The Changes in Yield Response Factor, Water Use Efficiency, and Physiology of Sunflower Owing to Ascorbic and Citric Acids Application Under Mild Deficit Irrigation

The predicted global warming and climate changes indubitably has adverse effects such as drought, salinity and nutrient deficiency on growth and productivity of economic crops (Saudy and Mubarak 2015; Abd El-Mageed et al. 2022). Such effects are most pronounced in arid and semi-arid regions, which suffer from water scarcity (Souri and Hatamian 2019). Accordingly, irrigation strategies that ensure the efficient and rational use of water and nutrients must be adopted (Saudy and El-Bagoury 2014; Saudy and El-Metwally 2019, 2022; Saudy et al. 2020a). In this respect, deficit irrigation strategy could save water and increase water use efficiency (WUE), resolving the contradiction of water needs and supply in semi-arid regions (Cui et al. 2009). Moreover, several studies proved that deficit irrigation had significant effects on water saving and increase in WUE, clarifying the priority of deficit water tactic in crop irrigation programs under water limitation in arid environment as in Egypt (El-Bially et al. 2018; El-Metwally and Saudy 2021a; El-Metwally et al. 2022a; Saudy et al. 2022b). However, using water less than normal represents abiotic stress (drought) on crop plants. Decrease in leaf water potential, promoting stomatal closure, decrease in leaf photosynthesis, and weakness in nutrient uptake are the most physiological phenomena associated with water deficit (Bresson et al. 2015; Yan et al. 2016; El-Metwally et al. 2021; Abd-Elrahman et al. 2022; Makhlouf et al. 2022). Decrease in sunflower yield was reported by Saudy et al. (2021a); El-Bially et al. (2022a, b) as 22.4–42.8% owing to water deficit stress.

Sunflower (Helianthus annuus L.) is one of the main oil crops in the world. Among oilseed crops, sunflower is the fifth most cultivated annual crop (FAO 2017). Sunflower seed has high oil (36–52%) and protein (28–32%) contents (Rosa et al. 2009) and extracted oil has low cholesterol and high unsaturated fatty acids (Qahar et al. 2010). Moreover, sunflower can adapt to different climatic and soil conditions (Kaleem et al. 2011).

Therefore, in particularly economically valuable plants, many approaches have been employed to induce stress tolerance with enhancing the crop growth and development (Saudy et al. 2018, 2021b). Ascorbic acid (ASC) is a plant nonenzymatic antioxidant and mediates biotic and abiotic stress, since it is the first line of plant defense against oxidative stress (Sharma et al. 2019). Since ASC is mostly a substrate of ascorbate peroxidase, an essential enzyme of the ascorbate-glutathione pathway, it acts as a protector against the oxidative stress by removing several free radicals (Bilska et al. 2019; Sharma et al. 2019).

Citric acid (CA) is a weak organic acid found in many fruits and is used as preservative agent in human food being an antioxidant. Inside the plant cell, citric acid plays an important role in the intermediary metabolism, being a component of the tricarboxylic acid cycle or Krebs cycle (Omar et al. 2018). In some plant species as cotton (Gebaly et al. 2013) and bean (El-Tohamy et al. 2013), CA ameliorated the adverse effect of drought.

The current study hypothysized that sunflower plants have different significant response to both ASC and CA under drought conditions. Therefore, the main objective of this article is to evaluate the influence of different levels of ascorbic and citric acids on physiological and agronomic traits, yield response factor, and irrigation water use efficiency of sunflower under mild water stress compared to full irrigation.

Drought as abiotic stress is considered as one of the most important consequences of climate change, which is an obstacle to crop productivity. Drought stress has tremendous impacts on plants, which could involve imbalance, stress damage, growth delay, and low availability of nutrients (Mubarak et al. 2021). Several chemical compounds are exogenously applied to improve grwoth and health of plants under both normal or stress conditions (Noureldin et al. 2013; Saudy et al. 2020b, 2022a; El-Metwally and Saudy 2021b; Elgala et al. 2022; El-Metwally et al. 2022b). The current study investigated some drought response mechanisms in sunflower under application of ASC and CA. As presented in Table 2, decreasing irrigation water by 15% than normal led to reduction in chlorophyll content and increase in proline content. Such findings are in accordance with those found by El-Bially et al. (2018) and Mostafa (2020) who reported that photosynthetic pigments of sunflower leaf (chlorophyll a and b) had large decline, but proline content increased due to drought stress. It has been reported that reactive oxygen species (ROS) overproduced in plant cells exposed to environmental stresses (Hatamian et al. 2020; Souri et al. 2019) can damage membrane and other essential macromolecules like photosynthetic pigments, proteins, DNA and lipids. Biochemical damage was measured due to production of ROS which eventually led to poor growth and metabolic damage of the plant (Zafar et al. 2015). High ROS levels cause serious dysfunction in many processes such as hormonal equilibrium, gene expression, pathways of signaling, photosynthetic efficiency, protein inactivation, inhibit the action of multiple enzymes involved in metabolic pathways and decrease grain yield, resulting in lipid and DNA oxidation (Huang et al. 2012; Choudhury et al. 2017). It has been proved that plant growth regulators and osmoprotectants like free amino acids, sugars, and polyamines have protective roles against drought (Chan et al. 2013). In this regrad, proline accumulation in stressed plants has been well established to play a key role as osmoregulation defense mechanism, leading to prevent the cell osmotic pressure and survive in the extreme conditions (Souri and Bakhtiarizade 2019; Souri and Tohidloo 2019). In this connection, Manivannan et al. (2007) reported that water stress caused reduction in activity of proline oxidase so, proline content increased. Moreover, a large proportion of the drought-responsive proteins are involved in photosynthesis. Rubisco (Ribulose bisphosphate carboxylase) is a vital enzyme associated with carbon fixation (Feller et al. 2008). Rubisco similarly showed decreasing abundance under drought stress, indicating that drought negatively affects the key protein of the photosynthetic apparatus (Wang et al. 2017).

