No abstract

Salicylic acid (SA) acts as an endogenous signal molecule responsible for inducing abiotic stress tolerance in plants. In this study, the role of SA in improving drought tolerance in two maize cultivars (Zea mays L.) differing in their tolerance to drought was evaluated. The plants were regularly watered per pot and grown until the grain filling stage (R2) under a rainout shelter. At stage R2, parts of the plants were treated with SA, after which drought stress was applied. Leaf samples were harvested on the 10th and 17th days of the drought. Some antioxidant enzyme activity, such as the superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), hydrogen peroxide (H 2 O 2 ) and malondialdehyde (MDA) content, was measured during the drought period. Exogenous SA prevented water loss and delayed leaf rolling in comparison with control leaves in both cultivars. As a consequence of drought stress, lipid peroxidation, measured in terms of malondialdehyde content, was prevented by SA. SA pretreatment induced all antioxidant enzyme activities, and to a greater extent than the control leaves, during drought. SA also caused a reduction in the ascorbate (ASC) and glutathione (GSH) content in two maize cultivars. The H 2 O 2 level was higher in SA pretreated plants than the controls in both cultivars. Pretreatment with SA further enhanced the activities of antioxidant enzymes and the concentrations of non-enzymatic antioxidants in the tolerant cultivar compared with the sensitive cultivar. Results suggested that exogenous SA could help reduce the adverse effects of drought stress and might have a key role in providing tolerance to stress by decreasing water loss and inducing the antioxidant system in plants with leaf rolling, an alternative drought protection mechanism.

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.

-Effects of leaf rolling (LR) on maize photosynthesis under severe drought stress were studied in two cultivars with opposite drought responses, Batem 56-55 (drought tolerant) and Batem 51-52 (drought sensitive). Drought stress and artifi cial prevention of leaf rolling (PLR) were applied at grain fi lling stage for 30 days. LR in Batem 56-55 occurred later than in Batem 51-52. Leaf water potential (Ψ leaf ) did not change in Batem 56-55 but decreased in Batem 51-52 at LR. Maximum quantum yield of photosystem II (F v /F m ), effective quantum yield of photosystem II (Φ PSII ) and electron transport rate (ETR) of the cultivars decreased during LR more signifi cantly in Batem 56-55 in comparison to Batem 51-52. The same was observed for the decrease in net photosynthetic rate (P N ), stomatal conductance (g s ), transpiration (E) and intracellular level of CO 2 (C i ). Rubisco activity and content were reduced at LR, but were less affected in Batem 56-55 than in Batem 51-52. Ear and kernel weights also decreased at LR. All parameters at PLR were more reduced than those of LR. These results implied that LR was an important and necessary mechanism protecting photosynthesis and reducing yield loss under drought stress by maintaining the leaf hydration, preventing loss of the photosynthetic pigments, sustaining the activity of PSII, keeping the stomata open, and conserving the activity of Rubisco.