Abstract
ABSTRACTPlants are exposed to high light intensity, high leaf temperatures and high air-to-leaf water vapor pressure deficit (ALVPD) during the day. These environmental stresses cause stomatal closure and photoinhibitory damage, leading to midday depression of photosynthesis. Chloroplast positioning is essential for the efficient operation of photosynthesis. However, chloroplast behavior before, during, and even after the midday depression of photosynthesis remains unknown. We investigated changes in the intracellular positioning of chloroplasts and photosynthetic traits under a diurnal pattern of light. Sorghum leaves were exposed to a 12-h regime of light mimicking the natural light environment, with constant leaf temperature and ALVPD. Net photosynthetic rate (Pn) showed a diurnal pattern, and midday depression in Pn was observed at 3.8 h of irradiation. Depression in Pn was attributed to stomatal limitation because the decrease in Pn was in accordance with the decrease in stomatal conductance. The maximum efficiency of photosystem II decreased with the increase in light intensity and remained low after 12 h of irradiation. Bundle sheath chloroplasts swelled after 8 h of irradiation, representing the accumulation of starch. Conversely, mesophyll chloroplasts exhibited avoidance response after 4 h of irradiation, and the avoidance position was maintained during the remainder of the daytime. These data suggest that chloroplasts are subject to light stress during and after the midday depression of photosynthesis. The intensity of natural light is excessive for most of the day and this light stress induces chloroplast avoidance response and depression of photosynthesis.
Highlights
The chloroplast is the site of photosynthesis, the process which converts sunlight to chemical energy in the form of energy-rich carbohydrates
After 4 h treatment, soon after photosynthetic rate (Pn) reached maximum, we found M chloroplasts distributed to the anticlinal walls, parallel to the direction of the incident light (Figure 5(b))
The proportion of the total surface area of bundle sheath (BS) chloroplasts to BS cell surface area increased as irradiation time advanced, and there were significant differences in the proportion after 8 h and 12 h treatment compared to that before treatment. These results indicate that the high-intensity light induces the avoidance response of M chloroplasts in sorghum as is the case in other plant species such as Arabidopsis thaliana
Summary
The chloroplast is the site of photosynthesis, the process which converts sunlight to chemical energy in the form of energy-rich carbohydrates. A difference in the M chloroplast response under high light intensity among C4 species has been identified: M chloroplasts in maize move toward cell walls parallel to light direction whereas those in finger millet aggregate to the BS side (Maai et al, 2011; Yamada et al, 2009). The former chloroplast response is analogous to the avoidance response and the latter is considered to be a specific response in some C4 species.
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