Low-salinity tolerance of two high-temperature resistant strains in Pyropia yezoensis

The growth and maximum photon quantum yield of photosystem II (F_v/F_m) of the blades of two high-temperature resistant strains (YZ-4 and TM-18) and one wild-type strain (WT, used as control group) in Pyropia yezoensis were analyzed at high-temperatures or low-salinities, in order to screen out strains resistant to the two abiotic stresses. The results showed that YZ-4 and TM-18 blades grew faster than those of WT for 50-85 d under the optimal culture conditions (18 °C and salinity of 26). The ratios of F_v/F_m and contents of photosynthetic pigments were also higher than those of WT. The absolute growth rate and F_v/F_m of every strain showed high positive correlations. In addition, WT blades began to form spermatangia around 70 d, their absolute growth rate and F_v/F_m decreased significantly. When cultured at high-temperatures, F_v/F_m and absolute growth rates of the blades of every strain decreased. The higher the temperature or the longer the stress lasted, the greater the decline would be. F_v/F_m of WT, YZ-4 and TM-18 decreased by 56.7%, 43.2% and 28.7%, respectively, after 35 d of high-temperature stress at 24 °C. The growth of WT was completely stagnated after 15 d of high-temperature stress at 25 °C. Meantime, growth rates of YZ-4 and TM-18 were 0.51 and 0.84 cm/d, respectively, indicating that they were veritable high-temperature resistant strains. The effects of low-salinity stress on the blades were similar to those of high-temperature stress. After 35 d of low-salinity stress at salinity of 9, F_v/F_m of WT, YZ-4 and TM-18 decreased by 46.2%, 42.0% and 32.0%, respectively. The absolute growth rates of WT, YZ-4 and TM-18 were 0.12, 0.10 and 0.90 cm/d, respectively. Besides, growth of the blades of WT and YZ-4 cultured at salinity of 5 was greatly affected, in that the blades were deepened in color, curled and rotted after 15 d under stress, while TM-18 did not rot at the same salinity of 5 for 25 d. These results indicated that TM-18 was more tolerant to high-temperature and low-salinity stress than WT and YZ-4, and the internal reason was that the decrease of F_v/F_m of TM-18 blades was small. It indicated indirectly that F_v/F_m could be used as a new index for breeding of stress-resistant strains in P. yezoensis.