Changes in Carbohydrate Content of Wheat Plants during the Process of Hardening for Drought Resistance.

The process of hardening for drought resistance results in many changes in the vegetative organs of plants. PRINGSHEIM (7) has noted that pumpkin seedlings, grown under conditions of insufficient moisture, had an increased osmotic pressure in the cells and a higher resistance to drought than check plants grown in a moist soil and humid atmosphere. Since then a number of investigations have been conducted dealing directly or indirectly with the physiology of hardening. FREY (1), for example, has established that under conditions of sufficient soil moisture, plants which previously had suffered from lack of water supply were transpiring more intensely than normal plants grown in a moist soil. Studies by TUMANOV (9) have shown that sunflower plants, which had been hardened by repeated wilting, transpired 73 per cent. more moisture than non-hardened plants; while buckwheat, which had suffered from periodic drought, had an increased transpiration of 35 per cent. The same investigator states that plants which had been exposed to recurrent wilting exhibited greater intensity of assimilation after their return to a normal state. For buckwheat this increase was 50 per cent. over normal plants. TUMANOV observed also that hardened plants had smaller cells and consequently a larger number per unit leaf area and a denser network of veins. The reaction between various parts of the plant during development was likewise changed under the effect of hardening. The leaf blade was more developed than the stem, and the root system was larger than the aboveground portion of the plant. VASSILIEV (11) has noted that the stomata were fully open when wheat was grown with a continuous optimum soil moisture of 40 per cent. of the total moisture capacity of the soil. The same wheat grown in soil saturated to 70 per cent. of its capacity suffered from insufficient water supply and the stomata of the leaves were almost closed when the moisture content in the soil was decreased to 40 per cent. These investigations present morphological evidence about the hardening process. But such gross studies of the nature of hardening, as for example the xerophytic structure, the increased capacity of gas exchange (through stomata), and a more intensive transpiration and assimilation,