Effect of high temperature and strong light after sustained heavy rainfall on the photosynthesis and root metabolism in kiwi trees

Abstract: 【Objective】This study examined the influence of weather with sustained rainfall followed by high temperature and strong light intensity on physiology and biochemistry of kiwi trees in summer in Shanghai region in order to understand the cause of tree death in such a weather.【Methods】The experiment adopted artificial rainfall and simulated high temperature and strong light environment and used delicious‘Xuxiang'kiwi trees as the test materials. The plants were placed in a rainfall simulator with a precipitation capacity of 0.58 mm·h^-1 at an interval of 1 hour for five days. Then the plants were moved into a small greenhouse and exposed to high temperatures (38-40 ℃) and strong light (50 000-100 000 lx) for 4 days. Soil water contents in the treatments of different rainfall time followed by high temperature and strong light were measured. Moreover, leaf net photosynthetic rate (P_n) , transpiration rate (T_r) , stomatal conductance (G_s) , and intercellular CO₂ concentration (C_i) of the kiwi leaves were measured at 10:00 am every day via CIRAS-2 portable photosynthesis system. The damage to each leaf of the kiwi plants was quantified, and damaged leaves were classified into yellowing, wilting and withering leaves. Then root samples were taken to laboratory to measure the membrane leakage using conductometric method and root activity with TTC methods. Besides, malondialdehyde (MDA) content in the samples was detected using thiobarbital acid method and proline content with sulfosalicylic acid method. Nitroblue tetrazolium (NBT) reduction method was used to determine the content of superox-ide dismutase (SOD) . Peroxidase activity was determined with guaiacol method, and catalase with ultraviolet absorption method.【Results】During the artificial rainfall weather, before the commerce of high temperature and strong light, soil moisture increased and then leveled off, and the damage on the leaves increased sharply. The increase of soil moisture due to sustained rainfall caused the leaves to suffer from high temperature and strong light. The subsequent decline in soil moisture did not weaken the damage on the leaves. The results indicated that high temperature and strong light could be the key factors to damage the leaves, while the increase of soil water content caused by sustained rainfall was only the predisposing factor. The plants exposed to the stress of high temperature and strong light after sustained rainfall, had significantly (p < 0.05) reduced leaf net photosynthetic rate (P_n) , transpiration rate (T_r) and stomatal conductance (G_s) , and increased intercellular CO₂ concentration. The decline in photosynthetic rate may be caused by non-stomatal limitation. The damage on the photosynthetic apparatus caused by the stress of high temperature and strong light in a short time after rainfall led to the decline of photosynthetic rate. The longer the time of rainfall was, the faster the root vitality decreased during the exposure to high temperature and strong light. On the fourth day of high temperature and strong light treatment, root viability was significantly lower than the control group (p < 0.05) . The activities of SOD, POD and CAT in the root increased significantly in trees under the stress of high temperature and strong light after rainfall. The trend of SOD activities in R3 group decreased after an initial increase. The SOD activities in R4 and R5 continued to decline, but maintained higher than in the control group in the end by 32.21% and 19.33%, respectively. The POD activities in R1-R5 groups were 11.91%, 36.17%, 119.18%, 96.35% and 97.89% higher compared with the control group, respectively. The CAT activities in R1-R5 groups were 58.53%, 76.80%, 131.51%, 150.26% and 205.86% higher than in the control groups, respectively. The CAT activities in R4 and R5 reached their maximum values in the second day and then decreased. Although there was no significant linear correlation between leaf damage and root activity, they displayed a negative correlation. The longer the duration of rainfall was, the more serious damage water-logging caused on root system. During the exposure to high temperature and strong light, soil moisture decreased and the water-logging effects on root system had not disappeared completely. It triggered accumulation of free radicals, resulting in the death of the root system.【Conclusion】Extended rainfall intensifies water-logging stress effect on kiwi trees, and the subsequent high temperature and strong light create additional stress, which weakens the defense capability of the plant. On top of this, the stress of high temperature and strong light aggravates the damage on the root and photosynthetic apparatus. Finally, the stress damages the cellular structures and physiological metabolism in the root and weakens root activity and thus causes leaf wilting and withering.