Cool Night Temperatures Alter Leaf Starch and Photosystem II Chlorophyll Fluorescence in Cotton

Cotton ( Gossypium hirsutum L.) growth and reproduction are sensitive to cool temperatures, but few studies are available on specific metabolic responses to low temperature stress. We studied the relationship between starch levels and Photosystem II (PSII) function in cotton plants grown in growth chambers to determine if low temperature stress at night alters metabolic processes during the day when temperatures are optimal. Predawn starch levels in fully expanded leaves were three to four times higher on an area basis, and PSII chlorophyll fluorescence was 43 to 54% lower in leaves of two varieties of cotton (T25 and Paymaster HS26, averages for three plants of each variety) when grown with a day/night temperature of 28/20 °C, compared with plants grown at a constant temperature of 28 °C. Cotton plants that were grown at a constant 28 °C had 4.4 (T25, n = 3) to 6.6 (HS26, n = 3) times more starch on a leaf‐area basis after exposure to one night at 20 °C. When cotton plants that were grown with a day/night temperature of 28/20 °C were exposed to one night at 28 °C, starch was depleted to levels found in control plants that were grown at a constant 28 °C. The response of chlorophyll fluorescence was reciprocal to that of leaf starch when night temperature was changed. Shading, to reduce starch in six plants of each variety grown with 20 °C night temperatures to levels found in plants grown with 28 °C night temperatures, resulted in increased PSII chlorophyll fluorescence of 200%. We conclude that a night temperature of 20 °C caused an accumulation of leaf starch that was associated with reduced PSII function, even when plants experienced optimal temperatures during the day.