NON-HORMONAL STIMULATORS AND INHIBITORS OF PLANT GROWTH AND DEVELOPMENT

1. Plants contain growth regulators that are non-hormonal in nature. These regulators change in concentration during ontogeny and when applied exogenously, can either stimulate or depress growth. While the bulk of either the phenolic or terpenoid regulators are localized within the vacuole, they can also be found within other cellular compartments where they may act upon metabolic pathways, modifying either cell multiplication or elongation. 2. Non-hormonal growth regulators may affect the synthesis and/or destruction of phytohormones, mainly indole-3-acetic acid (IAA). These regulators behave non-specifically, modifying the actions of auxins, gibberellins and cytokinins upon growth. 3. A variety of both uncertainties and unresolved contradictions exist that have prevented a thorough elucidation of the mechanisms of actions of both phenolic and terpenoid regulators. These uncertainties and unresolved contradictions include lack of data regarding compartmentalization of many of the inhibitors. This raises the question of whether their intracellular concentrations become elevated sufficiently to affect metabolic pathways in vivo. Exogenously applied regulators of non-hormonal nature usually interfere with growth only at high concentrations. Therefore, the possibility cannot be excluded that under these conditions, reactions occur within the cell that are absent in vivo. 4. The specific properties of natural non-hormonal regulators are similar in certain respects to phytohormones. For example, both of them may be biogenetically bound within metabolic centres: shikimate (phenolics, indoles, alkaloids), bi-benzi (coumarins) or acetate-mevalonate (terpenoids, fluorens, sesquiterpenes, cytokinins). In addition, both non-hormonal regulators and phytohormones exhibit biological activity in growth bioassays. 5. Non-hormonal regulators may possess a number of useful purposes, e.g. test substances such as fusicoccin permit the investigation of the mode of action of phytohormones, specific inhibitors blocking special forms of growth and protectors of phytohormone activity in culture.     

THE EFFECT OF CERTAIN BENZAZOLE COMPOUNDS ON PLANT GROWTH AND DEVELOPMENT

Klingensmith , M. J. (Colgate U., Hamilton, N. Y.) The effect of certain benzazole compounds on plant growth and development. Amer. Jour. Bot. 48(1): 40–45. Illus. 1961.—A number of benzazoles, in particular benzimidazole, benzothiazole, and benzotriazole, were examined for their effects on growth of seedlings and established plants. Benzothiazole was the most active in repressing elongation of the primary root of cucumber. Benzimidazole and benzotriazole were about 1/10 as active. Adenine was without effect in reversing the benzazole-induced inhibition of cucumber root elongation and, in fact, supplemented the inhibition caused by benzimidazole and benzothiazole. Application of benzotriazole to the root medium of bean, coleus, tomato, oat and wheat caused a pronounced inhibition of internodal elongation with a stimulation of axillary development. Distinct morphological changes were observed which did not correspond to those produced by other growth regulators. Application of benzimidazole to the root medium of several genera of plants resulted in injury to laminar tissue followed by desiccation, with no concomitant effect on the stem. Application of benzothiazole to the root medium induced development of adventitious roots in bean and tomato. This compound also caused initiation of roots on cultures of tobacco-stem segments, while not suppressing bud development. The benzazoles tested seem to be of a different class of compounds from other growth regulators which have been studied. The responses elicited by treatment with each of the 3 benzazoles are distinctly characteristic and are dependent on the structure of the azole portion of the molecule.     

CALCIUM AND PLANT GROWTH

Calcium as a plant nutrient is characterized by its relatively high content in the plant coupled with a requirement not much higher than that of a micro nutrient element and an exceedingly uneven occurrence in soils. The difficulties in defining its actions are accentuated by a weak biochemical activity. In ecological conditions the secondary consequences of variations in calcium content may be more striking than the direct ones. Electron-microscopical studies have revealed that calcium is required for formation and maintenance of lamellary systems in cell organellae, a fact which might suffice to explain its indispensability for meristematic growth. Calcium is required for cell elongation in both shoots and roots; the common experience that it inhibits shoot elongation is certainly due to calcium additions far above actual requirement. It must be assumed for a rational interpretation of cell elongation that the fundamental mechanism is the same in shoots and roots. The one action which can be ascribed with certainty to calcium is a stabilizing of the cell wall with an increase in rigidity, an effect which, with over-optimal supply, may lead to growth inhibitions. The function is, however, necessary for the normal organization of cell walls. Calcium has, on the contrary, no significant effect on the synthesis of cell wall compounds but appears to act on their proper incorporation into the cell wall. The growth-active calcium may be bound not only to pectins but also to proteins and nucleoproteids in or in close contact with the cell wall. The supposition that calcium interacts directly with auxin in the cell wall has not been verified and does not seem very probable. There are reasons to believe that the points of action of calcium and auxin in the cell wall differ, auxin inducing growth by wall loosening and calcium establishing new wall parts. For submerged organs it may be necessary to consider an indirect effect of calcium on growth by its regulation of cytoplasmic permeability and thus affecting the exudation of growth-active compounds. The ecological problem is to characterize calcifuges (acid soil plants) from calcicoles (base soil or calcareous soil plants). Growth inhibitions on acid soils depend upon poisoning by A1³⁺ and Mn²⁺. Opinions differ as to what extent this can be antagonized by calcium. Lime-induced chlorosis in calcifuges depends upon iron deficiency or iron inactivation in the plant. No acceptable explanation is given, but it might be related to an interaction of calcium carbonate, phosphorus, and iron. A hypothesis that it is linked to formation of organic acids is not tenable in the given form. Plants react to the calcium ions in the concentrations found in soils. Calcifuges have a low calcium-optimum for growth and show growth inhibition at high concentrations. Calcicoles have a high optimum for growth. Calcifuges are resistant to aluminium poisoning. Attempts made to explain the differences in calcium uptake and generally in salt uptake are tentative only, and relevant data are lacking.     

LENGTH OF THE LIGHT-DARK CYCLE AND PLANT GROWTH

Tukey , H. B., Jr ., and H. J. Ketellapper . (California Inst. Tech., Pasadena.) Length of the light-dark cycle and plant growth. Amer. Jour. Bot. 50(2): 110–115. Illus. 1963.—It has been shown that the length of the light-dark cycle which causes maximal growth of tomato, pea, peanut, and soybean plants is close to 24 hr for cycles consisting of equal periods of light and darkness. The exact optimum for tomato plants was determined by temperature; the optimal cycle length was 20 hr at 30 C and 27–30 hr at 14 C. Such an interaction between temperature and cycle length was not found in pea plants, because peas were less sensitive to cycle length than peanuts, tomatoes, and soybeans and did not respond to changes in cycle length of 2–3 hr. The response to cycle length was not influenced by the conditions in which the seedlings had been raised prior to the treatment. Seedlings raised in a 16-hr light, 8-hr dark regime responded in the same manner as those raised in continuous light. The response to cycle lengths of 18, 24, 36, and 48 hr was not changed qualitatively by the temperature during the growth determination. Small changes in cycle length had no characteristic effects on the rates of photosynthesis, respiration or stem elongation. Stem elongation showed a rapid and initial increase in rate when the light was turned off. It was concluded that plants possess an endogenous time-measuring device with a period of 24 hr. For maximal growth to occur the external periodicity must be synchronized with the endogenous period of the plant. Efforts to obtain direct evidence for this hypothesis were not successful since no overt rhythms could be found in tomato plants.