Effect of silicon and cycocel application on yield, yield components and biochemical traits of two wheat (Triticum aestivum L.) cultivars under drought conditions

Wheat is a major staple food crop in the world. Although all abiotic stresses adversely affect the wheat growth and production, water scarcity imposes the most severe effects on this crop. Drought stress is also believed to affect the uptake, transport and accumulation of key inorganic nutrients in plants. Silicon occurs abundantly in soils, but in the field of plant growth the other inorganic elements for plant growth are more important than Silicon. However, Silicon plays an effective role in plants under stressful conditions. Silicon entails useful effects in plants under water-deficit treatments, with respect to drought-induced regulation of metabolic processes and water relations. Of several plant growth regulators, Cycocel (CCC) is believed to be very effective in masking the adverse effects of different abiotic and biotic stresses on crops as well as being an essential component of the signal-transduction pathways operating in plants exposed to environmental cues including drought stress. Exogenously applied CCC is believed to affect absorption and transport of nutrients, stomata regulation, growth and photosynthetic rate, chlorophyll synthesis and transpiration. The literature has little information on the role of Silicon and CCC applied in combination in alleviating drought-induced injurious effects on plants. Therefore, in the present study, we appraised the effects of exogenous Silicon and CCC applied individually or in combination on wheat growth and grain yield under water-deficit conditions.

This study was carried out in Tiran province, Isfahan, Iran during 2018-2019, as a split-split plot design in randomized complete block with three replications. The main plots considered irrigation regimes (full irrigation and 40% field capacity irrigated), sub-plots considered Silicon and cycocel application (control, 3.6 g l-1 Silicon, 210 mg l-1 Cycocel, and 3.6 g l-1 Silicon + 210 mg l-1 Cycocel) and sub-sub plots considered two wheat cultivar (Sirvan and Chamran). All measurements based on fresh plant samples were done before the grain-filling stage. Measurements included relative water content, soluble sugars and soluble proteins; activities of peroxidase, ascorbate peroxidase, catalase, superoxide dismutase, levels of hydrogen peroxide, malondialdehyde, concentrations of Ca, K and Mg and leaf water potential. At maturity, yield and yield components were measured. Statistical analysis of data was performed using the MINITAB statistical software.

Drought stress caused a considerable reduction in yield and yield components and relative water content of both cultivars. Application of Silicon and cycocel effectively improved these parameters in water-deficit treatments. Moreover, water-limited conditions markedly promoted the activities of key antioxidant enzymes including peroxidase, ascorbate peroxidase, catalase and superoxide dismutase, while enhancing the accumulation of soluble sugars, potassium, magnesium and calcium in leaf tissues. Application of Silicon and cycocel further enhanced the activities of the key antioxidant enzymes and accumulation of osmolytes, and decreased the levels of H2O2 and malondialdehyde in drought stressed plants. Synergistic effects of Silicon + cycocel application on yield and physiological parameters were apparent compared with Silicon or cycocel applied separately. Water-stress alleviation and yield improvement in the wheat cultivars by Silicon and cycocel application was attributable to partly improved osmotic adjustment and antioxidant activity as well as to more favorable water status under stress conditions.

Foliar application of Silicon, CCC and especially the combination Silicon + CCC, markedly improved grain yield and yield components of the two wheat cultivars under water-deficit. In Silicon, CCC and Silicon + CCC treatments, grain yield was 15.63%, 16.60% and 24.32% higher respectively, than with no foliar application under water stress in cv. Chamran, and 10.25%, 16.02% and 19.25% higher in cv. Sirvan. The results of the study highlight the role of Silicon and CCC application in regulating water-stress response of wheat, suggesting that Silicon and CCC are involved in physiological activities. These results showed positive effects of Silicon and CCC in terms of increased antioxidant activity as well as relative water content and leaf water potential. In addition, Silicon and CCC stimulated the active accumulation of some osmolytes in leaves of water-stressed wheat plants, which suggests enhanced osmoregulation ability. The synergistic effects of Silicon + CCC application on yield and physiological parameters were greater than of Silicon or CCC applied separately. Therefore, proper application of Silicon and CCC might result in increased production of wheat, particularly in dryland areas.