Effects Of Elevated CO₂ And Heat Stress On Growth And Development Of Tea Plant

The atmospheric CO₂ concentration has been increased from 260²80 μmol mol-1 to 380 μmol mol-1 within 100 years. The study shows that it is still increasing at a speed of 1² μmol mol-1 per annum. The elevation of atmospheric CO₂ results global warming which could be noticed by the occurrence of extreme high weathered temperature. These elevated CO₂ and high temperature heat stress cause tremendious effect on plant. Therefore, the study on effect of elevated CO₂ and heat stress on growth and development of tea plant has been of increasing interest these days. Therefore, we carried out the experiment of elevated CO₂ concentration, high temperature and exogenous EBR treatment to investigated the effect of elevated CO₂ and heat stress on primary and secondary metabolism of tea plant by methods of gas exchange, photosynthetic fluorescence and respiratory rate determination, High Performance Liquid Chromotography（HPLC）, antioxidant enzyme activity, correlation and quality analysis and gene expression in present study. Some of the major results were described as follows:1. We studied the developmental changes in carbon and nitrogen metabolisms of tea leaves in different leaf position, analyzed the variation trend of major nitrogenous substances and quality composition under different leaf position. These were described through the relationship between the quality and photosynthetic and respiratory metabolism. Here we showed that photosynthetic capacity [ΦPSâ…¡] of tea leaves increased gradually with leaf maturity along with chlorophyll content（TChl）. While respiration rate（RR） together with total N content（TN） decreased persistently, total C remained unchanged during leaf aging. Analyses of major N-based organic compounds revealed that decrease in TN was attributed to significant decrease in the content of caffeine and amino acids（AA） in mature leaves. Furthermore, soluble sugar（SS） decreased but starch content increased with leaf maturity, indicating that source-sink relationship was altered with leaf development. Detailed correlation analysis showed that ΦPSâ…¡ was negatively correlated with RR, SS, starch, polyphenol, total catechins and TN but positively correlated with TChl, while RR was positively correlated with TN, SS, TP and caffeine but negatively correlated with TChl and starch content. Our results suggest that biosynthesis of chlorophyll, catechins and polyphenols is closely associated with photosynthesis and respiration in different leaf position that greatly influences the relationship between primary and secondary metabolism in tea plants.2. We studied the response of photosynthetic and quality metabolism of tea plant under elevated CO₂ concentration environment, confirmed the phenomenen of photosynthetic acclimation, and explored the mechanism of changes of quality metabolism from the gene expression level. We found that the growth of tea plant was significantly promoted, leaf fresh weight per unit area increased, root-shoot ratio also increased significantly, the fresh weight of tea leaves, stem and root increased, the content of chlorophyll a, chlorophyll b and carotenoid were increased, but the ratio of a/b has no significant change after elevated CO₂ treatment. In short term, net photosynthetic rate of tea leaves increased, Vc,max and Jmax improve, ΦPS â…¡ increased significantly; after 12 days, photosynthetic adaptation phenomenon appeard but net photosynthetic rate stopped increasing. After treatment, content of tea polyphenols, catechins and amino acids in tea leaves increase significantly, however caffeine content decreased. The expression of catechin synthesis related genes increased generally, and the expression of caffeine synthesis related genes（TIDH, sAMs, TCS1） decreased, the expression of some amino acids synthesis genes increased, but some decreased.3. We studied the effect of heat stress on photosynthetic system of tea leaves and its mechanism. Heat stress caused obvious alterations in the leaf phenotype. Net photosynthetic rate declined gradually following heat stress which was accompanied with significant decreases in the maximum carboxylation rate of Rubisco and the maximum RuBP regeneration rate. Heat stress decreased Fv/Fm, Y（I） and Y（II） rapidly, but increased Y（NO） and Y（NA）, indicating that both photosystem II and photosystem I were damaged by heat stress. ΦPSâ…¡, ETR（II） and ETR（I） were also decreased significantly by heat stress, suggesting that heat stress possibly blocked the electron transduction. In tea leaves, photosystem I was more tolerant to heat stress, while photosystem II was relatively sensitive.4. We studied the physiology mechanism of exogenous 24-epibrassinolide-induced heat resistance in tea plant. Foliar application of EBR could significantly increase Pn, Gs, Vc,max, Jmax and Fv/Fm in tea leaves under heat stress. Moreover, EBR remarkably increased activities of APX, CAT, SOD and POD, but decreased MDA content in tea leaves following heat stress. These results suggest that exogenous EBR mitigates heat stress-induced photosynthetic inhibitions in tea leaves. It is due to the fact that it eliminates the non-stomatal limitations and promote the photosynthetic carbon reaction. Furthermore, EBR strengthened the antioxidant system effectively to scavenge harmful levels of reactive oxygen species and thus alleviating heat stress in tea plant.