Exposure of barley plants to low Pi leads to rapid changes in root respiration that correlate with specific alterations in amino acid substrates

The majority of inorganic phosphate (Pᵢ) stress studies in plants have focused on the response after growth has been retarded. Evidence from transcript analysis, however, shows that a Pᵢ‐stress specific response is initiated within minutes of transfer to low Pᵢand in crop plants precedes the expression of Pᵢtransporters and depletion of vacuolar Pᵢreserves by days. In order to investigate the physiological and metabolic events during early exposure to low Pᵢin grain crops, we monitored the response of whole barley plants during the first hours following Pᵢwithdrawal. Lowering the concentration of Pᵢled to rapid changes in root respiration and leaf gas exchange throughout the early phase of the light course. Combining amino and organic acid analysis with¹⁵N labelling we show a root‐specific effect on nitrogen metabolism linked to specific substrates of respiration as soon as 1 h following Pᵢwithdrawal; this explains the respiratory responses observed and was confirmed by stimulation of respiration by exogenous addition of these respiratory substrates to roots. The rapid adjustment of substrates for respiration in roots during short‐term Pᵢ‐stress is highlighted and this could help guide roots towards Pᵢ‐rich soil patches without compromising biomass accumulation of the plant.