Showing papers in "Annual Review of Plant Biology in 1967"

Mechanisms of Foliar Penetration of Solutions

Abstract: From a classical point of view green leaves appear to be organs whose sole purpose is the production of organic materials by photosynthesis. While gases like CO2 and O2, both necessary for and arising from assimilation and respiration, are exchanged by stomata and intercellular spaces, water and mineral salt s are supplied by the roots. However, during the last few decades it has become more and more evident that absorption of inorganic and or­ ganic materials can also take place through the surfaces of leaves. This is rather surprising at first sight, since these surfaces are covered by the cuticle, which has been considered impenetrable. The same is true for excretion. Aside from gases and water vapour, it was thought that other materials, which are no longer useful to the plant or which are surplus, could not leave the assimilating organs but must be stored within the vacuoles or in special tissues. It has been shown, however, that some export of substances takes place from roots and even more from leaves which may sometimes be impor­ tant for the plant itself and perhaps for plant communities. There are numerous reports on foliar spraying, foliar abso rption and foliar leaching, many of which have already been discussed in former reviews in this series (8, 69, 107). Since then, the number of substances applied to leaves has grown enormously; more and more growth regulators, pesticides and nutrients are used for theoretical and practical purposes. The results of such work have been reviewed primarily in relation to applied problems during the last few years (11-13, 17, 36, 44- 46, 108, 109) . While practical aspects were emphasized, the location of the sites of foliar absorption and excretion and the nature of the underlying mecha­ nisms have more recently been investigated. Since substances to be absorbed or excreted by leaves must pass through a cell wall covered by a cuticle, the process of penetration may be somewhat different from that of absorption by root cells, which do not possess an outer layer structurally comparable to that of the leaves. The purpose of this review is to present current views, to reveal gaps in the existing knowledge and to reflect on both. No attempt has been made to refer to the whole body of information available at present. The amount of published work has made it necessary to select only literature directly relevant to the matters considered below. Further literature is cited in reviews referred to above. To understand the mechanisms of foliar penetration it is necessary to find out what the pathways are through which penetrating solutions pass

Freezing Stresses and Survival

Abstract: Protecting biological materials from deleterious effects of freezing is im­ portant and involves widely diverse sciences. Contributions are found in the literature of food sciences, enzymology, microbiology, pathology, and med­ icine. The literature of the physical sciences contributes theories of water structure and provides a basis for evaluating energy relationships. Perhaps man's most ancient interest involves winter survival, particularly survival of food-producing plants (75, 136, 155). The world literature concerning winter hardiness of plants is estimated to be about 5000 references (136). Winter hardiness is a complex property of a plant involving many inter­ acting factors. Even resistance to direct effects of freezing is complex. Many types of stress occur, depending on the type of plant, the tissue under consid­ eration, and the conditions under which the plant is tested (2, 57, 59, 71, 108, 130, 147, 150). No single unifying theory can be expected to explain resis­ tance to freezing injury. By definition, freezing involves redistribution of water with respect to state. Water molecules associate through extended hydrogen bonding and thus introduce a new phase which also causes redistribution of the remaining liquid. The pattern of redistribution, with respect to both location and state, determines the type and extent of freezing stress. Many patterns of redistri­ bution occur. Some of the macro redistribution patterns can be followed rather easily. Micro redistribution patterns involve exact knowledge of inter­ molecular associations, are mainly theoretical, and usually are based on con­ siderations of statistical mechanics. Because of the many possible combina­ tions of freezing processes and their interrelationships with a great variety of histological, cytological, and macromolecular features, it is necessary to think of resistance to injury from freezing in terms of specific situations. In each situation, the initial limiting injury or effect that occurs at the highest temperature must be defined. Subsequent injuries which occur in an organism already mortally injured are meaningless.

Electric Fields in Plants

Abstract: In the past 180 years much has been written on the intimate relationship between electricity and life. Animal and plant materials of many kinds have been found to have electric fields associated with them and to be affected by externally applied fields. The earlier work on plants has been reviewed on a number of occasions (1, 4, 5, 41,51, 53, 54). Since living cells contain aqueous phases which are separated from each other and from the external medium by membranes through which some ions in solution pass more easily than others, it is not surprising that electric fields are produced. The interior of the cell is maintained at a different ionic composition from that of the environment (and in the case of most plant cells at a much higher concentration) by processes requiring the continual expenditure of metabolic energy. These processes are conveniently described in terms of ionic or molecular pumps. Ions diffusing through a membrane establish an electric field which adjusts its magnitude so that the total flux of positive ions through the membrane (as determined by concentration differ­ ences and electric potential difference across the membrane, ion pumps, and possibly bulk water movement) balances the flux of negative ions exactly. Almost always the interior of the cell is more negative in potential than the external solution by 70 to 150 mV. The causes of macroscopic fields in and around bulk tissue will be considered later. Most of the recent investigations of plant electricity have been carried out at the cellular level. Attempts have been made to examine in detail the interrelationships among the electrical properties of the cellular membranes, the potential differences and ion concentration differences across them, and the fluxes of ions through them. Because of the difficulties of working with cells of normal size, most investigators have used the unusually large coeno­ cytic cells of certain fresh water and marine algae. One aim of these studies is to determine which ions cross a membrane passively (their movement being determined by concentration differences and electric potential differ­ ences alone), and which ions are being pumped as well. This information is essential to a fuller understanding of the mechanisms of ion accumulation by cells. Very good accounts of the electrochemical theory of ion movement in plant cells were given by Briggs, Hope & Robertson (3) in 1961 and by