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Phosphorus Concentrations in Wheat Leaves in Relation to Phosphorus Toxicity¹

The relationship between P concentration and the development of P toxicity symptcms in leaves was followed in young wheat plants growing on a siliceous sand at two levels of CaH₄(³²PO₄)₂ and of NH₄NO₃

³²P accumulated in the terminal region of the first leaves prior to the appearance of necrotic symptoms. When symptoms developed they coincided closely with regions of ³²P accmnulation. NH₄NO₃ slowed down the rate of ³³P accumulation in leaf tips and alleviated P toxicity symptoms.

P concentrations in and near necrotic areas of leaf tips ranged from 4 to 5% dry weight and most of this could be expected to be present largely (80% or more) as inorganic orthophosphate salts. Phosphate concentration in the cell sap could reach values of 200 mM with an osmotic pressure of about 10 atmospheres. It is suggested that under certain conditions this high phosphate concentration might disturb the water relations of the leaf ceils, thus causing the observed injury.

Key Words: Phosphorus distribution • Salt injury

² Ph.D. Student and Reader in Plant Nutrition, respectively.

Received for publication September 8, 1969.