Response And Adaptation Of Cyclobalanopsis Gilva Seedlings To Drought Stress

Cyclobalanopsis gilva, a member of Fagaceae, is an evergreen broad-leaved tree native to the central subtropical region. It is one of well-known hardwood timber tree for its wood hardness, high density and pleasant texture, and also a kind of valuable ornamental tree used for urban greening and garden. In the original distribution region, seasonal drought affects the regeneration of C. gilva. Meanwhile, under the stress of human disturbance, the occurrence of large patches of natural forests of C. gilva became rare. To solve this problem, one-year-old seedlings of C. gilva in container originated from Qingyuan of Zhejiang Province, and Dongkou and Jingxian of Hunan Province treated with different soil water contents were investigated. The morphology, physiological properties, growth and biomass of seedlings from these three provenances to drought stresses were examined and their drought tolerances were summarized to understand the adaptation mechanisms of C. gilva to drought stress and the difference between their drought tolerances. The drought tolerance rating system should provide the basic information for resource selection and cultivation management. Major research results are as follows:1. Leaf anatomical structure and chloroplast ultrastructure of C. gilva had a significant response to drought stress. Drought treatments reduced the total thickness of leaf, the thickness of upper epidermis and lower epidermis of three provenances significantly (P<0.05). The change of palisade tissue among provenances was different. The thickness of palisade tissue of Qingyuan and Jingzhou decreased significantly (P<0.05), while the value for Dongkou didn’t change significantly. As the drought stress intensified, the stomatal of lower epidermis became smaller, the stomatal density increased for all three provenances, the chloroplasts expanded, the grana lamellae arrangements became disordered, grana structure was destroyed, and most of starch grains attached to chloroplasts were degradated and fused. Among three provenances, the total thickness of leaf, and the thickness of the palisade tissue, upper and lower epidermis of Dongkou had the minimum variation, of which the stomatal size and damage degree of chloroplast ultrastructure were smallest, but stomatal density was highest.2. The tissue of C. gilva seedlings became adapted to drought stress by increasing the bound water contents and osmotic regulatory substances, and regulating the activity of protective enzymes and contents of endogenous hormones. With the drought stress increasing, the bound water content, the ratio of bound water to free water, soluble sugar and proline contents in leaves from three provenances increased significantly (P<0.05) compare to that of control. The activity of peroxidase and catalase increased with the increasing drought stress, and they had peaks at the moderate drought, and then decreased at the severe drought. Compared with other provenances, C.gilva seedling from Dongkou had relatively higher peroxidase and catalase activity under all drought treatments. In addition, the drought stress resulted in significant decreases in contents of indole-3-acetic acid, gibberellins, brassinolide, and zeatins and significant increases in contents of abscisic acid and methyl jasmonate in leaves of seedlings from all three provenances (P<0.05).3. Photosynthetic physiological process and key parameters of C. gilva also had a significant response to drought stress. The contents of chlorophyll a and chlorophyll b and the ratio of chlorophyll a/b of the three provenances decreased significantly under drought stress (P<0.05). Drought stress also reduced net photosynthesitic rate, transpiration rate and stomatal conductance of C.gilva seedlings. Among the three provenances, compared to the control, the decrease in net photosynthetic rate of Dongkou (28.47%) under severe drought was the lowest one (Qing Yuan and Jingzhou were 36.63% and 31.82% respectively). The light response curves showed that the net photosynthetic rate of seedlings from three provenances growing under different drought stresses had similar trend:net photosynthetic rate increased rapidly when the photosynthetic active radiation (PAR) was lower than 200 umol·m-2·s-1, and then increased slowly when PAR reached a certain degree.4. Drought stress had significant effects on height growth, diameter growth and biomass allocation of C. gilva seedling. As the drought stress increased, the increment of height, stem diameter, and biomass of the seedlings had a trend of decrease, but the proportion of root biomass in total biomass increased. Compared to the control, the proportion of root biomass in total biomass of Qingyuan, Dongkou, and Jingxian increased by 11.74%,17.50%, and 13.49% respectively under severe drought stress.5. Principal component analysis and subordinate function analysis were used to evaluate comprehensively drought-resistance ability of three provenances seedling. The results showed that comprehensive value of Dongkou, Jingzhou, and Qingyuan were 0.721、0.582 and 0.479 respectively. Therefore, the drought-resistance capacity were inferred as follows:Dongkou> Jingzhou> Qingyuan. Based on the results of principal component analysis, using stepwise regression method, the mathematical evaluation model for C.gilva drought-resistance was obtained as Y=2.407+1.964X1 — 9.281X2—0.107X4—1.098X5+0.014X7 (X1, activity of peroxidase; X2, the ratio of bound water to free water; X4, proline content; X5, net photosynthesitic rate; X7, indole-3-acetic acid content, R2=0.952).