Differing Precision Irrigation Thresholds for Kale (Brassica oleracea L. var. acephala) Induces Changes in Physiological Performance, Metabolites, and Yield

Abstract Kale originates from the eastern Mediterranean and Asia Minor, being cultivated worldwide and is considered as global cash crop to high nutrients as well as cold tolerance in the fall, winter, and spring. Although kale production is gradually increasing, there is limited research on physiological and metabolism changes by different levels of irrigation water. Here, we comprehensively investigated yield, metabolism, and physiological changes by different levels of precision irrigation based on soil moisture sensor. Kales were grown at three different levels of volumetric water content (VWC) which included 0.15, 0.25, or 0.35 m3 m3, corresponding to well-watered, intermediate drought, and drought stressed conditions. There was a 22.5% increase in fresh weight when comparing the 0.15 to 0.35 m3 m3 VWC treatments. Leaf transpiration, net CO2 assimilation rate, intercellular CO2 concentration, and stomatal conductance to water vapor were significantly different between 0.15 and 0.35 m3 m3 VWC treatments. Neoxanthin and antheraxanthin were significantly decreased when comparing the 0.15 to 0.35 m3 m3 VWC treatments. Additionally, aspartic acid and glutamate were highest concentration at 0.35 VWC, implying well-watered condition has a high assimilation capacity from nitrate and ammonium in the kale plant leaf tissue. Glycine, histidine, proline, fructose, and glucose were increased as VWC decreased, indicating that these metabolites are indicators of drought stress. Linoleic acid, glucoiberin, progoitrin, sinigrin, and total phenolic content were the highest concentration in the 0.25 m3 m3 VWC, indicating moderate drought stress may increase these specific metabolites. From the pathway analysis, drought stress significantly increased alanine, aspartate, and glutamate metabolism and glycine, serine, and threonine metabolism. Deficit irrigation that induce drought stress impacted kale plants by decreasing physiological (A, E, and gs) parameters and metabolite (carotenoids, chlorophylls) concentrations, while increase stress induced metabolites such as proline and glutamate. Thus, this study utilized a comprehensive research methodology that connects morphophysiological and biochemical metabolite information and demonstrates how kale plants may integrate growth in drought conditions.