Response of photosynthetic activity to different temperature and light intensities in Saccharina japonica sporophyte

In order to explain the influence of temperature and light intensity on the growth process of Saccharina japonica sporophyte and explore the physiological response mechanism of the temperature and light environment from the perspective of physiological ecology. In this study, based on the growth parameters of S. japonica sporophyte, we set up four water temperature gradient (6, 10, 14 and 18 °C) of S. japonica sporophyte in a laboratory experiment, and determined the chlorophyll fluorescence parameters at night with 9 photochemical light gradients (0, 25, 70, 133, 230, 317, 421, 582, 786 μmol photons/(m²·s)). Results showed that: ① under the condition of 6 °C, S. japonica sporophyte fluorescence parameters (F_v/F_m) and (F_v/F₀) maximum value was 0.71 and 2.40 respectively; when water temperature 18 °C, its (F_v/F_m) and (F_v/F₀) minimum value was 0.65 and 1.85 respectively. ② The maximum values of photochemical quenching and non-photochemical quenching of S. japonica sporophyte occur at 18 °C (0.92 and 3.29, respectively). ③ The light response curve of kelp first increased and then decreased with the enhancement of PAR. ④ The maximum leaf length growth rate, leaf width growth rate and dry weight growth rate of S. japonica sporophyte were 1.34 cm/d, 0.33 cm/d and 1.01 g/d, respectively. These results indicated that the change of dry weight growth rate was consistent with the change of light response curve under different water temperature conditions, and high temperature inhibited the photosynthetic efficiency of S. japonica sporophyte. When the ambient photosynthetic effective radiation was larger than the light saturation point (E_m), the relative electron transfer rate of kelp decreased and photosynthesis was inhibited.