Characterization of Photosynthetic Phenotypes and Chloroplast Ultrastructural Changes of Soybean (Glycine max) in Response to Elevated Air Temperatures.

Matthew T. Herritt,Felix B. Fritschi

doi:10.3389/fpls.2020.00153

Abstract

Heat stress negatively affects photosynthesis in crop plants. Chlorophyll fluorescence provides information about the efficiency of the light-dependent reactions of photosynthesis and can be measured non-destructively and rapidly. Four soybean (Glycine max) genotypes were grown in controlled environments at 28/20°C (control), followed by imposition of control, 38/28°C, and 45/28°C day/night temperature regimes for 7 days. Coordinated chlorophyll fluorescence, gas exchange, and chloroplast ultrastructure measurements were conducted over the course of the 7-day temperature treatments and revealed contrasting responses among the different genotypes. Although generally similar, the extent of the impact of elevated temperatures on net photosynthesis differed among genotypes. Despite dramatic effects on photosynthetic light reactions, net photosynthetic rates were not reduced by exposure to 45°C on the 1st day of treatment imposition. Temporal dynamics of light reaction characteristics over the course of the 7-day heat-wave simulation revealed distinct responses among the genotypes. Similarly, chloroplast ultrastructure examination identified contrasting responses of DT97-4290 and PI603166, particularly with respect to starch characteristics. These changes were positively associated with differences in the percent area of chloroplasts that were occupied by starch grains. Elevated temperature increased number and size of starch grains on the 1st day of DT97-4290 which was coordinated with increased minimum chlorophyll fluorescence (F0) and reduced leaf net CO2 assimilation (A). Whereas on the 7th day the elevated temperature treatment showed reduced numbers and sizes of starch grains in chloroplasts and was coordinated with similar levels of F0 and A to the control treatment. Unlike starch dynamics of PI603166 which elevated temperature had little effect on. The genotypic differences in photosynthetic and chloroplast ultrastructure responses to elevated temperatures identified here are of interest for the development of more tolerant soybean cultivars and to facilitate the dissection of molecular mechanisms underpinning heat stress tolerance of soybean photosynthesis.

Highlights

Plants have the ability to cool their leaves below that of the air temperature
Because plants were well-watered throughout the experiments, the imposition of these air temperature treatments, coupled with the control of relative humidity at 50% during the photoperiod, allowed for significant transpirational cooling which resulted in average leaf temperatures of 29.1, 31.9, and 34.5°C in the 28, 38, and 45°C treatments, respectively (Figure 1)
General responses in gas exchange and chlorophyll fluorescence were similar, genotypic differences in the temporal dynamics over the course of the 7-day heat treatment were observed for numerous traits

Summary

Introduction

Plants have the ability to cool their leaves below that of the air temperature. In conditions where leaf cooling cannot keep pace with increasing air temperatures, elevated leaf temperatures can lead to reductions in photosynthesis (Fischer et al, 1998). Central to the light-dependent reactions are photosystems I and II (PS1 and PS2). The photosystems are the central hubs where the energy from absorbed light is used to excite electrons. The photosystems, cytochrome b6f and the mobile electron carriers as well as the bulk of the accessory proteins involved with the light-dependent reactions are located either within or near the thylakoid membrane (Maxwell and Johnson, 2000; Friso et al, 2004)

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Discussion

Conclusion