Photosynthetic capacity and assimilate transport of the lower canopy influence maize yield under high planting density

Abstract

Photosynthesis is a major trait of interest for the development of high-yield crop plants. However, little is known about the effects of high-density planting on photosynthetic responses at the whole-canopy level. Using the high-yielding maize (Zea mays L.) cultivars "LY66," "MC670," and "JK968," we conducted a 2-yr field experiment to assess ear development in addition to leaf characteristics and photosynthetic parameters in each canopy layer at 4 planting densities. Increased planting density promoted high grain yield and population-scale biomass accumulation despite reduced per-plant productivity. MC670 had the strongest adaptability to high-density planting conditions. A physiological analysis showed that increased planting density primarily led to decreases in the single-leaf area above the ear for LY66 and MC670 and below the ear for JK968. Furthermore, high planting density decreased chlorophyll content and the photosynthetic rate due to decreased canopy transmission, leading to severe decreases in single-plant biomass accumulation in the lower canopy. Moreover, increased planting density improved presilking biomass transfer, especially in the lower canopy. The yield showed significant positive relationships with photosynthesis and biomass in the lower canopy, demonstrating the important contributions of these leaves to grain yield under dense planting conditions. Increased planting density led to retarded ear development as a consequence of reduced glucose and fructose contents in the ears, indicating reductions in sugar transport that were associated with limited sink organ development, reduced kernel number, and yield loss. Overall, these findings highlighted the photosynthetic capacities of the lower canopy as promising targets for improving maize yield under dense planting conditions.

Figure 1.

Biomass accumulation in each maize canopy layer among plants grown at several planting densities. A to E) Biomass accumulation at the silking stage for A) LY66 in 2019, B) MC670 in 2019, C) LY66 in 2020, D) MC670 in 2020, and E) JK968 in 2020. F to J) Biomass accumulation at the maturity stage for F) LY66 in 2019, G) MC670 in 2019, H) LY66 in 2020, I) MC670 in 2020, and J) JK968 in 2020. D1 to D4 represent 75,000, 105,000, 120,000, and 135,000 plants/ha, respectively. The lowercase letters indicate statistical significance groups at P < 0.05 (2-way ANOVA with post hoc Lsd test). The data are presented as the mean ± Se from 3 or 4 biological replicates per group.

Figure 2.

Biomass transfer in each maize canopy layer among plants grown at several planting densities. A to E) Biomass transfer before the silking stage in (A) LY66 in 2019, (B) MC670 in 2019, (C) LY66 in 2020, (D) MC670 in 2020, and (E) JK968 in 2020. A negative value indicates that the DW was higher at maturity than at the silking stage. The transfer amount was calculated from biomass accumulation per plant. D1 to D4 represent 75,000, 105,000, 120,000, and 135,000 plants/ha, respectively. The lowercase letters indicate statistical significance groups at P < 0.05 (2-way ANOVA with post hoc Lsd test). The data are presented as the mean ± Se from 3 or 4 biological replicates per group.

Figure 3.

Leaf area at each leaf position among plants grown at several planting densities. A to E) Green leaf area at each leaf position at the silking stage in (A) LY66 in 2019, (B) MC670 in 2019, (C) LY66 in 2020, (D) MC670 in 2020, and (E) JK968 in 2020. The first visible complete leaf was the seventh leaf from the bottom at the silking stage. The numbers 7 to 21 indicate the 7th to 21st leaves, respectively, from the bottom of the plant. The black dotted lines represent the ear position. D1 to D4 correspond to 75,000, 105,000, 120,000, and 135,000 plants/ha, respectively. The data are presented as the mean ± Se from 3 biological replicates per group.

Figure 4.

