Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
9 THE INDUCTION AND ENDOGENOUS CONTROL OF COLD ACCLIMATION IN PLANTS c. J. Weiser, Department of Horticultural Science, University of Minnesota Man cultivates a scant 7.6% of the earth's land surface. Crop- production problems caused by low temperatures are serious now and will become more so as crop production is expanded to areas less suited to agriculture. An understanding of how freezing kills plant tissues, and of how some plants physiologically acclimate to resist freezing at certain times of the year, may provide the basis for ultimately de- veloping agricultural technology that will permit the efficient pro- duction of crops in areas of stress. Experiments on freezing rates have shown that intercellular freezing may or may not injure plants, depending on their hardiness; that intracellular freezing is almost invariably lethal; and that vitrified tissues can survive extremely low temperature. Ice and not low temperature per se is responsible for injury. Theories of injury such as disulfide bond formation, removal of water shells from sensitive proteins, mechanical disruption of proto- plasmic or membrane continuity, and salting out were discussed along with hypothesized ways in which tissues may avoid or survive freezing. The two-stage sequence of cold acclimation in a hardy woody species under natural autumn conditions was characterized. Controlled environment studies on Cornus stolonifera revealed that short days fol- lowed by freezing temperatures were necessary for hardening of the type observed in nature. Climatic races of c. stolonifera from various parts of North America differed in the timing of cold acclimation but not in their ability to resist extremely low temperatures by midwinter. The following sequence of events is postulated: (A phytochrome system detects short days in autumn) ~ (P phytochrome induces forma- tion of hormones involved in growth cessa~ion and Stage I of hardening) ~ (Autumn hormones activate DNA operons) ~ (New kinds of RNA regulate synthesis of proteins) ~ (Enzymatic and structural proteins bring about the physical and biochemical changes involved in Stage II of hardening). Research projects dealing with specific facets of these postulated events were discussed. Of special agricultural interest is the poten- tial for regulating the timing of acclimation in plants that have the inherent capacity to acclimate but the wrong endogenous timing to avoid winter injury in the area of production.
10 Drought EPIDERMP~ RESISTANCE TO EVAPORATION Paul E. Waggoner, Department of Soils and Climatology, Connecticut Agricultural Experiment Station Epidermal resistance to evaporation is mainly determined by sto- matal resistance. Theory and experiment quickly demonstrate the ef- fectiveness of changes in stomatal resistance in changing evaporation, but analysis of the effect of stomatal resistance upon evaporation from an entire canopy of leaves is more complicated. The canopy is divided into several horizontal strata. The sum of evaporation and sensible heat transfer in each equals radiation absorp- tion. The difference in leaf temperature and hence potential for water and sensible heat exchange between two strata is determined by the sum of the products of heat currents and resistances through epidermis, boundary layers, and air within the strata. These facts make it pos- sible to calculate evaporation and temperature and humidity micro- climates within a canopy. The calculations predict a change in evap- oration from a canopy when stomatal resistance changes moderately, and experiments with stomata-closing chemicals have verified this predic- tion.
11 PROTOPLASMIC RESISTANCE OF PLANTS TO DROUGHT E. J. Stadelmann, Department of Horticultural Science, University of Minnesota Drought resistance can be caused in two ways. First, it can caused by constitutional factors that lead to drought avoidance. ond, it can be caused by drought tolerance, or drought hardiness, which we mean the ability of the protoplasm of a plant to survive riods of low water supply. be Sec- by pe- Two groups of plants can be formed on the basis of the environ- mental water relation: 1. Poikilohydric plants. The water content of poikilohydric plants (e.g., lichens) varies with the atmospheric humidity. These plants are resistant to complete drought and continue growth as soon as moisture is again available. 2. Homoiohydric plants. As long as atmospheric conditions are not too unfavorable, homoiohydric plants maintain a more or less con- stant degree of hydration. These plants have only limited drought resistance. All crop plants belong to this group. The protoplasm contributes directly to drought resistance (1) by determining the degree of drought avoidance through control of the osmotic concentration in the cell sap and (2) by the structural ability of the protoplasm to withstand drought. Different criteria are used to measure the degree of drought resistance: (1) survival of the plant, (2) growth, and (3) survival of cells under desiccation conditions (desiccation resistance). If different criteria are applied, different degrees of drought resistance may be found for the same plant. Changes or differences in drought resistance in plants of the same species or variety are accompanied by distinct changes in physiochemical qualities of the living protoplasm: (1) Protoplasmic viscosity is al- ways higher in drought-resistant varieties, (2) Osmotic ground value of the same kind of cells is often higher in drought-resistant varieties, (3) Permeability for nonelectrolytes changes also. These protoplasmic qualities also change when a plant is submitted to drought. Evaluation of these changes in protoplasmic qualities may be used in screening pro- grams to determine the degree of hardiness.
