The Effects Of Heat Stress On The Growth Of Laminaria Japonica And The Preliminary Study On The Effective Mechanism

The aquaculture industry in China is in a growth phase and sea kelp is one of the most commercialized seaweed species so far. However, the key link restricting the production growth in artificial breeding is the heat-resistance and the disease-resistance decreasing. It is of important meanings to study the mechanisms of the heat-stressing occurrence and to find effective techniques to raise the heat-resistance in sea kelp farming.2 species of Laminaria japonica cultured widely in Shandong adlittoral: the heat-resistant strain of Laminaria japonica 901, shorter form of 901, and the comparatively heat-sensitive strain of Rongcheng No.l, shorter form of R.C, were used to analyze the changes of the activities of antioxidant system and the isozyme differentiations under various treating conditions.901 was exposed to high temperature to analyze its physiological and biochemical characteristics changes. Results demonstrated that the growth was inhibited and the photosynthesis rate decreased with the temperature increment. The infection rate of alginic acid decomposing bacteria increased simultaneously. Meanwhile, the reactive oxygen species contents and the membrane relative permeability increased in contrast that the phospholipid content decreased in microsomal membrane. Results indicated that high temperature could lead to membrane damages and such damage was really relative to the accumulation of reactive oxygen species that initiated deesterification.Reactive oxygen species had close correlation with antioxidant system ability. To investigate the reasons of heat-resistance, the ROS contents, the membrane stability and the antioxidant system ability of 901 and RC exposed to different living conditions were examined. Results showed that:When living in normal conditions, the total antioxidant ability and PPO, POD activities of 901 were much higher than that in RC at the level of t=0.1. However, Vitamin C (Vt.C) content and glutathione peroxidase (GSH-px) activity of 901 weremuch lower than that of R.C (*p<0.05). The other contents investigated, such as SOD, CAT, PAL activities and protein, polyphenol contents, showed no significant differences at 1=0.05 level.Then the materials were exposed to high temperature. ROS contents kept constant in 901 while accumulated rapidly in RC, suggesting that ROS might play an important role in membrane stability. The protecting enzymes had various ways to resistant the heat stress, some increased the enzymatic activities and some enhanced the stability. However, enzymatic activities in RC were more vulnerable to heat stress and decreased markedly compared with 901, hinting that heat might have stronger effect on enzymatic system of RC. This might be the reason of heat-resistant difference in 901 and RC. Result also showed the 50% nocuity time of the chl. could be used as the most sensitive indicator to indicate the heat-resistant ability of Lam inaria japonica.From the results above mentioned, we drew such conclusion that heat stress would first injured the cell membrane, causing the ROS increasing markedly. Then the antioxidant system was affected by the excessive ROS leading to the metabolism distorting. Thereafter, the disease would happen. The key factor deciding the heat-resistance was not the antioxidant system background but the changes according to the stress. The antioxidant system in sensitive strain was much vulnerable to heat stress compared to the resistant one.