Performances of growth, photochemical efficiency, and stress tolerance of young sporophytes from seven populations of Saccharina japonica (Phaeophyta) under short-term heat stress

Temperature is one of the primary factors determining the geographic boundaries of the kelp Saccharina japonica. To compare the resistance of the offspring from different populations to heat stress, seven populations including Zhen-farmed population (ZFP), Daban-farmed population (DFP), Fujian-farmed population (FFP), Yanza-farmed population (YFP), Benniu-farmed population (BFP), Gaolv wild population (GWP) and Qingdao wild population (QWP) were chosen from different farms along the coasts of China where intensive farming of this species has been carried out. Offspring from these populations were obtained in the laboratory. The young sporophytes (1-3 cm) were subjected for 7 days to different temperature treatments of 10, 21, and 23A degrees C, respectively. The results showed that blade elongation rates were retarded both at 21A degrees C and 23A degrees C in all the populations. The percentages of plants that developed white blade tip were more serious in ZFP, YFP, and DFP than in other populations. Offspring from QWP demonstrated the most outstanding capacity to resist high temperature, as judged by the results of the maximal photochemical efficiency (F (v)/F (m)), the potential photochemical efficiency (F (v)/F (o)), content of malondialdehyde and the regulation of antioxidant enzymatic systems. Next were the offspring from GWP and FFP. Offspring from ZFP had the least heat-stress tolerance. Three lines of evidence obtained in the present study further confirmed that QWP distributed at 36A degrees N, the most southern distribution limit of this species in the North-Western coast of the Pacific, was the most ideal source as parental plants for breeding high-temperature-tolerant cultivar.