Since drought affects plant physiology as well as nutrient availability and accumulation (Mubarak et al. 2021; Salem et al. 2021, 2022), crop traits and its potentiality to expliot irrigation water were influenced (Tables 2 and 3). Drought stress caused significant reduction in growth, yield traits as well as IWUE because irrigation is the key factor for obtaining high yield (FAO 2010). Herein, deficit water significantly reduced root length, stem length, total leaf area, fresh and dry weight of sunflower plants (Manivannan et al. 2007). Thus, drought stress has been shown to significantly decrease plant height (Sincik et al. 2013), stem diameter (Saeed et al. 2015), leaf area index (Furtado et al. 2016), head weight (Ibrahim et al. 2016), seed yield (Soleimanzadeh 2012), oil yield (Kassab et al. 2012) and water use efficiency (El-Bially et al. 2018). These reductions could be due to lowering content of chlorophyll in leaves (Table 2), which leads to a decrease in photosynthesis rate and the dry matter accumulation.

On the other site, plant cells have many effective defense mechanisms to remove the harmful effect of ROS (Birben et al. 2012). ASC and CA are non-enzymatic antioxidants that play an important role to protect plants from oxidative damage by scavenging and sweep of ROS (Prasad and Upadhyay 2011; Tahjib-ul-Arif et al. 2021). Therefore, enhancements in leaves content of chlorophylls a and b were obtained owing to the application of antioxidants (Table 2). Previous studies have shown that foliar application of ASC or CA significantly increased leaf pigmints of stressed plants (Amin and Ismail 2015; El-Mantawy 2017; Farid et al. 2017, 2019). On the other hand, proline content in leaves was reduced when ASC and CA were sprayed under drought stress as compared to control (see Table 3),. Proline stabilizes subcellular structures and molecules experiencing osmotic stress conditions by working as a molecular chaperone, maintaining the integrity of proteins. Previous studied have shown that foliar application of ASC or CA significantly decreased proline content in leaves (El-Bially et al. 2018; Mostafa 2020). Since antioxidants alleviate the effects of stress, ASC and CA significantly decrease leaves proline content as compared to control treatment (Mostafa 2020). CA foliar application improved the germination rate and root weight of sunflower plant by improving the activities of several antioxidants enzymes including superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase (Ondrasek et al. 2019). Using ASC and CA as foliar applications, especiallly with higher rates (ASC₃₀₀ and CA₅₀₀), counteraceted the harmful effect of drought stress and significantly increased growth and yield attributes. These results are in consistance with the findings of El-Mantawy (2017). Since ASC is a preventive agent and the important compound working nonenzymatic system reinforcement against ROS in plant (Qian et al. 2014), foliar application of ASC enhanced physiological and biochemical traits, productivity and water use efficiency of sunflower under abiotic stress (Saudy et al. 2021a).

Despite ASC and CA alleviated partially the adverse impacts of deficit water, sunflower plants still sensetive to low water supply. In this regard, Fig. 3 showed that Ky values exceeded the unit (higher than 1.0) clarifing the sensetivity of sunflower to drought. Ky indicates the relationship between relative yield and relative crop water consumption (Lovelli et al. 2007; Singh et al. 2010), with values > 1 indicating that the crop is very sensitive to water stress, values < 1 indicating that the crop is more tolerant to water stress, and values of 1 indicating that the relative yield reduction is equal to the relative water use reduction (Steduto et al. 2012). Accordingly, our findings refer to that sunflower plants require additional practices to increase their tolerance to deficit water stress. The value of Ky was affected by climate and soil conditions, irrigation method and applied amount of irrigation water (Aydinsakir et al. 2021). Candogan and Yazgan (2016) reported that Ky value was greater than 1 (1.21) and stated that soybean was sensitive to water stress.

It could be concluded that reduction in the economic product of sunflower (seed yield) could not completely be compensated by application of antioxidants, i.e. ascorbic and citric. However, the adverse effects of drought were partially alleviated with antioxidant especially ascorbic acid. Also, since sunflower markedly responded to increasing the antioxidant level up to 300 mg L⁻¹ for ascorbic acid and 500 mg L⁻¹ for citric acid, this opens the field to further studies to examine higher rates of each under drought stress in sunflower. Moreover, other agronomic practices should be tested and adopted along application of antioxidant since sunflower still sensetive to drought as proved from measuing of yield response factor.