EXCISED ROOT CULTURE

Abstract

Summary Detailed knowledge of the growth requirements and growth responses of excised roots is available for only a few species, of which tomato (Lycopersicum esculentum) is the outstanding example. Excised tomato roots have absolute requirements for thiamine and pyridoxine. The growth of some strains is further enhanced by niacin. The view is advanced that these requirements arise because the vitamins are synthesized in the roots at an inadequate rate rather than from a complete loss of synthetic ability. By use of possible precursors and analogues knowledge has been obtained relating to pathways of vitamin biosynthesis, the specificity of the vitamins and the ‘sparing action’ on vitamin requirements of certain metabolites. Excised tomato roots can assimilate nitrate nitrogen. Recently the development of techniques for the adjustment and stabilization of the pH of the culture medium and the use of a stable ferric ethylenediamine tetra‐acetate have made possible growth on ammonium nitrogen. Absolute requirements for the micro‐nutrient elements iron, copper, molybdenum and manganese have been demonstrated. Present evidence indicates that the micro‐nutrient requirements of excised roots are qualitatively similar to those of whole plants.The importance of certain iodine compounds in maintaining the healthy growth of excised tomato roots suggests that the status of iodine as a plant nutrient should be further studied. Sucrose is the only sugar capable of supporting a high rate of excised tomato‐root growth. Fructose and glucose, singly or together, only support a very low level of growth. Other sugars and all sugar alcohols tested do not support growth. Preliminary work suggests that organic acids will not prove to be satisfactory carbon sources. The specificity of sucrose as an effective carbon source does not apply to those monocotyledonous roots so far cultured. Studies have been made of liquid compared with solid medium and of the effects on excised tomato‐root growth of temperature and light. The standard technique developed for the culture of excised tomato roots does not make possible the culture of the excised roots of angiosperms in general. In the present state of our knowledge of the growth requirements of excised roots, those of many species and varieties remain intractable to culture. Excised root culture exposes variation from the inter‐specific down to the intra‐clonal level. Interesting variations between species and between strains within species, in the responses of their excised roots to vitamins, amino‐acids and auxins, have been studied. Excised root culture has shed light on the nutritive interrelationships between shoot and root and made possible a new approach to the study of certain aspects of general physiology. Study of the physiological differences underlying variation in culture is laying the foundations of a comparative physiology of excised root systems. Recent work has indicated the potential value of excised root cultures for studies in the biochemical genetics of higher plant cells. The present status of the excised root‐culture technique should, in the immediate future, permit of a rapid expansion of our knowledge of the physiological basis and genetic control of the natural variation expressed in the growth requirements and physiological responses of excised roots.