COS 67-3
Has removal of the shade avoidance response altered photosynthetic capacity in maize? Evidence from a genome wide association study
The C4 grass Zea mays (maize or corn) is the third most important food crop globally in terms of production. Breeding of maize over the last 400 years has selected for traits of agronomic value, one of which is suppression of the shade avoidance response (SAR) to maximize productivity in dense plantings. A yet unanswered question is whether selection to minimize the developmental and morphological components of the SAR has placed constraints on photosynthetic performance. In particular, it is unclear if suppression of the SAR results in reduced plasticity of photosynthesis in variable light conditions. To address this question, we quantified variation in photosynthetic performance in 384 diverse lines of maize following two weeks of growth from seed in either sun (red/far-red light ratio, R/Fr, >1.2) or shade (R/Fr = 0.2) conditions. Using a novel, multi-line imaging fluorescence technique coupled to gas exchange measurements, we estimated photosynthetic electron transport (ETR) and CO2 assimilation rates over a range of light intensities in the complete panel of maize lines.
Results/Conclusions
Light-limited ETR efficiency in maize varied by up to 11% across lines, with an average value of 0.24 ± 0.2 μmol e- m-2 s-1 irradiance unit -1, and as predicted, did not differ across maize lines grown in sun compared to shade. Light-saturated ETR modeled from light response curves ranged from 29 to 71 μmol e- m-2 s-1, and was on average 12% higher in maize plants grown in the sun compared to those grown in the shade. In 68% of the lines measured, however, light-saturated ETR rates did not differ significantly between plants grown in the sun and shade. This indicated an inability to maximize CO2 assimilation under higher light conditions due to reduced plasticity of the photosynthetic apparatus. The majority of lines exhibiting this response have been developed in the last 100 years, the time period in which breeding for SAR removal has been the most intense. Our results demonstrate that breeding to remove the SAR has caused the majority of global maize cultivars to behave as shade plants, where photosynthesis saturates at a low intensity, irrespective of light conditions during growth, raising questions on the trade-off between agronomic traits and photosynthetic performance in an agriculturally important species.