PHYSICO-CHEMICAL STUDIES ON THE NATURE OF DROUGHT RESISTANCE IN CROP PLANTS

Abstract

Factors affecting drought resistance include those concerned in absorption, transpiration, and wilt endurance. The colloidal properties of leaf-tissue fluids are believed to be important in water retention under droughty conditions. These and other physico-chemical properties have been determined in a number of cereals, grasses and other plants, in relation to known drought adaptations. Methods for the study of colloidal properties have also been investigated.Hydrophilic colloids bind water and increase the concentration of aqueous solutions, as shown by freezing-point determinations. Dextrose is less effective than sucrose in demonstrating this phenomenon. Concentration, quality, and state of dispersion or coagulation of colloids are factors affecting the degree of water binding.The extraction of plant-tissue fluids and the estimation of dry substance by a modification of the refractometric method are described. The hydrolysis of sucrose added to plant juice in the bound-water determination is shown experimentally not to introduce serious errors. Storage of leaf tissue or press juice for a few hours even at 0 °C. leads to changes in colloidal properties, but errors from this source are avoided by prompt handling.Cylindrical dialyser sacs of 100 cc. capacity were found to require about 48 hr. for dialysis of plant juice at 3 °C., with the water changed every hour. A gradual partial coagulation of colloids took place during dialysis, as shown by a decrease in refractive index, by an increase in gold number, and by sedimentation when centrifugalized. Hydration and dispersion were not always related, though both were affected by hydrogen ion concentration and by salts. Acid and alkaline salts stimulated hydration less because of the opposing "salt effect". In the presence of the natural crystalloids, hydration was usually greater than in dialysed juice.The osmotic pressure of the tissue fluids of crop plants has been found to vary with physiological scarcity of water, but is not a reliable index of drought resistance. Bound-water content has been found more dependable; the cultivated wheats and several grasses have been on this basis satisfactorily arranged in the order of their drought resistance. The contrasting behavior of two poplar species, with reference to leaf fall and frost resistance, has also been explained. The separation of the colloids by dialysis and the determination of their gold number have been used as aids in interpreting the bound-water values.The method of measuring imbibition pressure by direct pressure on masses of leaves proved unsuitable. The rates of water loss by evaporation from cactus segments and detached leaves of two grasses, under controlled humidity conditions, proved the remarkable ability of the cactus to retain moisture, but in the grasses showed no relation to drought resistance.