Growth, Physiological Responses and Water Use Efficiency of Eggplant to Foliar Spray of L-cysteine and Calcium Lactate under Deficit Irrigation Condition

Water stress is the most prominent abiotic stress limiting agricultural crop growth and productivity. Deficit irrigation stress as a consequence of the progressive decrease in water availability has been a hot topic regarding food security during the last two decades. Growth and development of plants are influenced by reduction in turgor that results in decreased nutrient acquisition from dry soil. Due to the threat of climate change, there is a need to limit the use of water resources in arid and semi-arid climates. It is therefore important to find new approaches to avoid crop productivity losses in ‘limited fresh-water’ areas. Eggplant (Solanum melongena) is an important non-climacteric fruit grown in tropical and subtropical regions. The estimated total world production for eggplants in 2019 was 51287210 tons. Foliar application of agro-chemicals has widely been used in agriculture as a rapid, low-cost and effective way for enhancing growth and productivity of many vegetable crops under water deficit conditions. Ca is an essential macronutrient for plant growth and development, and is considered as an important intracellular messenger, mediating responses to hormones, stress signals and a variety of developmental processes. Amino acids are important bio-regulators which have promotive effects on plant growth and productivity. Cysteine is the precursor molecule of glutathione, the predominant non-protein thiol, which plays an important role in plant stress responses.

The field experiment was carried out at the Research Farm of Agriculture Faculty, University of Zanjan, Iran, during 2021. The experiment was performed using a split plot based on a randomized complete blocks design with three irrigation regimes (60, 80 and 100 %ETc) as the main plot and foliar spray of two levels of calcium lactate (CaL) (1 and 2 g L-1) and L-cystein (Cys) (0.15 and 0.5 %) as the sub-plot in three replicates. Distilled water was used as a control. Eggplant (cv. Greta RZ) seedlings were transplanted at the 4-5 leaf stage with 35 cm spacing within row and 100 cm spacing between rows. After plant establishment, eggplants were sprayed with different concentrations of CaL and Cys at 6-7 leaf stage. Second and third times of foliar application were performed at 10 and 20 days after the first spraying. Irrigation treatments were applied one week after the first spraying. Number of branch per plant, plant height and leaf area were measured as growth parameters in harvest stage. Total chlorophyll and carotenoid contents, proline, leaf relative water content, stomatal conductance and electrolyte leakage were measured as physiological characteristics. Also plant fruit yield and water use efficiency (WUE) were evaluated.

As the results showed, plant growth, physiological indices and WUE significantly affected by deficit irrigation. Water deficit stress significantly reduced plant length, leaf area and chlorophyll content, and increased carotenoid content. Decreased plant growth may be due to limited cell division, cell enlargement due to turgor loss, and inhibition of various growth metabolites. Foliar spray of CaL and Cys improved plant growth, fruit yield and physiological indices. Leaf relative water content (LRWC) is among the main physiological criteria affecting plant water relations and have been used for assessing drought tolerance. Based on the findings, deficit irrigation caused a significant reduction in LRWC and stomatal conductance, which is due to the reduction of leaf water potential and the decrease of root water absorption rate under drought conditions. The highest value of LRWC and stomatal conductance were obtained in plants sprayed with CaL 2 g L-1 under irrigation 100 %ETc. Free proline is one of the most important osmolytes whose accumulation is a common physiological response to drought stress in higher plants and is probably associated with osmotic regulation and membrane stability under stress condition. WUE, the physiological parameter of crop, describes the relationship between plant water consumption and dry matter production. The proline content and WUE increased under deficit irrigation treatments; in particular with sever deficit irrigation (60 %ETc). The maximum proline content and WUE were observed in plants treated with CaL 2 g L-1 and Cys 0.5 % under deficit irrigation of 60 %ETc. The results showed that deficit irrigation led to a significant increase in electrolyte leakage compared to control, and foliar application of CaL and Cys decreased electrolyte leakage under normal and deficit irrigation conditions. Water deficit stress caused significant reductions in yield. The highest fruit yield was obtained under irrigation of 100 %ETc along with application of CaL 2 g L-1 and Cys 0.5 %.

Water deficit has been shown to adversely affect plant growth and fruit yield. Foliar application of CaL and Cys positively improved plant growth, physiological traits and fruit yield, as well as WUE. According to the results, application of CaL 2 g L-1 or Cys 0.5 % can be proposed to improve growth, fruit yield and WUE of eggplant.