Effects of length of growing season on biomass accumulation and reproductive investment of Vallisneria natans (Lour.) H. Hara

The above-and below-ground biomass and fruit production of Vallisneria natans plants were measured in relation to length of the growing season, based on six treatments (April/May/June planting in relation to October/November harvest) in a microcosm experiment. Above- and below-ground biomass generally increased with length of growing season. There were no significant differences between early and late harvest measures for each start date, except for the earliest planting date, where above-ground biomass was significantly less in November compared to October. There were no significant differences in the dry weight production of fruits between any of the treatments, nor in the ratio of unripe to mature fruits produced in each treatment. The biomass of mature fruits and the ratio of mature to unripe fruits were significantly higher (approximately double) amongst November harvested plants compared to October harvested plants across all start dates, suggesting reduced seed production as a result of early harvesting. There was a significant increase in the dry weight of mature fruits with length of growing season. Based on these results, shortening of the inundation periods experienced by Vallisneria natans plants in Yangtze River ephemeral wetlands (more so in autumn than in spring) will reduce annual biomass production, and advances in autumn desiccation date will reduce investment in viable seed production in this submerged macrophyte.