Effect of fusaric acid on biomass and photosynthesis of water- melon seedlings leaves

Wilt disease of watermelon caused by Fusarium oxysporum f. sp. niveum in long-term monoculture system has been much often found particularly in protected land cultivation system worldwide. Fusaric acid (FA), a fungal phytotoxin, was extracted from F. oxysporum f. sp. niveum isolated from diseased watermelon plant. The influence of FA on the biomass and leaves photosynthesis of watermelon seedlings was investigated in a laboratory study. Results obtained are listed as follows. Plant height, fresh weight, main root length and lateral roots of watermelon seedling exposed to FA (400 mg·L-1) for 12 hours and then grown in ordinary condition for nine days were decreased by 23.0%, 23.1%, 23.6% and 33.6% compared with control respectively. Cotyledons were wilted to death completely and necroses occurred on the first true leaf and the upper leaves had become crinkled and chlorosis, which was a typical symptom of wilting disease of watermelon. And thus physiological functions were inhibited and damaged strongly. Net photosynthesis rate, stomatal conductance, intercellular CO2 concentration, and chlorophyll (SPAD reading) of the seedlings decreased with the increasing of concentrations of FA and duration of exposure to FA. Compared with control, stomatal conductances of leaves treated with FA were declined by 87.4-98.4% and net photosynthesis rates of leaves were decreased by 51.8-92.1%. Intercellular CO2 concentration of the seedlings treated with FA was reduced by 66.6 -94.8%. Chlorophyll content in the leaves of the seedlings was decreased by 42.7-72.3%. Transpiration rate fell down to 86.8-96.3%. It was concluded that FA strongly reduced the chlorophyll content of watermelon seedlings leaves resulting to heavy suppression of leaf photosynthesis, which therefore affected seedlings growth finally leading to leaf wilting and necrosis.