Light stress-induced chloroplast movement and midday depression of photosynthesis in sorghum leaves

• Published in: Plant production science Vol. 23; no. 2; pp. 172 - 181
• Main Authors: Maai, Eri, Nishimura, Kazusa, Takisawa, Rihito, Nakazaki, Tetsuya
• Format: Journal Article
• Language: English
• Published: Kyoto Taylor & Francis 02.04.2020; Taylor & Francis Ltd; Taylor & Francis Group
• Subjects: Avoidance; Avoidance behavior; Chloroplast movement; Chloroplasts; Diurnal; diurnal variation; Environmental stress; high light; Irradiation; Leaves; Light; Light intensity; Luminous intensity; Mesophyll; midday depression of photosynthesis; Natural lighting; photoinhibition; Photosynthesis; Photosystem II; Radiation; Sheaths; solar radiation; Sorghum; starch; Stomata; Stomatal conductance; stomatal movement; temperature; Vapor pressure; vapor pressure deficit; Water damage; Water vapor
• ISSN: 1343-943X; 1349-1008; 1349-1008
• DOI: 10.1080/1343943X.2019.1673666

Plants 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 (P n ) showed a diurnal pattern, and midday depression in P n was observed at 3.8 h of irradiation. Depression in P n was attributed to stomatal limitation because the decrease in P n 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.