COOL NIGHT TEMPERATURES ALTER LEAF STARCH AND PHOTOSYSTEM-II CHLOROPHYLL FLUORESCENCE IN COTTON

Cotton (Gossypium hirsutum L.) growth and reproduction are sensitive t o cool temperatures, but few studies are available on specific metabol ic responses to low temperature stress. We studied the relationship be tween starch levels and Photosystem II (PSII) function in cotton plant s grown in growth chambers to determine if low temperature stress at n ight alters metabolic processes during the day when temperatures are o ptimal. Predawn starch levels in fully expanded leaves were three to f our times higher on an area basis, and PSII chlorophyll fluorescence w as 43 to 54% lower in leaves of two varieties of cotton (T25 and Payma ster HS26, averages for three plants of each variety) when grown with a day/night temperature of 28/20-degrees-C, compared with plants grown at a constant temperature of 28-degrees-C. Cotton plants that were gr own at a constant 28-degrees-C had 4.4 (T25, n = 3) to 6.6 (HS26, n = 3) times more starch on a leaf-ama basis after exposure to one night a t 20-degrees-C. When cotton plants that were grown with a day/night te mperature of 28/20-degrees-C were exposed to one night at 28-degrees-C , starch was depleted to levels found in control plants that were grow n at a constant 28-degrees-C. The response of chlorophyll fluorescence was reciprocal to that of leaf starch when night temperature was chan ged. Shading, to reduce starch in six plants of each variety grown wit h 20-degrees-C night temperatures to levels found in plants grown with 28-degrees-C night temperatures, resulted in increased PSII chlorophy ll fluorescence of 200%. We conclude that a night temperature of 20-de grees-C caused an accumulation of leaf starch that was associated with reduced PSII function, even when plants experienced optimal temperatu res during the day.