Growth and photosynthetic changes of Ulva prolifera in response to diurnal temperature variations.

The responses of Ulva species to diurnal temperature difference remain poorly understood. In this present study, we cultured Ulva prolifera under different diurnal temperature treatments with 22°C for photoperiod and 22, 20, 18, 16, 14, and12°C for dark period, respectively (noted as 22-22, 22-20, 22-18, 22-16, 22-14, and 22-12°C treatments). The growth, pigment contents, photosynthesis, superoxide dismutase (SOD) activity, soluble proteins, and sugars were measured. The main results were shown as follows: (1) The growth of U. prolifera was enhanced by the moderate diurnal temperature difference, and the highest growth rate was observed at 22-18°C. (2) Compared with 22-22°C treatment, the thalli grown under 22-18°C condition showed lower chlorophyll a (Chla) content, respiration rate (Rd), the ratio of Rd, and gross photosynthetic rate (Rd/Pg) as well as the net photosynthetic rate (Pn), while the lowest Pn was observed at 22-12°C. (3) The maximum quantum yield (Fv/Fm) was enhanced by diurnal temperature difference, while the effective quantum yield (Fv′/Fm′) decreased with the decreased in temperature in the nighttime. (4) With the increase of the diurnal temperature difference gradient, the SOD activity decreased and then increased, with the lowest value observed at 22-18 °C, and the soluble protein content showed similar trend. Then we cultured this species at 22-22°C and 22-16°C both under 250 and 60 μmol m−2s−1 conditions in order to study the combined effects of diurnal temperature change and light intensities. It was found that under both two light levels, 22-16°C-grown thalli showed higher growth rate, while the SOD activity was lower than that grown under 22-22°C condition. Overall, the suitable range of diurnal temperature difference for the growth of U. prolifera was about 4–6°C and also was mediated by light intensity. [ABSTRACT FROM AUTHOR]