Abstract
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.
Highlights
Photosynthetic rate primarily depends on the incident light intensity [8,10,19,44,45,46], other factors such as air temperature [9], humidity, and vapor pressure deficit play important roles that impose limitations [10,47,48,49,50,51,52,53]
In support of these leaf-level results, Harenda et al [8] found that carbon influx on a peatland was affected by photosynthetic limitation caused by a high vapor pressure deficit. These results indicate that a midday depression is a common phenomenon under high temperature for both terrestrial and aquatic plants, further studies are required of emergent aquatic plants
Compared with terrestrial plants, quantitative evidence of the effects of high temperature is limited for wild aquatic plants
Summary
Photosynthesis of plants greatly affects global [1,2] and regional [3,4,5,6,7,8,9,10,11,12,13,14,15,16] carbon cycles.Since ecosystem-level photosynthesis is the sum of single-leaf photosynthesis [16,17,18,19,20,21,22], understanding of stress responses of leaves to diurnal and seasonal environmental change is necessary to improve ecosystem carbon cycles modeling [6,8,9,11,12,23,24,25,26,27] and to increase agricultural [22,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41] and forestry [13,42,43] production. photosynthetic rate primarily depends on the incident light intensity [8,10,19,44,45,46], other factors such as air temperature [9], humidity, and vapor pressure deficit play important roles that impose limitations [10,47,48,49,50,51,52,53]. Midday depression of photosynthesis is a phenomenon where photosynthetic rate decreases during the midday hours when light is not a limiting factor for photosynthesis [6,8,11,21,50,54,55,56,57,58]. The degree of photosynthetic limitation is affected by the atmospheric environment surrounding leaves, and by the soil water content and resultant whole-plant water status [21,31,32,37,38,41,42,43,48,50,54,59,60,61,62,63,64,65]. Since plants in nature grow in habitats with varying degrees of water availability (i.e., from aquatic environments to deserts), it is important to investigate different plant species from different habitats with varying amounts of water availability
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