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

Abstract

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.

Producing Enhanced Yield and Nutritional Pigmentation in Lollo Rosso Through Manipulating the Irradiance, Duration, and Periodicity of LEDs in the Visible Region of Light

“…Through daily morphological and physiological adaptations, e.g., leaf angle and chloroplast movement, the plant is able to adapt to the fluctuating natural environment by predicting daily changes but also by anticipating regular natural events such as dawn (Dodd et al, 2015). In addition to circadian control, some processes respond cyclically to metabolic feedback; for example, in natural conditions, CO 2 assimilation usually follows a daily pattern characterized by an initial "photosynthesis activation" at dawn, a maximum CO 2 fixation at mid-morning, and a decline from midday (midday depression) (Koyama and Takemoto, 2014;Maai et al, 2019). After the midday depression, the photosynthetic activity declines until the dark when the nocturnal process of starch consumption is under circadian oscillator control (Haydon et al, 2013).…”

Chloroplasts in C3 grasses move in response to blue-light

“…Recently, chloroplast movements have also been shown in Sorghum ( Sorghum bicolor ) (Maai et al 2019 ). The experiments concerned midday depression of photosynthesis and chloroplast positioning in excess light.…”

“…Surprisingly, for many decades chloroplast movements have not been investigated in grasses, and in particular in cereals, in spite of their crucial agricultural importance. The first reports showing rearrangement of chloroplasts in cereals analyzed finger millet and sorghum (Maai et al 2011 , 2019 ). Recently, avoidance movement in barley has also been reported (Nauš et al 2016 ).…”

High Light and High Temperature Reduce Photosynthesis via Different Mechanisms in the C4ModelSetaria viridis

“…Under natural conditions, especially at the tops of canopies, direct sunlight can be very intense and thus oversaturate the photosynthetic mechanism in C4 plants. Sorghum leaves had reduced stomatal conductance and net CO2 assimilation rates after 4 h exposure to HL mimicking nature sunlight 9 . To improve C4 crop yields, it is crucial to holistically approach how C4 plants respond to HL or HT, two of the most influential environmental factors that can compromise C4 photosynthesis.…”

“…However, the short-term transcriptional responses of C4 plants to HL remain largely unknown. In sorghum leaves, HL induced the avoidance response in M chloroplasts and the swelling of BS chloroplasts (by cross section light microscope images), but the underlying mechanisms are unclear 9 .…”