 grasses, such as sugarcane and maize. However, the advantages of the C4 cycle are most evident under high light as the CO2 concentrating mechanism comes at the expense of additional ATP. Recent works have suggested a negative impact of shading across NADP-ME C4 grasses, causing a downregulation of maximal quantum efficiency of CO2 assimilation (ϕCO2,max). The mechanisms behind the loss in photosynthetic efficiency and whether these results apply for other C4 crops species and within germplasm of a species remain unclear. We analysed the photosynthetic acclimation to shade in four sugarcane genotypes with contrasting yield. To find out whether the effects of leaf history, i.e., shading a leaf developed under full sunlight and exposed later on to shade differ from a leaf fully developed under shading, these two types of leaves were evaluated. Shaded sugarcane plants showed decreased CO2 assimilation efficiency compared to plants grown under full sunlight. Based on the fluorescence measurements, it seems that this reduction coincided with a more reduced QA redox state, which could point to a metabolic limitation downstream of the light-dependent reactions. The results were similar for all genotypes and were observed regardless of whether leaves developed under shade or under full sunlight conditions and exposed subsequently to shade, suggesting that light is the main factor affecting photosynthetic efficiency. This study reinforces the notion that this negative impact of shade could reflect a common bottleneck across NADP-ME C4 grasses.