Net photosynthesis (P_n) in the leaves of each canopy layer among plants grown at several planting densities. A to E)P_n at the silking stage in (A) LY66 in 2019, (B) MC670 in 2019, (C) LY66 in 2020, (D) MC670 in 2020, and (E) JK968 in 2020. D1 to D4 represent 75,000, 105,000, 120,000, and 135,000 plants/ha, respectively. The lowercase letters indicate statistical significance groups at P < 0.05 (2-way ANOVA with post hoc Lsd test). The data are presented as the mean ± Se from 3 or 4 biological replicates per group.

Figure 5.

Total chlorophyll contents in leaves from each canopy layer among plants grown at several planting densities. A to E) Total chlorophyll contents in leaves at the silking stage in (A) LY66 in 2019, (B) MC670 in 2019, (C) LY66 in 2020, (D) MC670 in 2020, and (E) JK968 in 2020. D1 to D4 represent 75,000, 105,000, 120,000, and 135,000 plants/ha, respectively. The lowercase letters indicate statistical significance groups at P < 0.05 (2-way ANOVA with post hoc Lsd test). The data are presented as the mean ± Se from 3 biological replicates per group.

Figure 6.

Young ear development among plants grown at several planting densities. A, C, E) Representative (A) LY66, (C) MC670, and (E) JK968 ears at several time points after sowing in 2020. Images were digitally extracted for comparison. B, D, F) Quantification of ear length over time for (B) LY66, (D) MC670, and (F) JK968 plants. Scale bar = 1 cm. D1, 75,000 plants/ha; D4, 135,000 plants/ha. *P < 0.05, **P < 0.01 (Student's t-test). ns, not significant. The data are presented as the mean ± Se from 4 biological replicates per group.

Figure 7.

Levels of starch, sucrose, glucose, and fructose in young maize ears from plants grown at several planting densities. A to D) Levels of (A) starch, (B) sucrose, (C) glucose, and (D) fructose. Samples were analyzed at 72 and 77 DAS in 2020. D1, 75,000 plants/ha; D4, 135,000 plants/ha. The lowercase letters indicate statistical significance groups at P < 0.05 (2-way ANOVA with post hoc Lsd test). The data are presented as the mean ± Se from 3 or 4 biological replicates per group, each of which consisted of pooled samples from at least 3 plants.

Figure 8.

Correlation of yield components with physiological parameters at each canopy layer among plants grown at several planting densities. A and B) Correlation of yield with (A) kernel number and (B) biomass accumulation at maturity. C to G) Correlation of grain weight per plant with (C) biomass accumulation per plant at silking, (D) biomass accumulation per plant at maturity, (E) biomass transfer per plant, (F) FIPAR in Layer III, and (G) P_n. H and I) Correlation of biomass accumulation per plant at maturity in (H) Layer I and (I) Layer II with P_n. J) Correlation of grain weight per plant with chlorophyll content in Layer II. *P < 0.05, **P < 0.01 (Pearson correlation analysis). A to E) and G to I) A total of 54 replicates, each point represents 1 replicate of 1 planting density and 1 variety and 1 yr. F and J) A total of 18 replicates, each point represents 1 planting density of 1 variety and 1 yr. A and B) The data are based on the entire canopy. C to J) The data are based on different canopy layers.

Figure 9.

A schematic diagram showing the physiological mechanisms of yield losses or gains among plants grown at several planting densities. Representations of (A) reduced and (B) increased grain yield under high-density planting conditions. Increased planting density reduces the photosynthetic rate and leaf area in the lower canopy layer, thereby reducing assimilate accumulation. Moreover, increased planting density affects the glucose and fructose contents of young ears, impairing ear development. This ultimately reduces kernel number per ear and per-plant kernel weight, resulting in per-plant yield losses. Optimizing leaf morphology in the canopy layers in response to increased planting density could improve the photosynthetic rate and stimulate ear development, increasing yield. The dashed boxes represent the parts below and above the ear in the plant. The processes named in turmeric, blue, and black correspond to ear development, carbon metabolism, and yield, respectively. The black arrows represent the indication. The blue arrows (regular solid arrows and outline arrow) and purple outline arrow represent decreases and increases, respectively, in the indicated processes.