Effects Of Enhanced UV-B Radiation On Photosynthesis Of C₄ And CAM Plants

Enhanced UV-B radiation resulting from ozone depletion is one of the global environmental problems. Up to now, the effects of UV-B radiation on organisms have been studied extensively. However, the investigated materials in these studies were mainly C3 plants, some were C4 plants, few CAM plants were explored. Furthermore, most of these studies were conducted indoors with strictly controlled conditions, and the conclusions from these studies have many inconsistencies. In this experiment, the C4 plant-maize and the CAM plant-Neorgelia cawlinae Meyendor ffii were used as the experimental material. The maize and Neorgelia cawlinae Meyendor ffii were treated with UV-B radiation with the intensities of 0uw.cm-2 (CK), 10uw.cm-2 (R1) and 15uw.cm-2 (R2), the duration of UV-B treatment were ten hours per day(from AM8: 00 to PM18: 00). The effect of UV-B radiation on the photosynthesis was studied, and the results showed as follows:(l)The effects of UV-B radiation on photosynthesis of maize and Neorgelia cawlinae Meyendor ffiiThe results of the experiment show that the photosynthesis of maize seedlings was restrained because of the decrease of chlorophyll contents activity of Hill reaction stamotal conductance activity of PEPcase and ATP content caused by increased UV-B radiation, and it was more restrained under higher UV-B radiation. Though the activities of RuBPcase and PEPcase were decreased, the chlorophyll contents activity of Hill reaction stamotal conductance of mature maize were not affected, therefore the photosynthesis of grown-up plant was not affected. The reason is that the response to UV-B radiation in maize is different in different growth phases. The UV-B/PAR was high because of low flux of light in spring when the seedlings grown, moreover, all the structures and functions of the tender maize seedling leaves were not completely developed, hence, the photosynthesis of maize seedling was affected greatly. However, the UV-B/PAR was lower because of high flux of light in summer when the grown-up plant lived, this weakened the effect of UV-B radiation on maize,and the structures and functions of mature leaves were completely developed, consequently, the effect of UV-B radiation on photosynthesis of grown-up plant was not distinct. The photosynthesis of Neorgelia carolinae Meyendor ffii was not distinctly affected by enhanced UV-B radiation in a short-term, yet the effect and damage of enhanced High doses of UV-B radiation on Neorgelia carolinae Meyendor ffii were increased with the increase of exposure time, thus enhanced high dose of UV-B radiation was not beneficial to the growth of Neorgelia carolinae Meyendor ffii. However, enhanced low dose of UV-B radiation was beneficial to the growth of Neorgelia carolinae Meyendor ffii.(2) The effects of UV-B radiation on transpiration of maize and Neorgelia carolinae Meyendor ffiiThe results show that the effect of enhanced UV-B radiation on transpiration of maize seedlings was not distinct, and the transpiration of grown-up plant was increased under enhanced UV-B radiation. The transpiration of Neorgelia carolinae Meyendor ffii was not distinctly affect Hi by enhanced UV-B radiation in a short-term, yet it was increased initially and then decreased with the increase of exposure time, and the greater decrease with higher UV-B radiation.(3) The effect of enhanced UV-B radiation on productivity of maize The results show that the productivity of maize was decreased because of the photosynthesis of maize seedling, but the decrease was not prominent since the photosynthesis of grown-up plant was not affected.