Effect of Weather Conditions on Phytochemical Profiles in Organically Grown Cowpea (Vignaunguiculata L. Walp)

Cowpeas are prone to abiotic (heat and drought) and biotic (pathogens and insect pests) stresses, with the former representing the predominant challenge, causing poor growth and reduced yield globally under changing climatic conditions. Cowpea can synthesize phytochemicals to respond to these stresses; however, there is limited information on the impact of weather on phytochemical biosynthesis in the cowpea phyllosphere. Phytochemical profiles were determined via chromatographic and spectrophotometric analyses of leaf samples from six cowpea varieties grown during 2020–2021. A total of 10 fatty acid methyl esters (FAMEs) and 62 diverse metabolites were identified across varieties and seasons, with higher levels in 2020 under elevated temperatures and rainfall. The Queen Anne (QA) variety exhibited the maximum concentration of elaidic oleic acid (cis + trans), behenate, lignocerate, methyl laurate, and methyl palmitate (with the highest concentration at 258.415 µg/mL), and the Whippoorwill Steele’s Black (WP) variety predominantly exhibited diverse phytochemicals with high peak areas during 2020, including phenolic acids, phytohormones, alkaloids, flavonoids, and amino acids. While higher overall increases were observed in 2020, some compounds and varieties peaked in 2021, including FAMEs in the Colossus (CL) variety and other phytochemicals in QA. Flavonoid, flavone, and flavonol biosynthesis; phenylalanine metabolism; and tyrosine metabolism were significantly affected, leading to the accumulation of metabolites. Understanding plant–climate interactions will help farmers with variety selection and planting decisions. This study suggests that further research on the temperature mechanism for the biosynthetic pathways of these metabolites in the screened cowpea varieties is required.