Response Of Photosynthetic Characteristics Of Nitraria Tangutorum To Rain Addition

Photosynthetic characteristics of a typical desert plant Nitraria tangutorum under four rain addition treatments (+25%, 50%, 75% and 100% of local mean annual precipitation) were measured with Li-6400xt Portable Photosynthesis Analyzer at the eastern edge of the Ulanbuh Desert. Discussed the response process and mechanism of photosynthetic characteristics to rainfall. The main results are as follows:(1)Net photosynthetic rate (Pn), transpiration rate (Tr), the maximum net photosynthetic rate(Amax), apparent quantum efficiency(AQY) and dark respiration rate(Rd) increased, but water use efficiency (WUE) decreased with the rain addition treatments. Rain addition increased photosynthetic rate, light use efficiency, as well as consumption of photosynthetic products by respiration, but reduced plant's water use efficiency. Rain addition on the magnitude of photosynthetic production capacity is much larger than the magnitude of the loss, although Pn and Rd increased, and WUE decreased, photosynthetic capacity was still rising.(2) Diurnal variation of Pn, Tr, Gs were obvious single peak curves, the peak at 8:00 am. Net photosynthetic rate and other gas exchange indicators showed significant differences between 8:00-16:00 under different treatments. The results showed that photosynthetic capacity and water metabolism that 75% and 100% treatment was significantly stronger than the other treatments. WUE reach the maximum value of the day at six o'clock in the morning , WUE did not show significant differences between different treatments. Diurnal change of intercellular CO₂ concentration is obviously anti-parabolic type. And Stomatal limitation value of the diurnal variation is opposite to intercellular CO₂ concentration.(3) Seeing from the whole growing season, June's Pn was highest, followed by September, and May was last. June's Tr was highest, followed by July, and September was last. WUE in September was significantly higher than in other months. WUE of Nitraria tangutorum was little changed. In addition to September were significantly higher than other months, there were no significant difference from May to August. The value of WUE maintained between 2.96 mmol·mol-1 and 3.74 mmol·mol-1. Leaf area is small, chlorophyll content is low, photosynthetic capacity is not strong, so Pn is the lowest and there was no significant difference between different treatments in May.(4) Pn increased at first and then decreased with the gradual process of soil drought after rain addition. Pn reach peak at 7 day, then decreased. In the increasing process of Pn, 75% and 100% treatment increased by 34.5%, 30.0%, and 25% and 50% treatment increased by 28.8%, 16.2%. There was smallest change in control. In the decreasing process of Pn, the rate of decline of 75% and 100% treatment is less than other treatments. The rate of rise of 75% and 100% treatment is larger than other treatments. With the continued extension of treatment time, Pn can still maintain a high level, indicating that precipitation could increase the photosynthetic capacity of Nitraria tangutorum.75% and 100% treatment have more influence on photosynthesis.In short, photosynthesis of Nitraria tangutorum can be self-regulation to environmental conditions in a timely manner, maintains a high photosynthetic capacity to accumulate photosynthetic products, ensuring the needs of their own growth and survival. Therefore, rain addition changed the physiological characteristics of Nitraria tangutorum in varying degrees and the changes could be an adaptation strategy of the plant to environmental conditions and is a result of benefit photosynthesis.