Compensation of the brown alga Undaria pinnatifida (Laminariales; Phaeophyta) after thallus excision under cultivation in Matsushima Bay, northern Japan

In recent years, the crop yield of cultivated Undaria pinnatifida (Harvey) Suringar has not been able to meet commercial demand and so advances in cultivation technology are strongly needed to increase production. Interestingly, cultivation work has shown the yield of U. pinnatifida sporophytes may be increased significantly by thallus excision, which may cause compensatory growth of the remaining tissues. To test this hypothesis and clarify correlative mechanisms, we examined seasonal morphological characteristics, photosynthetic rates, nutrient uptake rates, and carbon and nitrogen contents of U. pinnatifida sporophytes after thallus excision at a point 30 cm from the meristem in late February and compared these parameters with control kelps grown without excision, which were cultivated together in Matsushima Bay, northern Japan. Compared to control kelps, the length and dry weight of blades increased significantly after excision, and the growth phase was prolonged for about 1 month. The photosynthetic rates, nutrient uptake rates, and carbon and nitrogen contents of excised kelps were significantly higher than those of the controls. After vegetative growth stopped, resources accumulating in thalli were translocated significantly to sporophylls for maturation, indicating maturation was not negatively affected by thallus excision. These results indicate U. pinnatifida exhibits a very strong compensatory ability in response to thallus excision and consequently, the yield could be increased due to an increase in harvest frequency.