Physiological Gradients in the Potato Tuber

The distribution of the major cations and anions, together with citric acid, malic acid and iron, has been studied in tubers of Solanum tuberosum cv. Golden Wonder, by analysis of longitudinal cores divided into 16 equal cylindrical sections. In immature daughter tubers the constituents studied were evenly distributed along the tuber, but in mature tubers a considerable polarity had developed. The concentrations of potassium, phosphorus and citric acid each rose from the heel to the rose end of the tuber. Iron showed a gradient in the opposite direction. These gradients were maintained in sprouting tubers, and although after 25 days the levels of these constituents were reduced, the patterns of distribution were unchanged. In the exhausted mother tubers all these constituents were much depleted and only potassium exhibited any pattern of polar distribution. Malic acid was barely detectable at any stage, and calcium, sodium, magnesium, chloride, sulphate and nitrate were always at low levels and showed no polarity of distribution. At each developmental stage potassium was correlated with the difference in electrical potential measured between cells at points across the tuber. A close correlation was apparent between potassium and citric acid content in young, mature and sprouting tubers. In all stages a considerable discrepancy existed between total anions and total cations and this was attributed to the presence of organic acids other than citric acid.