The effect of early season drought on chlorophyll a fluorescence in sugar beet (Beta vulgaris L.)

Two field experiments were conducted in 1998 and 1999 in the Khorasan Agricultural Research Center at Mashhad with the aim to study the effect of early season drought stress on a variety of sugar beet genotypes. The irrigation of the field was withhold for a period of about 50 days in order to impose a severe drought stress at the 8 to 10 leaf stage of sugar beet crop. The drought stress was evaluated by changes in chlorophyll fluorescence, measured in the fully expanded leaves of three beet genotypes in 1998 and nine genotypes in 1999, both in stressed and non-stressed plants. The results showed that in the 1998 experiment, both maximal (Fm) and variable (Fv fluorescence were reduced during the growth period, but the reduction was greater under drought conditions than under non-stress conditions. The quantum yield of photochemistry in Photosystem II (Fv/Fm) remained unaffected under non-stress conditions, but reduced under drought stress. In the 1999 experiment, initial fluorescence (Fo) was not affected by drought stress, whereas chlorophyll content of stressed plant was greater than that of non-stressed. Thus, drought stress had no destructive effect on the initial fluorescence, presumably due to increase in chlorophyll content of stressed sugar beet. Under stress conditions, Fv3 Fm and Fv/Fm were reduced for all genotypes, but the reduction rates varied between genotypes. The genotype 7219.P129 showed the highest reduction of Fv3 Fm and Fv/Fm under drought stress conditions, that suggests the highest sensitive to drought stress. In all genotypes, Fv/Fm recovered 16 days after stress relief, although, their reactions were different. Strong correlation (R2 = 0.75) between the white sugar yield and Fv/Fm has been found, suggesting that chlorophyll fluorescence can be used as a tool for screening sugar beet genotypes for early season drought stress.