Carbohydrate accumulation and utilization in soybean plants in response to altered phosphorus nutrition

We conducted a study to determine whether increased starch accumulation is the cause or the result of decreased growth in phosphorus‐deficient soybean plants (Glycine max [L.] Merr. cv. Ransom). Nitrate‐dependent plants were provided with nutrient solution containing either 0.05 or 1.00 mM phosphorus. On day 22 after transplanting the phosphorus concentration in solutions supplied to one‐half of the plants that had received 0.05 mM phosphorus was increased to 1.0 mM phosphorus and the phosphorus concentration in solutions supplied to plants that had received 1.0 mM phosphorus was decreased to 0.05 mM P. The other half of plants in each treatment remained at the same external phosphorus concentration to serve as phosphorus‐deficient or phosphorus‐sufficient controls. Onset of phosphorus deficiency decreased (1) photosynthetic capacity through negative effects on whole plant leaf area and photosynthetic rate per unit leaf area and (2) relative growth rate and leaf elongation rate. However, continuous increases in starch concentration in all organs during onset of phosphorus deficiency indicated that decreased nonstructural carbohydrate availability was not the primary cause of decreased growth. During onset of phosphorus deficiency, significant decreases in relative growth rate (3–8‐day interval) and in day and night leaf elongation rate (8 days) occurred before or at the same time as significant increases in stem, leaf and root starch concentrations. These results support the conclusion that disruption of metabolic functions associated with growth impaired utilization of available nonstructural carbohydrate in plants adjusting to phosphorus deficiency. After phosphorus‐deficient plants were transferred to the sufficient external phosphorus concentration, starch concentrations in leaves and stems decreased to those of phosphorus‐sufficient control plants within 3 days. These decreases in starch concentration in leaves and stems preceded measurable changes in relative growth rate (3–8‐day interval), day and night leaf elongation rate (8 days) or CO2 exchange rate (8 days). Therefore, when soybean plants recover from phosphorus deficiency, starch stored in leaves and stems is ready to be utilized in the synthesis of structural biomass during the time required for activation and/or development of additional photosynthetic capacity.