What is the critical leaf magnesium concentration to identify magnesium deficiency in Meyer lemon?

Citrus trees grown in Gisborne, New Zealand, often show symptoms of magnesium (Mg) deficiency and most have leaf Mg concentrations below the recommended minimum of 2.6 mg kg(-1). However, adequate fruit yields are still attained. We report an experiment to investigate the threshold leaf Mg concentration for physiological stress in Meyer lemon (Citrus x meyeri). Plants were grown in nutrient solutions containing 0-2 mM Mg in a glasshouse. After 6.5 months of growth various physiological indicators of plant growth and leaf function were measured. For most indicators, the optimum solution Mg concentration was 0.5 mM. Plants grown at lower concentrations showed reduced maximum quantum yield of photosystem II (F-v/F-m) and reduced stomatal conductance (g(s)) in the older leaves. The youngest fully expanded leaves from plants grown in the best performing treatment (0.5 mM Mg) had a Mg concentration (1.7 mg kg(-1)) considerably below the recommended minimum. Leaves containing <0.9 mg Mg kg(-1) showed a marked decline in gs and F-v/F-m. Our results challenge the notion of a simple threshold leaf Mg concentration that indicates deficiency in crops like citrus. Trees with leaf Mg concentrations well below the previously accepted thresholds physiologically appeared to be performing well. However, in the field, higher concentrations may be needed to supply Mg to maturing fruit if uptake is restricted by factors such as water stress or cool soil temperatures-so the threshold leaf Mg concentration for crop performance could vary with the amounts of leaves and fruit on each tree.