Manganese Deficiency, Soil Chemistry, and Root Dysfunction Drive Physiological and Metabolic Changes in Date Palm Under Field Conditions

Manganese (Mn) deficiency is a major factor underlying brittle leaf disease in date palm, yet its root-centered mechanisms under field conditions remain poorly understood. Nine mature palms (three per health category: healthy, asymptomatic Mn-deficient, and BLD-affected) were assessed for soil chemistry (pH, salinity), root Mn concentration and hydraulics, canopy pigments and chlorophyll fluorescence (Fv/Fm), as well as metabolic responses. Elevated soil pH and variable salinity significantly constrained root Mn uptake and water conductance, leading to a ~60% decline in root Mn, a 20% reduction in root water content, an 80% loss of chlorophyll, and a 26% decrease in Fv/Fm. These changes induced strong metabolic reprogramming, including a twofold rise in glucose, increased protein content, and a tenfold enhancement in peroxidase activity. Asymptomatic palms already displayed early declines in pigments and fluorescence, highlighting their diagnostic value. This study demonstrates that soil-driven Mn deficiency impairs root function and cascades to canopy physiology and metabolism, offering realistic avenues for rhizosphere management and early field diagnosis in arid oases.