Coordination Of Leaf Morpho-anatomical Traits, Photosynthesis And Leaf Hydraulic Conductance In Oryza

Enhancing crop production to meet the food demand by growing population is an important issue for agricultural research. Due to the fact that the arable land is declining with urbanizing, increasing the productivity of crops is the only way to face the challenge of the food security in the future. Photosynthesis is the basic process for biomass formation, thereby increasing the photosynthetic rate is the key step for high-yield breeding. This dissertation focuses on the coordination of leaf anatomical traits, biochemical traits, photosynthesis and hydraulic conductance in Oryza leaves. Furthermore, under natural conditions, the light, temperature and other environmental factors are highly variable, and the photosynthetic productivity of plants is affected by the response speed of photosynthetic rate to the environmental changes. Thus, the second object of the present study is to explore the mechanism of how photosynthesis adaptes to environmental changes.Based on those purposes, some pot experiments were conducted:(1) The gas exchange, chlorophyll fluorescence, leaf hydraulic conductance, leaf nitrogen content, Rubisco enzyme content and leaf structural traits were investigated in rice and wild relatives from seven Oryza genus to explore the coordination of leaf structures, functions and biochemical characteristics;(2) With different nitrogen levels, gas exchange, chlorophyll fluorescence, leaf nitrogen content, Rubisco enzyme content and leaf structural parameters were measured to reveal how photosynthetic rate respond to environmental changes. The main results are as follows:(1) There was a great variation in the photosynthetic rate across 11 different genotypes from seven genus Oryza. Further analysis indicated that genotypic variation also existed in leaf nitrogen content, Rubisco enzyme content, stomatal conductance and mesophyll conductance. The strong correlations between photosynthetic rate and stomatal conductance, and between photosynthetic rate and mesophyll conductance were observed, however, there was no relationship between photosynthetic rate and leaf nitrogen content per area, nor between photosynthetic rate and Rubisco enzyme content per leaf area. These results suggested that the variation of photosynthetic rate among genotypes is mainly caused by CO2 supplement despite that a great variation of leaf nitrogen content and Rubisco content per area appeared among genotypes.(2) Stomatal conductance is limited by stomatal features(i.e. size and density) and/or the opening status under a given environment. In this study, a great variation in stomatal density and size across the selected genotypes was found. Across the genotypes, a negative correlation between stomatal density and size was also observed. Interestingy, across the selected genotypes, the variation range of maximum stomatal conductance, which was calculated based on stomatal morphological parameters, was very narrow. Therefore, under natural condition the great variation of stomatal conductance among genotypes is not caused by stomatal density or size, but mainly determined by the degree of stomatal opening. This conclusion also was supported by the positive relationship between stomatal conductance and leaf hydraulic conductance across genotypes.(3) Beyond stomatal conductance, mesophyll conductance is another important limiting factor for photosynthetic rate. Identifying the major factors that caused a great variation of mesophyll conductance among species or genotypes is an important issue for plant physiologists and ecologists. The results of this study showed that genotypic variation of mesophyll conductance was caused by the leaf anatomy, particularly, mesophyll structure. Mesophyll conductance increased with increasing leaf porosity, decreased with the cell wall thickness, increased with the surface area cell wall facing to intercellular air space, and increased with the chloroplast surface area facing to intercellular air space. In addition, the results also showed that mesophyll conductance increases with leaf nitrogen content per leaf area, which may be caused by that high nitrogen enhanced the expression of water channel protein on the cell membrane thereby improving the CO2 permeability of the biological membrane.(4) Carbon assimilation and water transpiration are two basic functions of the leaf. In the current study, I found that the photosynthetic rate was tightly correlated with leaf hydraulic conductance in rice leaves. This correlation probably is caused by following reasons:(a) the opening status of stomata is determined by the capacity of water supplement of the leaves, thus affecting the stomatal conductance and(b) CO2 diffusion and water transport within the mesophyll may share a common pathway. Leaf hydraulic conductance is composed of two parts: hydraulic conductance inside xylem and outside xylem, and the contributions of these two parts to leaf hydraulic conductance were similar. Leaf hydraulic conductance inside xylem tightly was correlated with vein density and morphology; and leaf hydraulic conductance outside xylem is determined by mesophyll structural traits, including mesophyll porosity, cell wall thickness, cell wall surface facing to intercellular air space, and inter vein distance. Due to the mesophyll conductance to CO2 and leaf hydraulic conductance outside xylem is affected by the same structural traits, a very strong correlation between the two traits was observed in this study. Many previous studies found that the responses of stomatal conductance and mesophyll conductance to environmental changes are similar.The results of this study showed the close relationship between stomatal conductance and mesophyll conductance,which may be mediated by leaf hydraulic conductance.(5) Leaf mass per area which is a central trait in leaf economic spectrum, is constituted by leaf thickness and density. The contributions of leaf thickness and density on variation of leaf mass per area among species are still unclear, here, the results showed that genotypic variation of leaf mass per area is mainly determined by the variation of leaf density and the contribution of thickness can be ignored. In addition, the results also showed that leaf mass per area is related to the volume of mesophyll, but not to the epidermal tissue or veins.(6) Leaf photosynthetic rate is usually represented by a point measurement, however, a great variation of photosynthetic rate within leaves has been reported in some species. The results from the current study showed that variation of photosynthetic rate within rice leaves is similar with variation among genotypes. Photosynthetic rate increased from the leaf base to the tip in both genotypes and the heterogeneity of photosynthetic rate within leaves is associated with alteration of leaf structural features, leaf nitrogen content, Rubisco content and chlorophyll content per area. The heterogeneity of leaf structure and biochemistry within leaves may be a strategy for plants adapting to the micro-climate environment of canopy.(7) Many previous studies have found that nitrogen supplements can increase rice photosynthetic rate, stomatal conductance and mesophyll conductance, however, the mechanisms of nitrogen enhanceing mesophyll conductanceis still unclear. In the current study, I found that the enhancement of mesophyll conductance under high nitrogen condition is mediated by the adjustment of leaf anatomy, specifically, the surface area of chloroplasts facing to intercellular air space. Meanwhile, high nitrogen supplements also decreased cell wall thickness and increased cell wall surface area facing to intercellular air space. In addition, high nitrogen supplements may also increase the amount of aquaporins on the biological membrane, thereby increasing the efficiency of CO2 transmembrane diffusion. The upregulated aquaporin genes expression on biological membrane may explain the results that mesophyll conductance under high nitrogen condition was more sensitive to CO2, light and temperature changes in rice.(8) Chloroplast movement is an important reaction for plants to adapt environmental light. Here I found that the light-dependent chloroplast movement varied with species, where chloroplast movement was more obvious in soybean than in rice. Chloroplast movements can be affected by nitrogen supplement, which significantly enhances chloroplast size in both soybean and rice. Under zero nitrogen supplement conditions, due to the small size of individual chloroplasts, there was a significant light-dependent chloroplast movement in both rice and soybeans. However, under high nitrogen supplements, the light depended chloroplast movement disappeared in both species.