School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemImpact of Silicon on tolerance mechanisms of wheat (Triticum aestivum L.) under drought and heat stress environments( 2022)Bread wheat (Triticum aestivum L.) is widely cultivated and amongst the major staple food crops of the world. With a total grain production of around 760 million tonnes in 2020, the crop provides ~ 20 percent of the total dietary calories and protein to the human population around the world. However, global wheat productivity faces challenges due to climatic adversities, which are becoming more acute in most of the world's established agricultural regions, raising concerns for future food security. Abiotic stress factors such as drought and heat are the primary causes of crop grain yield reduction worldwide, and the frequency of their concomitant effects has increased in the semi-arid wheat belts of the world. Plant available silicon (Si) has been widely reported for its beneficial effects on plant development, productivity and attenuating physiological and biochemical impairments caused by various abiotic stresses. This research work investigated the impact of Si application on the morphological, physiological, and biochemical mechanisms in contrasting bread wheat cultivars at critical growth stages under individual and combined drought and heat stress. Firstly, a preliminary screening experiment was conducted to categorize 46 wheat genotypes, mostly from Australia, for terminal drought and heat stress tolerance. The stress tolerance level of each genotype was determined on the basis of morphological and physiological traits, including rapid, non-destructive infrared thermal imaging for computational water stress indices (Tc, CTD, Ig, Tdry, Twet, and CWSI). Multivariate data analysis on significant traits was performed to group drought and heat stress-tolerant and susceptible genotypes. Based on their overall performance, this study identified the top ten best and five lowest-performing genotypes for drought and heat stress tolerance. After identifying the drought and heat tolerance level of wheat cultivars, a series of experiments were conducted to evaluate the role of Si on drought and heat stress susceptible and tolerant wheat cultivars at critical growth stages. In this series, to evaluate the role of Si on the wheat root system and canopy physiology under drought stress, an experiment was conducted with two contrasting bread wheat cultivars (RAC875, drought tolerant; Kukri, drought susceptible) in the glasshouse. Results showed that compared with control (drought and no Si), Kukri had a significant increase in primary root length (PRL, 44 percent) and lateral root length (LRL, 28.1 percent) with Si treatment under drought stress compared with RAC875 having a significant increase in PRL (35.2 percent), but a non-significant increase in LRL. An increase in the wheat root system positively impacted the canopy physiology (photosynthesis, stomatal conductance, and transpiration) and computational water stress indices (Tc, CWSI, CTD, Ig) in Kukri and RAC875 under drought stress. These results showed that Si has the potential to influence below-ground traits, which regulate the moisture uptake ability of roots for a cooler canopy in tolerant and susceptible wheat cultivars under drought stress. Subsequently, a comparative study of drought tolerant (RAC875) and drought susceptible (Kukri) wheat cultivars investigated the impacts of pre-sowing Si treatment in attenuating the physiological and biochemical disruptions caused by pre-anthesis drought stress. The results showed, compared to the controls (drought and no Si), Si application significantly improved the relative water content (RAC875, 10.8 percent; Kukri, 18.1 percent), chlorophyll content (RAC875, 8.7 percent; Kukri, 12.7 percent), and chlorophyll fluorescence (RAC875, 10.1 percent; Kukri, 22.3 percent) in Si treated plants under drought stress. Similarly, the concentrations of various osmolytes and antioxidants increased with Si treatment in drought tolerant and susceptible cultivars under drought stress. Results showed that the impact of a Si-induced percent increase in physiological and biochemical traits was higher in Kukri (susceptible wheat cultivar) than tolerant wheat cultivar (RAC875) under pre-anthesis drought stress. Overall, these results showed that Si has the potential to enhance plant morphological, physiological, and biochemical traits and alleviate oxidative damage by improving antioxidant defense mechanisms, both in tolerant and susceptible wheat cultivars, during pre-anthesis drought stress conditions. Following this, another experiment was conducted to study the impact of pre-sowing Si treatment on the individual and the cumulative effects of drought and heat stress during later growth stages of contrasting wheat cultivars (drought and heat stress-tolerant, RAC875, Excalibur, ECH957, RAC622; drought and heat stress-susceptible, Kukri, CM59443). Results showed that Si treatment significantly improved various stress-affected morphological, physiological, and biochemical traits, including grain yield (tolerant wheat cultivars, > 40 percent; susceptible wheat cultivars, > 31 percent) and yield components. The highest averaged Si-induced increase in thousand-grain weight across all the stress treatments was observed in Excalibur (8.6 percent), followed by Kukri (6.9 percent) and CM59443 (4.9 percent). With Si treatment, osmolytes concentrations increased significantly by > 50 percent in tolerant and susceptible wheat cultivars. Similarly, computational water stress indices also improved with Si treatment under drought, heat and drought-heat combined stress in susceptible and tolerant wheat cultivars. The study concludes that Si treatment has the potential to mitigate the detrimental effects of individual and combined stress of drought, heat, and drought-heat combined stress at early grain-filling stages in susceptible and tolerant wheat cultivars in a controlled environment. Finally, to fully understand the role of Si under natural drought and heat-stressed field conditions, a field experiment was conducted at International Maize and Wheat Improvement Center (CIMMYT), Mexico. The study aimed to determine the effects of Si application on the performance of contrasting wheat cultivars (five checks, eight tolerant, and three susceptible to drought stress) in response to terminal drought and heat stress under field conditions. Results showed a significant (p < 0.05) Si effect on most of the measured agronomic and physiological traits, including improved grain yield (p < 0.01) under drought and heat-stressed environments. Relative grain yield gain with Si application among susceptible cultivars ranged from 7.8 percent to 61.4 percent, compared with 4 to 44 percent among the tolerant cultivars under drought and heat-stressed conditions. A significantly (p < 0.05) lower mean canopy temperature was observed in Si-treated plots compared with the control for both tolerant and susceptible cultivars. The stay-green phenotype at the mid-grain-filling stage, which was estimated using relative NDVI decay and relative rate of flag leaf greenness decay (based on SPAD measurements), was significantly (p < 0.01) enhanced in tolerant (9.7 percent) and susceptible cultivars (6.3 percent). Based on these findings, it can be concluded that Si application under field conditions significantly improved various agronomical and physiological traits both under drought and heat stress. Overall, the findings from the thesis revealed that Si has the potential to mitigate critical growth stage individual and combined effects of drought and heat stress in wheat cultivars through improved morphological, physiological, and biochemical attributes. The findings also revealed that Si application has the potential to enhance the tolerance level of susceptible wheat cultivars under critical growth stage drought and heat stress. This study suggests that Si addition as a nutritional element can potentially be a sustainable management strategy to mitigate individual and combined effects of drought and heat stress on susceptible and tolerant wheat cultivars in the rainfed cropping system.