Effects of planting density on light environment within canopy
As shown in Fig. 1, higher light intensity of the ear leaf compared with the fourth leaf below the ear was observed under all planting densities at both heading stage and grain-filling stage. Compared with the ear leaf, the light intensity of the fourth leaf below the ear decreased by about 30% at low planting density and by 80% at high planting density. As planting density increased, the decrease of light intensity became more pronounced in the fourth leaf below the ear compared to the ear leaf, so that at high planting density, the light intensity of the fourth leaf below the ear was 70% lower than at the low planting density (Fig. 1). In contrast, the ear leaf light intensity changed very slightly as density increased. These data indicate that close planting enhanced spatial heterogeneity of light environment.
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The removal of the tassel and 1–3 top leaves close the tassel (RTL) is generally believed that this method could improve light intensity penetrating the canopy. In this study, RTL increased the light intensity reaching the ear leaf by a small amount, but it significantly increased the light intensity at the fourth leaf below the ear (Fig. 2). This confirms that RTL can improve the light environment, especially at the lower canopy.
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Effects of planting density on chlorophyll content
The total chlorophyll content in the ear leaves from heading stage (July) to the grain-filling stage (August) was maintained at a relatively high level. The chlorophyll content decreased with increasing planting density, but the extent of the decrease was not significant (Fig. 3). The chlorophyll content was lower at the fourth leaf below the ear than at the ear leaf. As the planting density increased, the chlorophyll content of the fourth leaf below the ear was markedly reduced. It was about 35% lower at high planting density than at low planting density for the fourth leaf below the ear.
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RTL reduced the chlorophyll content in both the ear leaf and the fourth leaf below the ear, and more so with increasing planting density (Fig. 4). However, chlorophyll content decreased less in the fourth leaf below the ear compared to the ear leaf. This suggests that RTL has different effects on chlorophyll content at the ear leaf compared to the fourth leaf below the ear.
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Effects of planting density on gas exchange
Under low planting density, the photosynthetic rate and stomatal conductance of the ear leaf maintained a higher state at the heading (July) and grain-filling stage (August), and decreased slightly as density increased (Fig. 5). The fourth leaf below the ear had a lower photosynthetic rate and stomatal conductance than the ear leaf under all planting densities. The photosynthetic rate and stomatal conductance of the fourth leaf below the ear decreased gradually with increasing planting density.
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The photosynthetic rate and stomatal conductance (Fig. 6) of the ear leaf decreased after RTL, but there was no obvious change in photosynthetic rate with increased planting density. In contrast, the photosynthetic rate of the fourth leaf below the ear significantly increased after RTL. Compared with CK, the photosynthetic rate of the fourth leaf below the ear increased by 18.1%, 22.1% and 36.6% at low, medium and high planting densities, respectively. These results confirm that RTL can decrease the photosynthetic rate of the ear leaf, while increasing the photosynthetic rate of the fourth leaf below the ear.
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Effects of planting density on chlorophyll a fluorescence transient
The chlorophyll a fluorescence transient (OJIP curve) contains abundant information about the primary photochemical reaction of PSII and is widely used in the study of PSII function. At the heading stage, the chlorophyll a fluorescence transient of the ear leaf was slightly affected by the planting density, but at the grain-filling stage the relative fluorescence yield of J and I phases of OJIP increased significantly as density increased (Fig. 7A and Fig. 7B). For the fourth leaf below the ear, at heading stage, the relative fluorescence yield of J and I phases of OJIP increased with increasing planting density, and it did so significantly at the grain-filling stage.
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Fig. 8 shows how OJIP changed after RTL. The relative fluorescence yields of J and I phases in both the ear leaf and the fourth leaf below the ear decreased, indicating that RTL resulted in an increase in the quantum yield of electron transport. As planting density increased, this decrease was much larger in the fourth leaf below the ear compared with the ear leaf.
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PI(CSo) refers to the photosynthetic performance index on cross section basis, and reflects the function of PSII. At both the heading stage (July) and grain-filling stage (August), the PI(CSo) was higher in the ear leaf than the fourth leaf below the ear. Compared with the ear leaf, PI(CSo) in the fourth leaf below the ear reduced 10% and 25% under the high planting density condition at heading stage and grain filling stage, respectively (Fig. 9A, B). As the density increased, the PI(CSo) of both the ear leaf and the fourth leaf below the ear went down. Relative to low planting density, the PI(CSo) of the ear leaf under high planting density decreased by about 20%, while the PI(CSo) of the fourth leaf below the ear dropped 30% at least.
We also calculated ΨE0,which represents the quantum yield of electron transport beyond QA. In all cases, these measurements displayed similar patterns to PI(CSo) (Fig. 9C, D). After RTL, the PI(CSo) and ΨE0 of the ear leaf and the fourth leaf below the ear increased. The enhancement of PI(CSo) and ΨE0 in the ear leaf was relatively small, but the increase in the fourth leaf below the ear was significant (Fig. 10). These results suggest that although high planting density reduces the photosynthetic performance and electron transport efficiency of PSII, RTL improves these parameters.
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