Enhanced photosynthetic nitrogen use efficiency and increased nitrogen allocation to photosynthetic machinery under cotton domestication

Domestication involves dramatic phenotypic and physiological diversifications due to successive selection by breeders toward high yield and quality. Although photosynthetic nitrogen use efficiency (PNUE) is a major trait for understanding leaf nitrogen economy, it is unclear whether PNUE of cotton has been improved under domestication. Here, we investigated the effect of domestication on nitrogen allocation to photosynthetic machinery and PNUE in 25 wild and 37 domesticated cotton genotypes. The results showed that domesticated genotypes had higher nitrogen content per mass (N_m), net photosynthesis under saturated light (A_sat), and PNUE but similar nitrogen content per area (N_a) compared with wild genotypes. As expected, in both genotypes, PNUE was positively related to A_sat but negatively correlated with N_a. However, the relative contribution of A_sat to PNUE was greater than the contribution from N_a. Domesticated genotypes had higher nitrogen allocation to light-harvesting (N_L, nitrogen in light-harvesting chlorophyll–protein complex), to bioenergetics (N_b, total nitrogen of cytochrome f, ferredoxin NADP reductase, and the coupling factor), and to Rubisco (N_r) than wild genotypes; however, the two genotype groups did not differ in PNUE_p, the ratio of A_sat to N_p (itself the sum of N_L, N_b, and N_r). Our results suggest that more nitrogen allocation to photosynthetic machinery has boosted A_sat under cotton domestication. Improving the efficiency of nitrogen use in photosynthetic machinery might be future aim to enhance A_sat of cotton.