Excessive nitrogen impairs hydraulics, limits photosynthesis, and alters the metabolic composition of almond trees

Horticulture nitrogen (N) runoffs are major environmental and health concerns, but current farming practices cannot detect ineffective N applications. Hence, we set to recognize high N conditions and characterize their effects on the physiology of almond trees grown in drainage lysimeters. Water and nutrients mass balances exhibited that N benefitted almond trees in a limited range (below 60 mg N L−1 in irrigation), while higher N conditions (over a 100 mg N L−1) reduced evapotranspiration (ET) by 50% and inherently constrained N uptake. Respectively, whole-tree hydraulic conductance reduced by 37%, and photosynthesis by 17%, which implied that high N concentrations could damage trees. Through gas-chromatography, we realized that high N conditions also affected components of the citric acid cycle (TCA) and carbohydrates availability. Such changes in the metabolic composition of roots and leaves probably interfered with N assimilation and respiration. It also determined the proportions between N and starch in almond leaves, which formed a new index (N:ST) that starts at 0.4 in N deficiency and reaches 0.6–0.8 in optimal N conditions. Importantly, this index continues to increase in higher N conditions (as starch reduces) and essentially indicates to excessive N applications when it exceeds 1.1.