Oxygen enhances the recovery of Potamogeton maackianus from prolonged exposure to very low irradiance

Recovery ability in relation to carbohydrate content of Potamogeton maackianus growing in two dissolved oxygen concentrations (8 and 2 mg L−1) was investigated during 28 days exposure to very low irradiance (about 0.06 μmol m−2 s−1). Plant weight remained relatively constant (0.19 g dry wt plant−1) within the initial 21 days in the high oxygen treatment, but decreased to 0.14 g dry wt plant−1 at the end of the experiment. In low oxygen environments, plant weight was similar within the initial 14 days, but decreased to 0.08 g dry wt plant−1 at 21 day. During the experimental period, both soluble sugar and starch contents in shoots decreased with time. Compared to high oxygen treatment, plants in the low oxygen treatment depleted starch more quickly (25 versus 18 mg g−1 at 28 day) but remained a relatively high soluble sugar content (0.9 versus 1.8 mg g−1 at 28 day). After recovery in high light and high dissolved oxygen conditions for 1 week, plant growth rate, new branch number, stem elongation rate and leaf recruitment number were significantly higher in high oxygen than in the low oxygen treatments. These data suggest that the ability of the plant to recover from prolonged exposure to very low irradiance is related to the depletion level of carbohydrate stored in plant tissues, which is regulated by oxygen availability in the water.