Improving photosynthesis to increase grain yield potential: an analysis of maize hybrids released in different years in China

Abstract

In this work, we sought to understand how breeding has affected photosynthesis and to identify key photosynthetic indices that are important for increasing maize yield in the field. Our 2-year (2017-2018) field experiment used five high-yielding hybrid maize cultivars (generated in the 1970s, 2000s, and 2010s) and was conducted in the Xinjiang Autonomous Region of China. We investigated the effects of planting density on maize grain yield, photosynthetic parameters, respiration, and chlorophyll content, under three planting density regimens: 75,000, 105,000, and 135,000 plants ha^-1. Our results showed that increasing planting density to the medium level (105,000 plants ha^-1) significantly increased grain yield (Y) up to 20.32% compared to the low level (75,000 plants ha^-1). However, further increasing planting density to 135,000 plants ha^-1 did not lead to an additional increase in yield, with some cultivars actually exhibiting an opposite trend. Interestingly, no significant changes in photosynthetic rate, dark respiration, stomatal density, and aperture were observed upon increasing planting density. Moreover, our experiments revealed a positive correlation between grain yield and the net photosynthetic rate (P_n) upon the hybrid release year. Compared to other cultivars, the higher grain yield obtained in DH618 resulted from a higher 1000-kernel weight (TKW), which can be explained by a longer photosynthetic duration, a higher chlorophyll content, and a lower ratio of chlorophyll a/b. Moreover, we found that a higher leaf area per plant and the leaf area index (HI) do not necessarily result in an improvement in maize yield. Taken together, we demonstrated that higher photosynthetic capacity, longer photosynthetic duration, suitable LAI, and higher chlorophyll content with lower chlorophyll a/b ratio are important factors for obtaining high-yielding maize cultivars and can be used for the improvement of maize crop yield.

Keywords: Density-tolerance; Grain yield; Maize; Photosynthesis; Planting density.

Fig. 1

Radiation (MJ m⁻² day⁻¹), daily mean temperature (T_mean,  °C), and daily precipitation (mm) during the growth period of maize in 2017 (a) and 2018 (b). The data range of the silking and filling stages includes five varieties that were marked with arrow lines

Fig. 2

Photosynthetic characteristics of the ear leaf at the silking, filling, and maturity stages, as influenced by plant density in 2017. Different lowercase letters indicate significant differences (p < 0.05) between treatments. P_n, net photosynthetic rate; G_s, stomatal conductance; C_i, intercellular CO₂ concentration; T_r, transpiration rate. SL, SC704 at low density; SM, SC704 at medium density; SH, SC704 at high density; ZL, ZD958 at low density; ZM, ZD958 at medium density; ZH, ZD958 at high density; XL, XY335 at low density; XM, XY335 at medium density; XH, XY335 at high density; LL, LY66 at low density; LM, LY66 at medium density; LH, LY66 at high density; DL, DH618 at low density; DM, DH618 at medium density; DH, DH618 at high density

Fig. 3

Photosynthetic characteristics of the ear leaf at the silking, filling, and maturity stages, as influenced by plant density in 2018. Different lowercase letters indicate significant differences (p < 0.05) between treatments. P_n, net photosynthetic rate; G_s, stomatal conductance; C_i, intercellular CO₂ concentration; T_r, transpiration rate. SL, SC704 at low density; SM, SC704 at medium density; SH, SC704 at high density; ZL, ZD958 at low density; ZM, ZD958 at medium density; ZH, ZD958 at high density; XL, XY335 at low density; XM, XY335 at medium density; XH, XY335 at high density; LL, LY66 at low density; LM, LY66 at medium density; LH, LY66 at high density; DL, DH618 at low density; DM, DH618 at medium density; DH, DH618 at high density

Fig. 4

Dark respiration rate of maize ear leaves in 2017 (a) and 2018 (b). Different lowercase letters indicate significant differences (p < 0.05) between treatments. R_d, dark respiration rate

Fig. 5

Pigment content and chlorophyll a/b ratio of the ear leaf during the growing season in 2017. a Amount of chlorophyll a; b Amount of chlorophyll b; c Amount of carotenoids; d chlorophyll a/b ratio. Different lowercase letters at the same growth stage indicate significant differences (p < 0.05) between treatments

Fig. 6

77K fluorescence emission spectra of the ear leaf at the filling stage in 2017. a–c 77 K fluorescence emission spectra after excitation at 435 nm. The chlorophyll concentration was adjusted to 10 μg/ml. The values were normalized to the emission at 685 nm. d–f 77 K fluorescence emission at 685 nm and 735 nm and the F735/F685 ratio. Different lowercase letters at the same growth stage indicate significant differences (p < 0.05) between treatments

Fig. 7

Trends in grain yield, net photosynthesis, and chlorophyll a/b ratio at the silking and filling stages over the hybrid release year. Y, grain yield; PnS, P_n at silking stage; PnM, P_n at maturity; A/BS, chlorophyll a/b at silking stage; A/BF, chlorophyll a/b ratio at filling stage. Data from previous studies, including maize varieties released between the 1970–1990 decades, were integrated into this figure (Zhang et al. ; Ding et al. ; Zhang et al. ; Li et al. 2015)

Fig. 8

Relationship between grain yield and main physiological indices during the growing stage in 2017 (a) and 2018 (b). Y, grain yield; YP, yield per plant; TKW, 1000-kernel weight; MLAI, maximum LAI; DMPS, dry matter per plant at silking stage; DMPM, dry matter per plant at maturity; PnM, P_n at maturity; A/BS, chlorophyll a/b ratio at silking stage; A/BF, chlorophyll a/b ratio at filling stage; R/FRF, R/FR at filling stage. ∗, ∗∗ Significance at the 0.05 and 0.01 probability level, respectively