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Salicylic acid (SA) is one of the important signal molecules modulating plant responses to environmental stress. In this study, the effects of exogenous SA on leaf rolling, one of drought avoidance mechanisms, and antioxidant system were investigated in Ctenanthe setosa during long term drought stress. The plants were subjected to 38-day drought period and they were treated with or without SA (10 -6 M) on the 25th, 27th and 29th days of the period. Leaf samples were harvested on the 30th, 34th and 38th days. Some antioxidant enzyme activities (superoxide dismutase, catalase, ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione reductase), reactive oxygen species (hydrogen peroxide and superoxide) and lipid peroxidation were determined during the drought period. Treatment with SA prevented water loss and delayed leaf rolling in comparison with control leaves. Exogenous SA induced all antioxidant enzyme activities more than control leaves during the drought. Ascorbate and glutathione, a-tocopherol, carotenoid and endogenous SA level were induced by the SA treatment. Levels of reactive oxygen species were higher in SA treated plants than control ones on the 34th day. Their levels on the 38th day, however, fastly decreased in SA treated plants. SA treatment prevented lipid peroxidation while the peroxidation increased in control plants. The results showed that exogenous SA can alleviate the damaging effect of long term drought stress by decreasing water loss and inducing the antioxidant system in the plant having leaf rolling, alternative protection mechanism to drought.

Hydrogen peroxide (H 2 O 2 ) functions as a signal molecule in plants under abiotic and biotic stresses. Leaves of detached maize (Zea mays L.) seedlings were used to study the function of H 2 O 2 pretreatment in osmotic stress resistance. Low H 2 O 2 concentration (10 mM) which did not cause a visual symptom of water deficit (leaf rolling) was applied to the seedlings. Exogenous H 2 O 2 alone increased leaf water potential, endogenous H 2 O 2 content, abscisic acid (ABA) concentration, and metabolite levels including soluble sugars, proline, and polyamines while it decreased lipid peroxidation and stomatal conductance. Osmotic stress induced by polyethylene glycol (PEG 6000) decreased leaf water potential and stomatal conductance but enhanced lipid peroxidation, endogenous H 2 O 2 content, the metabolite levels, and ABA content. H 2 O 2 pretreatment also induced the metabolite accumulation and improved water status, stomatal conductance, lipid peroxidation, ABA, and H 2 O 2 levels under osmotic stress. These results indicated that H 2 O 2 pretreatment may alleviate water loss and induce osmotic stress resistance by increasing the levels of soluble sugars, proline, and polyamines thus ABA and H 2 O 2 production slightly decrease in maize seedlings under osmotic stress.

Photochemical efficiency of PSII of Ctenanthe setosa was investigated to understand the photosynthetic adaptation mechanism under drought stress causing leaf rolling. Stomatal conductance (g s ), the levels of photosynthetic pigments and chlorophyll (Chl) fluorescence parameters were determined in leaves that had four different visual leaf rolling scores from 1 to 4, opened after re-watering and mechanically opened at score 4. g s value gradually decreased in adaxial and abaxial surfaces in relation to scores of leaf rolling. Pigment contents decreased until score 3 but approached score 1 level at score 4. No significant variations in effective quantum yield of PSII (Ф PSII ), and photochemical quenching (q p ) were found until score 3, while they significantly decreased at score 4. Non-photochemical quenching (NPQ) increased at score 2 but then decreased. After re-watering, the Chl fluorescence and other physiological parameters reached to approximately score 1 value, again. As for mechanically opened leaves, g s decreased during drought period. The decrease in adaxial surface was higher than that of the rolled leaves. NPQ was higher than that of the rolled leaves. Ф PSII and q p significantly declined and the decreases were more than those of the rolled leaves. In conclusion, the results indicate that leaf rolling protects PSII functionality from damage induced by drought stress.