Molecular Soil Biology 2024, Vol.15, No.5, 227-235 http://bioscipublisher.com/index.php/msb 232 4.3 Effect of water stress on yield Drought condition negatively impacted bulb development, resulting in production of significantly fewer and smaller bulbs than wet condition (Table 2). This shows that water availability plays a crucial role in bulb formation and development in A. cepa. In an earlier experiment, Shu-aib et al. (2023) found that morning and evening daily irrigation of onions produced the highest yield of 28.68 tons/ha, followed by morning daily with 28.12 tons/ha. In morning and evening alternate days however, the lowest yield of 25.80 tons/ha was recorded. According to research results, regular watering of is necessary for the growth of onion, as water stress at particular stages can adversely impact on growth resulting in reduction in yield (Ortola and Knox, 2015; Shu-aib et al., 2023). Higher yield under regular watering than water deficit has also been confirmed by Pejic et al. (2011) and Temesgen et al. (2018) who stated that there was an improvement in the yield of onions, particularly with regards to the marketable bulbs as regular supply of water to the onion plants (morning and evening) resulted in improved growth and yields of the onion bulbs. Moges (2021) indicated that variation in the level of irrigation water application had a significant effect on onion growth and yield. Similarly, Tolossa (2021) observed that onions with treatments of deficit irrigation and skipping of irrigation in one growth stage had a lower yield than those that had full irrigation. They recorded the highest total onion yield from a control treatment (full irrigation). Deficit by 75% and above resulted in the highest yield reduction. Islam and Takagaki (2012) likewise reported negative effect on garlic bulb development under different moisture regimes resulting in smaller bulb sizes. Likewise, onion could sustain little water deficits and its bulb yield declined with increasing drought (Wakchaure et al., 2018). Yield reduction might be due to water deficit as a result of early maturity without attainment of full growth potential. This concurs with the results of Tolossa (2021) who observed that onion plants that received an optimum amount of irrigation water or a slightly less amount, had prolonged days to maturity whereas those, which received only one- fourth of full irrigation requirement throughout the growing season, had maturity earlier. This emphasises the importance of managing water availability for optimal bulb formation and development in onion cultivation. Islam and Takagaki (2012) likewise reported negative effect on garlic bulb development under low moisture regimes resulting in smaller bulb sizes. Additionally, due to drought conditions on a worldwide scale, about 21% decline in the yield of Triticum aestivumL. and 40% decline in the yield of Zeamays L. were recorded (Daryanto et al., 2016; Zhang et al., 2018). It was postulated that while encountering drought stress, plants seize more NaCl in the leaf, lowering its osmotic potential and reducing the root hydraulic conductance and leaf tissue (Vysotskaya et al. 2010; Bhat et al. 2020). Overproduction of reactive oxygen species (ROS), caused by decrease in CO2 assimilation rates and excessive light absorption is another common occurrence under this condition, causing lipid peroxidation, DNA mutation, and cell damage (Kumar et al., 2019). Hence, drought conditions can severely affect the overall growth and yield of plants, increase leaf and flower abscission, frequent senescence and cell necrosis (Ahmed et al., 2015; Sahin et al., 2018). Under soil water deficiency, cotton yield was reduced as a result of reduced canopy absorption of photosynthetically active radiation, decreased efficiency in the use of radiation, and reduced harvest index (Earl and Davis, 2018). Waterlogged condition induced detrimental effect on bulb production bringing about fewer and smaller bulbs than other conditions. This has been corroborated by earlier records of Grzebelusi et al. (2017) on bulb size of A. sativum. Likewise, grain yield of maize was reported to be reduced by waterlogging (Huang et al., 2022; Hu et al., 2023). Pod yield in Arachis hypogaea also decreased under waterlogging (Zeng et al., 2023). In a recent research, grain yield of maize was reportedly reduced by 29.1% by waterlogging through decreased grain weight and numbers (Yang et al, 2024). Waterlogging is known to significantly reduce crop yield, causing plants to show chlorosis and necrotic spots on leaves and also reduces dry matter accumulation, leaf catalase activity and ultimate yield. It was stated that in plants, oxygen deficiency is the first response to waterlogging limiting growth and ultimately lead to root death and stomatal closure and then to a restrained decrease in gas exchange limiting photosynthesis (Kirnak et al., 2002).
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