Foliar potassium nutrition and water stress: Molecular and physiological adaptation mechanisms via the proline biosynthesis pathway in maize

A field study was conducted at the Anbar University College of Agriculture research station during the autumn season of 2024 to evaluate the effects of irrigation intervals and foliar potassium application levels on the expression of the pyrroline-5-carboxylate synthase (P5CS) gene, the activity of the pyrroline-5-carboxylate synthase enzyme, proline accumulation and selected growth and yield traits of maize. The experiment followed a split-plot arrangement within a randomized complete block design (RCBD). The results revealed that extending the irrigation interval significantly enhanced the expression of the P5CS gene, with the 15-day irrigation interval (W2) recording the highest gene expression levels. This increase in expression positively influenced the activity of the pyrroline-5-carboxylate synthase enzyme, which reached 3.383 absorbance units min-1 g-1 fresh weight. Consequently, proline accumulation increased, with an average content of 52.36 µmol g-1 fresh weight. However, prolonged irrigation intervals adversely affected most growth and yield traits. The control treatment (W0) recorded the highest averages for plant height (196.00 cm), leaf area (4780.66 cm2 plant-1), chlorophyll content (55.14 SPAD units), number of kernels per ear (361.55 kernels ear-1) and individual plant yield (126.55 g plant-1). Conversely, foliar potassium application mitigated the adverse impacts of water stress by reducing the plant’s reliance on increased proline accumulation. Foliar potassium supply suppressed the expression of the P5CS gene, leading to reduced enzymatic activity and lower proline content under high potassium level (K2), with average values of 1.353 absorbance units min-1 g-1 fresh weight and 27.67 µmol g-1 fresh weight, respectively. Moreover, the high potassium level (K2) improved several growth and yield traits, recording the highest averages for leaf area (4242.22 cm2 plant-1), chlorophyll content (49.21 SPAD units), grain number per ear (276.77 grains plant-1) and plant yield (101.88 g plant-1).