Phosphorus fertilization induces nectar secretion for honeybee visitations and cross-pollination of almond trees.

Precise phosphorus (P) application requires a mechanistic understanding of mineral effects on crops' biology and physiology. Photosynthates assimilation, metabolism, and transport require phosphorylation, and we postulated that P is critical for the bloom and fruit-set of almond trees that rely on stored carbohydrate reserves. Hence, we studied the growth-physiology and carbohydrate dynamics in two-year-old almond trees irrigated with P concentrations between 1 and 20 mg L -1. Almond trees attained maximal photosynthesis, transpiration, and growth by 6 mg P L -1 in irrigation. Nevertheless, almond trees continued to extract P in 10 and 15 mg P L -1 irrigations, which corresponded to bigger starch reserves and yields. We attributed the augmented productivity to increased fruit-set (59% between 6 and 15 mg L -1 P), caused by more frequent (29%) honeybee visits. High P improved pollinators' visitation by enabling almond trees to utilize more of their starch reserves for nectar secretion (which increased by ~140% between 6 and 15 mg P L -1). This work illuminates P fertilization's benefits to plant-pollinator mutualism, critical to almond productivity, and reveals novel indices for optimal P application in almond orchards.