Phthalanilic Acid with Biostimulatory Functions Affects Photosynthetic and Antioxidant Capacity and Improves Fruit Quality and Yield in Cowpea ( Vigna unguiculata (L.) Walp.)

The widespread application of biostimulants with a growing trend represents sustainable practices aimed at improving growth and yield and alleviating stresses in green agricultural system. Phthalanilic acid (PPA), with biostimulatory functions, has been increasingly applied to fruit and vegetable production. However, its specific biostimulatory effects on growth and development of cowpea ( Vigna unguiculata ) plants is still unclear. In this study, the regulatory function of foliar spraying PPA at the flowering timing in morphometric (length, width, single pod weight and yield), physiological (relative electrical conductivity), and biochemical (antioxidant enzymes activity, photosynthetic pigment, malondialdehyde, vitamin C, soluble protein, and soluble sugar content) parameters of cowpea plants were investigated. In general, PPA treatments exhibited higher antioxidant enzymes activities (with an increase of 11.89–51.62% in POD), lower relative conductivity (with a decrease of 22.66–62.18%), increased photosynthetic pigment levels and amounts of free proline (with an increase of 24.62–90.52%), and decreased malondialdehyde. Furthermore, the length, width and weight of single pod, podding rate (with an increase of 19.64%), vitamin C, soluble protein (with an increase of 18.75%), and soluble sugar content were increased by 200 mg·L −1 PPA. These data, together with an increased yield of 15.89%, suggest that PPA positively regulates the growth and development, improving fruit quality and yield, especially at 200 mg·L −1 . This study indicates that PPA has biostimulatory effects in cowpea production and shows application prospect in field cultivation.