The photosynthetic response of cultivated juvenile and mature Undaria pinnatifida (Laminariales) sporophytes to light and temperature

We investigated the effects of photosynthetically active radiation (PAR) and temperature on the photosynthesis of juvenile (JS) and mature sporophytes (MS) of a temperate edible brown alga Undaria pinnatifida cultivated in northeastern Japan, using optical dissolved oxygen sensors and pulse amplitude modulation (PAM)–chlorophyll fluorometry. Photosynthesis–PAR experiments at 8, 15, and 22 °C revealed that the maximum net photosynthesis differed with temperature. Values for compensation and saturation PAR indicated a high affinity for a low-light environment. The photosynthetic response of U. pinnatifida to temperature revealed an optimum temperature for oxygenic photosynthesis of 18 °C, which was lower than other laminarian kelps and inconsistent with the wide distribution range of this U. pinnatifida. High temperature and irradiance tolerance differed between MS and JS. Notably, Fv/Fm of MS remained above 0.5 at 28 °C after 168 h of incubation under dark conditions, whereas Fv/Fm was reduced to 0 for the juveniles. Six hours of constant exposure to 200 (low) and 1000 μmol (high) photons m−2 s−1 at 8, 15, and 22 °C induced chronic photoinhibition in all both MS and JS. In contrast to MS, the JS did not recover from photoinhibition in either of the PAR treatments, regardless of a period of darkness. The physiological performance of U. pinnatifida sporophytes indicate an affinity to low light, especially for JS. We expect that these finding will lead to further improvements in commercial production methods to prevent the withering of juvenile sporophytes.