Understanding plant physiological responses to high temperature is an important concern pertaining to climate change. However, compared with terrestrial plants, information about aquatic plants remains limited. Since the degree of midday depression of photosynthesis under high temperature depends on soil water conditions, it is expected that emergent aquatic plants, for which soil water conditions are always saturated, will show different patterns compared with terrestrial plants. We investigated the diurnal course of the photosynthetic light-response curve and incident light intensity for a freshwater emergent plant, buckbean (Menyanthes trifoliata L.; Menyanthaceae) in a cool temperate region. The effect of midday depression was observed only on a very hot day, but not on a moderately hot day, in summer. The diurnal course of photosynthetic light-response curves on this hot day showed that latent morning reduction of photosynthetic capacity started at dawn, preceding the apparent depression around the midday, in agreement with results reported in terrestrial plants. We concluded that (1) midday depression of emergent plants occurs when the stress intensity exceeds the species’ tolerance, and (2) measurements of not only photosynthetic rate under field conditions but also diurnal course of photosynthetic light-response curve are necessary to quantify the effect of midday depression.

Understanding the mechanisms underlying seed dispersal is a fundamental issue in plant ecology and vegetation management. Several species demonstrate myxospermy, a phenomenon where the seeds form mucilage after absorbing water. Mucilage is thought to act as a glue, enabling seeds to attach to the external surfaces of dispersing agents. However, there have been no quantitative investigations of the efficacy of this function of seed mucilage. We performed a trampling and walking experiment to investigate the seed dispersal of a perennial herb, Asian plantain (Plantago asiatica L.), which forms polysaccharide mucilage upon hydration. Our experiment showed that: (1) after trampling, more seeds of P. asiatica attached to shoes in wet conditions (after rainfall), in which seed mucilage was created, than in dry conditions (no rainfall); and (2) after walking for 1000 m, more seeds remained attached to shoes in wet conditions than in dry conditions. Our results indicate that mucilage promotes the adherence of seeds to the surface of vectors. We therefore provide the first empirical evidence that seed mucilage facilitates epizoochory and human-mediated dispersal.