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.     

藻类与植物生长物质

高等植物和藻类的发育之间具有相似性.尽管藻类具有多样性,但人们认为它们是比高等植物低级的生物类群,它们的形态比高等植物简单,但多数的发育过程却是相似的1。       

METABOLISM OF PLANT GROWTH REGULATORS

Comparison of the metabolism of ¹⁴C carboxyl-labelled 2,4-D in detached leaves of different varieties of apple showed that the 2,4-D-resistant Cox decarboxylated 57% of the 2,4-D in the leaf in 92 hr., whereas the susceptible Bramley's Seedling decarboxylated only 2% in the same time. Comparatively high rates of decarboxylation were found with all varieties of apple having Cox in their parentage; varieties unrelated to Cox showed low rates of decarboxylation. Similar high rates of decarboxylation were found in resistant varieties of strawberry and in Syringa vulgaris. Sixteen other species tested showed low rates of decarboxylation, irrespective of whether they were resistant or susceptible to 2,4-D.
The rate of decarboxylation of 2,4-D in the mature strawberry leaf was proportional to the amount of 2,4-D in the leaf. The rapid fall in decarboxylation rate in a detached leaf was due chiefly to reduction in the amount of 2,4-D present, and not to any falling off in the metabolic activity of the leaf. Young leaves decarboxylated 2,4-D at about half the rate of fully grown or senescent leaves. Oxidation of the methylene carbon proceeds at less than half the rate of that of the carboxyl carbon and is accompanied by an increase in the phenol content of the leaf which agrees closely with the amount of 2,4-D demethylated.
As previously reported for currants, part of the 2,4-D entering the leaf was bound and part inactivated by the formation of water-soluble compounds. The reaction of strawberry variety Talisman to 2,4,5-trichlorophenoxyacetic (2,4,5), 2-chloro-phenoxyacetic (2-CPA), 4-chlorophenoxyacetic (4-CPA) and 2-methyl-4-chlorophenoxyacetic (MCPA) acids was similar to that previously reported for red currant. Leaves of Cox's Orange Pippin apple and Talisman strawberry were able to decarboxylate all these phenoxy-acids except 2-CPA.