Understanding and the management of spatial and temporal patterns for the soil water and its availability and deficiency for spring wheat is a critical aspect to stabilize yield and food security. Drought has negative effects on the growth and development of most crops and severely limits the wheat quality and quantity of yield (Yang et al., 2020). Whereas, excessive soil moisture also exerts negative influences on the growth and development of crops. Therefore, proper and well-organized irrigation scheduling according to climatic conditions, soil type, and the growing crop is an effective approach to overcome water deficiency for growth improvement during the different growth stages of wheat (Gautam and Pagay, 2020).
Why it is important to give attention to deficient irrigation?
The shortage of irrigation water due to industrialization, urbanization, and contamination is threatening food security all over the globe (Al-Ghobari and Dewidar, 2018). The quantification of biochemical and physiological responses in spring wheat is difficult due to the lack of novel approaches and temporal, and spatial variation (Negrão et al., 2017). The practices for opting for water-saving techniques should be focused to understand the mechanisms behind the biochemical, and physiological processes.
Factors affecting irrigation requirements
The effects of deficient irrigation and partial root-zone irrigation are also dependent on soil type, climatic conditions, specific varieties of spring wheat, and growth stages (He et al., 2013). The main mechanisms behind improved yield production and quality due to deficient irrigation, and partial root-zone irrigation are the development of drought hardiness, stimulated osmotic adjustment, enhanced signal transduction of guard cells, optimized control over the stomatal gas exchange, and reduced luxurious loss of transpiration (Yactayo et al., 2013; Parvathi et al., 2013).
Effectiveness of deficient irrigation to increase water use efficiency
Deficient irrigation is an effective irrigation approach to increase water use efficiency (WUE) and to reduce irrigation water requirements as in this way spring wheat plants can minimize water consumption by reducing soil evaporation, and leaf transpiration. Although a deficient irrigation system is helpful to reduce water consumption, it may cause physiological disorders such as poor shoot, and root growth, and alterations in root morphology, and root systems (Rahil, and Qanadillo, 2015).
Although the anatomy of plant leaves is complex, it plays a crucial functional role for overall growth and development. Water deficient conditions also reduce the level of water contents in the leaves that causes the activation of hydraulic signaling and triggers the partial closing of stomata, and significantly reduces the expansion of leaf area. The maintenance of water potential is greatly dependent on the intensity of applied water-deficient conditions, deficient irrigation, and partial root-zone irrigation (Hernandez-Santana et al., 2016).
Effects of deficient irrigation during different growth stages
Deficient irrigation during flowering, and grain filling stages is also responsible for reduced yield due to sinking capacity, reduced grain setting, oxidative damage to assimilated translocation, and photo-assimilatory machinery, and accelerated leaf senescence (Farooq et al., 2014). Physiological responses of spring wheat to the stress associated with deficient irrigation, and partial root-zone irrigation have a direct relation to the opening, and closing of stomatal rhythms, stomatal density, and fluctuation in the size of stomatal cells. Stomatal behavior of plants under deficient irrigation, and partial root-zone irrigation causes the regulation of chemical signaling responsible for providing water to the plants.
This signaling process is also affected by the abscisic acid (Pál et al., 2018). Furthermore, changes in the pH of xylem sap have stomatal density changes have also been observed under both types of deficient irrigations. There is redistribution of various inorganic ions in the compartment of leaves so that control over stomatal regulation and water loss is a difficult approach but the negative effects are significantly reduced due to defense mechanism and protective biochemical molecules and signaling (Sun et al., 2013).
Effects of deficient irrigation on spring wheat
Interaction between deficient irrigation and spring wheat planting patterns are key features affecting the growth, and development of wheat (Ali et al., 2017). Partial root-zone irrigation is a newly developed method of irrigation mainly intended to improve WUE without negatively impacting the growth and yield of spring wheat. In this way, the evaporation loss from the soil is declined by 50%. Results of various scientific studies have shown that deficient irrigation causes the stimulation of plant roots to interact and uptake more water via plant roots during drought conditions.
Whereas partial root-zone irrigation causes high water stress on side of the roots of spring wheat and does not provide water for optimal plant growth. Resultantly, there is an increased production of abscisic acid and thereby significantly increase the water use efficiency (Barideh et al., 2018). The increased production of abscisic acid causes the maintenance of water status in the plants by the partial closure of stomata to minimize the transpiration rates. Although this stomatal closure affects the photosynthetic processes there is no significant negative effect on the final yield of spring wheat (Vishwakarma et al., 2017).
Effects of deficient irrigation on wheat enzymes
The increased production of ABA also causes the triggered production of reactive oxygen species (ROS) and reduces the synthesis of cytokinin that interrupts the normal functioning of other signaling molecules. However, there is the regulation of proline production and antioxidant defense system mechanism. Resultantly the elevated level of stress on plant growth is reduced and this technique can be used to improve wheat tolerance against drought, and phenotyping plasticity (Wang et al., 2013). The production of cytokinin along with the defensive mechanism of ABA helps to tolerate drought by improving WUE, photosynthetic rates, and activation of the defense system. Cytokinin helps for the modulation of enzymatic activities such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in the leaves. Therefore, there is no major damage to the tissues, organs, and metabolism of plants (Chang et al., 2016).
Benefits of deficient irrigation
Furthermore, there is a positive relationship between stomatal closure and abscisic acid synthesis under deficient irrigation, and partial root-zone irrigation. Enhanced responses of antioxidants defense mechanism in the wheat crop have a direct association with stomatal closure and thereby helps to improve WUE and quality and quantity of grains. However, the physiological mechanism behind the positive association between cytokinin, and abscisic acid is still unknown (Batool et al., 2019). Furthermore, a synergistic relationship between stress signaling has also been confirmed by scientific findings. The activation of CAT and POD causes removal of hydrogen peroxide, whereas production of SOD caused catalysis of O2– to the dismutation of hydrogen peroxide (Ni et al., 2018).
The production of non-enzymatic substances such as monoaldehyde, proline, and soluble sugars in both roots and leaves of wheat is also helpful to reduce the damage caused by osmotic stress. The production of non-protein thiol, carotenoids, anthocyanins, and polyphenols is also helpful to decrease the osmotic potential of leaves that in turn helps to tolerate the reduced water levels (Meenakshi et al., 2013). The dry portion of the roots in the partial root-zone irrigation causes the maintenance of abscisic acid production whereas the wet portion of plant roots ensures the regular supply of water to the spring wheat plants.
The abscisic acid produced under the water-stressed conditions is carried to leaves via xylem vessels where it acts as a regulator for the osmotic adjustments. Practical analysis has reported that the use of partial root-zone irrigation is most effective to improve water use efficiency and crop yield than deficient irrigation. The water use efficiency of spring wheat is directly related to transpiration rate, stomatal conductance, and intercellular carbon concentrations (Flexas et al., 2013; de Santana et al., 2015). Moreover, these irrigation methods are also helpful to reduce the weeds population in the wheat field and are helpful to conserve nutrients and moisture.
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