IntroductionThe factors which have been reported to influence stomatal movement are: (1) light; (2) turgidity of the leaf; (3) temperature; (4) physiological shock; and (5), under artificial conditions, certain chemical agents. On the efficacy of light and water supply there is general agreement, but the conditions of their respective dominance are not well understood and the mechanism of their action is still the subject of diverse theories. As regards temperature and shock, investigators differ on the question of whether these factors have any important effect at all on the stomata of plants in
Observations in a micro-freezing apparatus of isolated tissues of the cortex of hardy and non-hardy plants of Catalpa and Cornus species, and of the epidermis of red cabbage, reveal that there are two modes of freezing of plant cells, intracellular and extracellular.In intracellular freezing, ice crystals form first in the protoplasm and then in the vacuole. In extracellular freezing, ice forms outside the cells from water in the cells. The resulting dehydration of the cell causes its collapse, the opposite walls coming together and squeezing the contents to the periphery. Intracellular freezing is fatal to all cells by visible mechanical disruption of the protoplasm and vacuole. It is facilitated by rapid freezing and occurs less easily and less frequently in hardy tissues and in trees and shrubs than in non-hardy and herbaceous tissues. Extracellular freezing induced through slow cooling is fatal to all unhardy cells in trees and herbs at all temperatures below the freezing point, and to cells of hardy cabbage only at − 10 °C. to − 15 °C., but not to cells of hardy trees and shrubs.Both types of ice formation have been observed in intact plants of red cabbage frozen in a refrigerator.The behavior of hardened plants shows that intracellular freezing tends to be prevented in them by an increased permeability to water. In regard to extracellular freezing, from the behavior of the cells on freezing and in micrurgy, a mechanical injury hypothesis is presented.
IntroductionIn the foregoing papers of this series (25, 26) we have given an account of our more intensive researches on the physiology of cold resistance, approached always through a study of living cells. We now describe a number of lesser excursions into the same field, and combine with this a survey of the whole problem of hardening in the light of those changes which have been found to accompany it. The relation of proved hardening changes to the mechanism of resistance must remain hypothetical unless we know the type of injury which has to be resisted. This, however, is still a problem, and evidently a complex one. The immediate cause of death is not always the same. Sometimes it is only indirectly related to temperature, as in soil heaving, smothering by ice, and physiological drought. Often there is a time factor which would seem to involve a mechanism different from that responsible for immediate killing.Though we confine our attention to the more direct and immediate action of frost, the problem is still complicated because, as we shall see, the mode of injury varies with conditions, such as the rate of freezing or the rate of thawing, and also with the type of plant. Very tender plants are killed merely by chilling to temperatures which are still above the freezing point of their juices, or even above 00 C., but most plants of temperate regions suffer no harm unless ice forms in their tissues, and they may be supercooled with impunity. The well-known resistance of dry seeds and spores to the extreme cold produced by liquid air or liquid hydrogen shows that low temperature per se is not fatal. There is also some evidence, though rather indirect, that the amount of injury to a particular tissue is more or less proportional to the amount of ice formed in it (1, 31). Whatever the mechanism of frost injury, apparently any change which reduces or prevents ice-formation will have a hardening effect, and certain theories of hardening are based entirely on this type of resistance. But tissues do freeze, and the major problem before us is how hardening enables a plant to endure an amount of freezing that is fatal in the unhardened state. It is this problem that depends on the mechanism of injury for its solution, and it will therefore be discussed in relation to theories of the same. These fall naturally into two main groups: Those that regard injury as an effect
IntroductionThe causes of dehydration of cells are evaporation, frost, and the osmotic action of hypertonic solutions. In addition to loss of water and consequent diminution of cell volume, there are other effects peculiar to the respective agencies, so that we cannot assume on theoretical grounds that the injury which each of them may cause is effected in the same way. One type of frost injury indeed has no counterpart in the action of drought or hypertonic solutions; namely, disruption of the protoplasm by intracellular ice formation. This, however, occurs only under rather special conditions and at the moment of freezing. It is usually fatal to hardy as well as tender cells. The protection which is developed against intracellular freezing on hardening is an increase of cell permeability which tends to prevent its occurrence (7,8). This type of freezing, is necessarily absent when marked resistance is displayed. Usually ice forms in the intercellular spaces and it is possible that here too it may produce injury otherwise than by dehydration.Peculiar to hypertonic solutions is their chemical or physico-chemical effect on the plasma membrane. The least toxic, namely sugars and balanced mixtures of Na and Ca salts, are most likely to injure through dehydration. Another difference between osmotic and other modes of dehydration is that generally the protoplast contracts and separates from the wall in the former and wall and all collapse in the latter. Thus the mechanical stresses to which the protoplasm is exposed during contraction and expansion are somewhat different in the two cases.To throw light on the question of whether frost injury and plasmolysis injury result mainly from the special effects of these agents or from those that they share with drought we may determine the degree of correlation in the resistance of different types of cell to the respective agents.In the first place it seems to be the rule that extreme resistance to cold,
The old hypothesis that light induces stomatal opening through its photosynthetic activity has been variously tested in a number of recent investigations. The results are strangely contradictory. First, reference may be made to tests based on the relative efficiency of different regions of the spectrum in promoting stomatal opening, though these tests are not very recent. The only systematic studies in which strictly quantitative methods have been used to equalize light intensity are those of PAETZ (11), SIERP (15), and HARMS (6).. Paetz found that the order of activity was red light > blue > green, whereas Sierp and Harms found that red light was regularly the least active and that in most species green light was as active as blue.It has long been known that a reduction in the normal CO2 content of the air tends to induce t&mKtalTopening and-an increase in CO2, Up to a point, above normal has the opposite effect. It has, however, only recently been demonstrated (HEATH,7,9) that the sensitivity of stomata to small changes in CO2 concentration below normal is such that stomatal response to variations in light intensity could conceivably be due primarily to the effect of the latter on CO2 concentration within the leaf.The only investigation in which measurement of photosynthesis rate has been combined with estimation of stomatal aperture is that of WILSON (18). Approaching the problem from various angles, he obtained results which seem quite opposed to the photosynthetic theorv. Both in the behavior of plants in the open and under certain artificial conditions, stomatal opening and photosynthesis often showed no correlation, while complete absence of chlorophyll from leaves, including guard cells, did not entirely prevent stomatal response to light. On the other hand, regarding the long unsettled question of whether guard cells are capable of photosynthesis, the most
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