EFFECTS OF THE PHYSICAL ENVIRONMENT ON THE PHYSICO-CHEMICAL PROPERTIES OF PLANT SAPS, AND THE RELATION OF THESE PROPERTIES TO LEAF TEMPERATURE

Abstract

Prefreezing with solid CO2 gave the best results and therefore was used in all experiments with the exception of the ones on bound water. In that case grinding of the tissue previous to extraction was followed. Moisture content of a plant sap may be read by a refractometer and corrected by a factor determined in the customary manner. The increase in osmotic pressure of a stored plant juice varies with the time and temp. of storage. The osmotic gradient within the plant depends on time of sampling, showing no close relationship with the leaf temp. gradient, while the younger leaves are always cooler than the older ones. Periodical wilting induced a more xeromorphic structure, lower water content, and higher osmotic pressure in soy bean plants but when grown under uniform conditions little varietal difference was observed. Bound water and osmotic pressure determinations were made on the expressed sap of milo and several cabbage varieties with the conclusion that the elaboration of hydrophilic colloids under greenhouse conditions when the plants were subjected to decreasing soil moisture resulted in differential cold resistance, indicated by bound water determinations and confirmed by actual freezing tests. Osmotic pressure concentrations may be largely the result of environment; the percentage of bound water in the fresh juice seems to be the best index of hardiness.