Effects Of Planting Density On Photosynthetic Function And Regulative Mechanisms In Higher Plants

Gas exchange, chlorophyll a fluorescence, anatomical structure as well as proteomics were used to study the effects of planting density on growth strategies and photosynthetic characteristics, and investigate the effects of mutual shading on the regulation of photosynthesis as well as the systemic regulation of photosynthetic function under field conditions. The main results are as follows:1. Population density is one of the most important impact factors in crop production. The aim of this study was to analyze the effects of population density on photosynthetic characteristics and growth strategies of M. haplocalyx Briq. The results revealed that:(1) the plant height, stem diameter, branch number, leaf number, leaf area and leaf thickness declined along with elevation of population density. (2) The increase of population density induced the decrease of stomatal density, photosynthetic area and net photosynthetic rate (Pn), whereas the maximum quantum yield of photosystem II photochemistry (Fv/Fm) kept constant. (3) The biomass of leaf, stem and root as well as the total biomass of single plant reduced markedly, while the biomass of population elevated slightly with the increase in population density. However, the biomass allocation of single plant was similar to population. Accordingly, we suggested that high population density could depress photosynthetic capacity via affecting leaf morphology and structure; the M. haplocalyx Briq could compete for mineral nutrients preferential through enhancing the root mass ratio (RMR) with the increase of population density.2. In the field, close planting inevitably causes mutual shading and depression of leaf photosynthesis. To clarify the regulative mechanisms of photosynthesis under these conditions, the effects of population density on leaf structure, gas exchange and proteomics were carefully studied in field-grown sorghum. In the absence of mineral deficiency, (1) close planting induced a significant decrease in light intensity within populations, which further resulted in much lower stomatal density and other anatomical characteristics associated with shaded leaves; (2) sorghum grown at a high population density had a lower net photosynthetic rate (Pn) and stomatal conductance (Gs) than those grown at a low population density; (3) approximately 62 protein spots changed their expression levels under the high population density conditions, and 22 proteins associated with photosynthesis were identified by mass spectrometry. Further analysis revealed the depression of photosynthesis caused by mutual shading involves the regulation of leaf structure, absorption and transportation of CO2, photosynthetic electron transport, production of assimilatory power, and levels of enzymes related to the Calvin cycle. Additionally, heat shock protein and oxygen-evolving enhancer protein play important roles in photoprotection in field-grown sorghum. A model for the regulation of photosynthesis under mutual shading was suggested based on our results.3. The photosynthetic characteristics of developing leaves of plants grown under artificial conditions are, to some extent, regulated systemically by mature leaves; however, whether systemic regulation of photosynthesis occurs in field-grown crops is unclear. To explore this question, we investigated the effects of planting density on growth characteristics, gas exchange, leaf nitrogen concentration and chlorophyll a fluorescence in field-grown sorghum (Sorghum bicolor). Our results showed that close planting resulted in a marked decline in light intensity in the lower canopy. Sorghum plants grown at a high planting density had lower net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (E) than plants grown at a low planting density. Moreover, in the absence of mineral deficiency, close planting induced a slight increase in leaf nitrogen concentration. The decreased photosynthesis in leaves of the lower canopy at high planting density was caused mainly by the low light. However, newly developed leaves exposed to high light in the upper canopy of plants grown at high planting density also exhibited a distinct decline in photosynthesis relative to plants grown at low planting density. Based on these results, the photosynthetic function of the newly developed leaves in the upper canopy was not determined fully by their own high light environment. Accordingly, we suggest that the photosynthetic function of newly developed leaves in the upper canopy of field-grown sorghum plants is regulated systemically to a certain extent by the lower canopy leaves. The differences in systemic regulation of photosynthesis were also discussed between field conditions and artificial conditions.In summary, we suggested that high population density could depress photosynthetic capacity via affecting leaf morphology and structure; the plant could compete for mineral nutrients preferential through enhancing the root mass ratio with the increase of population density. In field, the depression of photosynthesis caused by mutual shading involves the regulation of leaf structure, absorption and transportation of CO2, photosynthetic electron transport, production of assimilatory power, and levels of enzymes related to the Calvin cycle. Moreover, the photosynthetic function of newly developed leaves in the upper canopy of field-grown plants is regulated systemically by the lower canopy leaves. Additionally, leaf nitrogen content may not be the key factor in the systemic regulation of photosynthetic performance. Accordingly, this study provides new insight into photosynthetic regulation under field conditions and enhances our understanding of systemic regulation of photosynthetic as well as the optimization of close planting.