Photosynthetic characteristics combined with metabolomics analysis revealed potential mechanisms of cucumber (Cucumis sativus) yield reduction induced by different phosphorus stresses

Improper application of phosphorus fertilizer is common in vegetable production, and phosphorus stress affects cucumber growth and reduces yield. However, the underlying physiological and metabolic mechanism of cucumber yield reduction remains unclear. Therefore, photosynthetic capacity, reactive oxygen metabolism and metabolomics analyses were performed in cucumber leaves under phosphorus stress. The results showed that compared with the control (CK), moderately low phosphorus (MLP), severely low phosphorus (SLP) and severely high phosphorus (SHP) treatments significantly reduced the biomass and yield of cucumber and inhibited the photosynthetic capacity of leaves. The SLP, moderately high phosphorus (MHP) and SHP treatments all caused an overproduction of H2O2 and O-2(center dot-), resulting in significant oxidative damage. In addition, low phosphorus (LP) treatments disturbed nitrogen metabolism, and high phosphorus (HP) treatments disturbed carbon and nitrogen metabolism in cucumber leaves. Compared with CK, the content of most sugars and amino acids decreased significantly under LP treatments; this decrease intensified under SLP treatment, and the lack of nutrients seriously inhibited yield formation. In addition, the malic acid content increased in the leaves, which might be the feedback regulation method used by cucumber in response to phosphorus deficiency. Under HP treatments, most sugars, amino acids and organic acids in the leaves increased, and cucumbers accumulated compatible solutes (sugars and amino acids) to cope with the decreased soil osmotic potential, which resulted in excessive carbon and nitrogen resources being trapped in cucumber leaves rather than being allocated to yield. These results indicated that the LP and SHP treatments inhibited the photosynthetic capacity of cucumber, disturbed the reactive oxygen metabolism and physiological metabolism in the leaves, and ultimately affected cucumber yield.