the translocation of minerals in trees - Canadian Forest Service

More documents

Recommendations

Info

on the fact that the living cell is not in equilibrium with the environment (Steinbach, 1951). Concerning this aspect, the studies of Epstein and Hendricks (1955) on ion transport in the roots of higher plants have shown that ions move freely into and out of an "outer" space of the roots. This takes place by diffusion and exchange, independently of the simultaneous active transport of the same ions which results in their transfer into an "inner" space where they are no longer exchangeable with the same or other ions. Epstein (personal communication) considers that these inner and outer spaces in the root do not represent distinct tissues but rather parts of cells. The outer space is intracellular and is identified with the cytoplasm. Since entry into and exit from this space is by diffusion, it develops that the outer membrane of cytoplasm is quite permeable to ions, and therefore is not the membrane which operates the active metabolic mechanism of ion transport. These exchange and diffusion processes are reversible, non-selective, non-metabolic, and come to equilibrium approximately within an hour after immersion of the roots in a new solution. Kramer and Wiebe (1954) noted that the meristematic region of the roots accumulates large amounts of minerals but very little is translocated from it to other parts of the plant. They concluded that most of the minerals translocated out of the roots are absorbed by the root-hair zone, or region of differentiation. RELATIVE ROLES OF XYLEM AND PHLOEM IN TRANSLOCATION There has been conflicting evidence as to the relative roles played by xylem and phloem in the up\vard translocation of minerals. Some physiologists (Curtis and Clark, 1950) do not consider any difference in the mechanisms of transport of inorganic and organic substances, except that the latter are more usually transported and required by different parts of the plant in larger amounts. Curtis (1925) found removal of the bark (ringing) would interfere with the upward transfer of solutes. In his experiments, divided stem were used, \\'here water was supplied to the top by one set of roots and nitrogen by another set. Results indicated that if the roots supplied with nitrogen were connected to the tops by xylem only, there was little transfer of nitrogen, while if they were connected by phloem only, considerable transfer occurred. Diffusion alone could not account for this rapid movement, and Curtis thought that protoplasmic streaming within the cells may have been a factor. Evidence showing that there is no direct relation between water absorption and salt absorption was given by Muenscher (1922) in his work on the effect of transpiration on· ab orption of salts by plants. Mason, Maskell, and PhiJIis (1936) verified this work and mentioned that oxygen deficiency, induced locally, could reduce transport, but not always, since the transpiration stream may carry enough oxygen. It is considered that minerals can move upward through the phloem, especially if the leaves have not expanded. There are conflicting observations on the importance of the phloem in upward transport from roots to leaves. Stout and Hoaglund (1939), using radioactive tracers, found that when the xylem and phloem were separated, upward transport took place almost entirely in the xylem. Curtis and Clark (1950) compared these negative results for phloem transport with the positive results of Biddulph and Markle (1944). The latter authors allowed their plants to stand overnight to recover from the shock of having the phloem separated from the xylem and they found phosphorus movement upward and downward through the phloem. It is probable that either phloem or xylem alone can transport the required minerals. Curtis and Clark (1950) summarize mineral transport thus: "that the xylem is concerned with mineral transport more (1) in herbaceous plants than in woody; (2) in those plants showing active root pressure than in those without root pressure; (3) near the base of the plant than in the 6
upper part; (4) when there is an excess of the particular element than when it is deficient; (5) more, especially of nitrogen, when the roots have a low carbohydrate content than when it is high; (6) possibly more of the salts like calcium, zinc, and iron, that are not greatly accumulated in living cells; and (7) possibly more of the elements that are less likely to be carried in combination with organic molecules" . ADVANTAGES AND METHODS OF USING RADIOISOTOPES IN TRANSLOCATION STUDIES In older methods of research on translocation, dyes )vere used which were injected into the tree, and could be detected visually when the tree was cut down. This technique was applicable more to the study of water movement than to that of translocation of minerals. Other workers used poisons and they hoped that the movement of a poison would be accompanied by external symptoms such as a change in appearance of the foliage. It was not until the radioisotope became available that investigators could tag an element which would emit detectable radiation, and thus permit its observation without ·killing the tree. There are several kinds of radiations. According to the present atomic theory, the atom is made of a nucleus about which revolve particles known as electrons. The mass of the atom is found primarily in the nucleus which, for present purposes, consists of protons having a positive charge and neutrons having no charge. The neutrons affect the mass but not the chemical properties of the element, and atoms which vary in nuclear mass but have the same chemical behaviour are called isotopes. When these emit radiations which may be detected by various types of counters, they are referred to as radioisotopes. Radiations are of three types depending on the characteristics of the radioisotope. (1) Alpha rays: are helium nuclei or alpha particles which have a relatively large mass compared with electrons but possess a very limited penetration power. (2) Beta rays: are negative electrons emitted from atomic nuclei; these have a limited penetration power but give good resolution in autoradiography. (3) Gamma rays: are electromagnetic radiation similar to X-rays; these are very penetrating and may be easily detected by external monitoring. The last two types of radiations are most suitable for biological research and problems pertaining to forestry. Radioisotopes are useful in biological research in several ways. They are very good analytical tools which increase the sensitivity and ease of analysis available through conventional methods. They also permit the measurement of ion fluxes in a given direction even when there is no net flux of that ion, or when the net flux is in the opposite direction. In tree investigations the use of the radioisotope permits detection within the tree by external monitoring. This has certain advanLages over older methods since the tree does not have to be sacrificed in order to determine the translocation of the element within the tree. The major minerals required by trees, as indeed by plants in general, include, in addition to carbon, hydrogen and oxygen, thirteen mineral elements-potassium, calcium, magnesium, nitrogen, phosphorus, sulphur, iron, manganese, zinc, copper, molybdenum, boron and chlorine. Of these, only nitrogen and boron have no radioisotopes suitable for tracers. Several other elements such as sodium, cobalt, and rubidium, although not known to be essential, posse s suitable isotopes and are of physiological interest. Growing plants in nutrient 7
Page 1 and 2: CANADA Department of Northern Affai
Page 3: CONTENTS PAGE INTRODUCTION. . . . .
Page 8 and 9: solutions and determining the miner
Page 10 and 11: FIGURE 1. Monitoring movement of ru
Page 12 and 13: FIGURE 2. Autoradiograph of longitu
Page 14 and 15: SUMMARY AND CONCLUSIONS It appears
Page 16: STEDIBACH, H. B. 1951. Permeability

the translocation of minerals in trees - Canadian Forest Service

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?