Ecophysiological Responses Of Porphyra Haitanensis (Rhodophyta) To Solar Ultraviolet Radiation

Porphyra haitanensis Chang et Zheng, as an economic and important red alga, isendemic to the Southern part of China and is widely farmed. Therefore, itsbiological studies have been of general interests. In its natural habitats, P.haitanensis grows in the eulittoral zone, experiencing high temperature, high solarirradiance and repeated desiccation during low tides. However, little is known aboutits ecophysiological responses to solar UV radiation (UVR: 280-400 nm). Thisstudy investigated the physiological responses of Porphyra haitanensis to UVR andrelated protective mechanisms, which were compared among its different life-cyclestages. The main results are as follows:The rates of photosynthetic carbon fixation of desiccated P. haitanensis thallidecreased with water loss, and were further reduced by solar UVR at water loss levelshigher than 20ï¼…. UVR also negatively affected the carbon fixation rate of theconchocelis, with higher inhibition in the vegetative phase than in the sporangialphase. High levels of both photosynthetically active radiation (PAR: 400-700 nm)and UVR reduced the photochemical efficiency of PSâ…¡, and the inhibition was higherin conchocelis stage than in thallus stage. Although P. haitanensis conchocelisexhibited high sensitivity to solar UVR, living under the shell-inner layer (0.06 mmthick), that shielded off 50-80ï¼…UVR, could provide them effective protection.Solar UVR affected the germination of P. haitanensis conchospores andsubsequent morphogenesis and growth of sporelings. Germination of P. haitanensisconchospores was faster when solar UV-B (280-315 nm) was excluded. Solar UV-Binhibited the growth of sporelings via decreasing cell division and cell growth andhindering cellular transverse division. UV-A (315-400 nm) showed significantpositive effects on the morphogenesis, enhancing the formation of transverse-dividedcells.Contents of UV-absorbing compounds (UVAC, peak at 336 nm) in of P.haitanensis thalli decreased steadily when maintained submerged, and periodicdesiccation (3 h) of the thalli per day sustained the contents of these compounds.The correlation was well established between the UVAC and water loss in the thalliwithin a water loss range of 70-96ï¼…. The thalli with more UVAC showed highertolerance to UVR, with less inhibition of photochemical efficiency when exposed toUVR. Contents of UVAC were different among different parts of female and male thalli, and the parts containing more UVAC showed higher tolerance to UVR.In conclusion, the responses of P. haitanensis to UVR were significantlydifferent among different life-cycle stages. The physiological tolerance to UVR washigher in thallus stage than in conchocelis stage, and higher in sporangial conchocelisthan in vegetative conchocelis, and positively correlated with the contents ofUV-absorbing compounds.