Effects of low-energy N+ ion beam irradiation on photosynthetic and transpiration in rice under enhanced UV-B radiation

Abstract: With the rapid economic development occurring since the 20th century, human activities have exacerbated the deterioration of the environment, and one of the most prominent environmental problems is the global reduction of the stratospheric ozone layer that results in increasing levels of mid-ultraviolet (UV-B, 280-320 nm) radiation on the earth's surface. The effect of enhanced UV-B radiation on higher plants has been studied intensively. Many studies showed that enhanced UV-B radiation could damage the photosynthetic system by destroying the photosynthetic organs, degrading the photosynthetic pigments, restraining the process of photosynthetic electron transportation, reducing the CO2 assimilation rate, and so on. Numerous scholars have even reported some ways to moderate the damage on plants caused by enhanced UV-B radiation such as nitrogen supply, doubled CO2 supply, He-Ne laser radiation, NaHSO3 spray, heavy metals addition, and so on. All of ways above had moderated the damage caused by enhanced UV-B radiation, while more ways should be found and applied in this field. Low-energy N+ ion implantation has been widely used in the mutation breeding of plant and microorganism due to its higher mutation frequency and wide mutation spectrum since the early 1980s. There are some reports showing that appropriate doses of low-energy N+ ion beam treatment could inspire the anti-oxidative enzyme system and change the physicochemical characteristics in plant cells on some level, which could regulate its defense system to deal with the outside stress. However, there was no report about the interactions between enhanced UV-B radiation and low-energy N+ ion beam radiation on rice. The effects of low-energy N+ ion beam irradiation on diurnal variations of photosynthesis and photosynthetic pigment contents in rice under enhanced UV-B radiation were investigated in the study. The experiment materials were pre-treated by three different doses of low-energy N+ ion beam that was conducted in Henan provincial at a key laboratory of ion beam bio-engineering, Zhengzhou university, China. Then the test materials were planted in an experimental field and uniformly managed routinely. One month later, the rice seedlings were implanted into the planting boxes provided by the laboratory. After turning green, the seedlings were treated with UV-B radiation for two months (from 8:00 to 17:00 per day) except for rainy or cloudy days. The dose of enhanced UV-B radiation was 16.46 kJ/m2·d (a 20% difference in ambient UV-B, Zhengzhou, China) which was detected by an ultraviolet irradiation detector and normalized at 297nm to obtain effective radiation. Two months later, the photosynthetic parameters (Pn,Tr,Gs,Ci) were determined with a portable photosynthesis system (CB-1102, Beijing Yaxin Liyi Science and Technology Co., Ltd. China) every two hours from 7:00 a.m. to 17:00 p.m. on a cloudless day, and the photosynthetic pigment content was determined too. The results were as follows: the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs),intercellular CO2 concentration (Ci), and the content of chlorophyll a (Chla), chlorophyll b (Chlb) were decreased while the efficiency of the water application (WUE), the content of carotenoids (Car) and the Chla/b were enhanced by UV-B stress. The combined treatment of enhanced UV-B radiation and low-energy N+ ion beam radiation (3.0×1017 N＋/cm2) increased the photosynthetic parameters (Pn,Tr,Gs,Ci) and the photosynthetic pigment content. It was suggested that the appropriate dose of low-energy N+ ion beam treatment (3.0×1017 N＋/cm2 in this study) may alleviate the damage caused by the enhanced UV-B radiation on rice. In addition, the test results provided a valuable reference for exploring the combined effect of ion beam biotechnology and various mutagenic sources in the physiological characteristics of rice deeply.