METABOLIC PRIORITIES WITH RESPECT TO GROWTH AND MINERAL UPTAKE IN ROOTS OF HORDEUM, TRITICUM AND LYCOPERSICON

Reduction in the supply of photosynthate to the roots of tomato, barley, and wheat plants was achieved indirectly by lowering the intensity of sunlight striking the foliage of test plants. The decrease in sugar and starch concentrations in the roots was verified by appropriate extraction and colorimetric analysis, and a corresponding reduction in the total respiratory rate of the roots was confirmed using an oxygen tension monitor. Other processes measured directly include the rate of uptake of potassium, the mitotic quotient in the root tip-a measure of growth-and the rate of accumulation of dry matter in the root. The study demonstrated that of the metabolic activities observed, root growth is the process first limited when the supply of photosynthetic fuel is decreased. Root growth was severely inhibited under conditions that did not significantly affect either the active uptake of potassium per gram of root or total respiration per gram of root. With greater restriction of photosynthesis, growth was completely halted while the uptake of potassium was strongly decreased and total respiration was still affected only moderately. As the light intensity is reduced significantly, most of the reduced energy supply in the root appears to be used in support of processes critical to maintenance of the organ and the organism. IT HAS BEEN SHOWN that the magnitude of root respiration is primarily controlled by the availability of soluble carbohydrate supplied by the shoot as an energy source in the root. For example, removal of photosynthetic tissues and competition by floral development for photosynthates are commonly followed by decreases in the supply of available carbohydrates in roots (May, 1960; Weinmann, 1961; Evans, 1971; Ryle and Powell, 1975). The respiratory rate in roots is diminished after defoliation (Woodford and Gregory, 1948; Sakurai et al., 1960; Crapo, 1973), or in response to a reduced intensity of photosynthetically active radiation striking the foliage (Huck et al., 1962; Osman, 1971; Schafer, 1971; Hanson, 1977). Thus, it appears that the total respiratory rate of a root decreases when the supply of photosynthate is decreased. The rate of respiration, determined as consumption of oxygen or evolution of carbon dioxide, is a gross measure which reflects a myriad of metabolic processes. Normally, part 1 Received for publication 27 November 1979; revision accepted 20 August 1980. The authors express their gratitude to Dr. T. L. Rost for his advice and to Dr. E. Epstein for his counsel and for making available the facilities for the ion uptake experiments. Support by an NSF Graduate Fellowship (N.L.C.) is gratefully acknowledged. 2 Present address: Department of Natural Sciences, York College of the City University of New York, Jamaica, New York 11451. of the energy released from respiration is used for metabolic work and a reduction in fuel should correspond to a decrease in biological work. However, the response of respiration to reduced carbohydrate supplies provides no information with respect to the reactions of individual metabolic processes. As the active uptake of minerals is supported by metabolic energy (Woodford and Gregory, 1948; Epstein, 1972; Clarkson 1974), its rate should decline when carbohydrate supplies to the root diminish. Root growth is always reduced when the amount of soluble carbohydrate in the root is decreased (Weinmann, 1948; Jameson, 1963; Wardlaw, 1968; Ryle and Powell, 1975). The relative sensitivity of these two processes to a reduction in carbohydrate supply is not known, however. This study addresses the question how the available energy is apportioned among energydependent processes in the root under conditions of reduced carbohydrate supplies. Based on a review of the available literature (Crapo, 1975), it is assumed that a direct causal relationship exists between the level of carbohydrate and the total metabolism, expressed as rate of respiration, in roots. The supply of carbohydrates in roots can be manipulated by altering the rate of photosynthesis. MATERIALS AND METHODS-Tomato (Lycopersicon esculentum (L.) Merr., var. Marglobe) and barley (Hordeum vulgare L., var.