A New Method of Determining Drought Resistance.

Abstract

Present methods for testing resistance of plants to frost and drought require long observations in the field under conditions which are unsatisfactory for such an investigation, due to the great variation from year to year of weather conditions and other factors. Attempts to substitute laboratory tests for field experiments in measuring resistance of plants to freezing started with sugar cane some years ago in the auithor's laboratory. A laboratory test was developed for measuring the resistance of sugar cane to frost. The method was based on the measurement of the drip that cane segments exuded upon thawing, after having been frozen in the laboratory under standard conditions (5). It was shown that the drip given by various frozen products after thawing was closely related to the abilitv of these materials to withstand freezing storage changes (2, 3, 4). It was considered probable that frost hardiness and drought resistance are correlated and that a quantitative determination of the former property might therefore serve as a measure of the latter. It was one purpose of the present work to test this suggestion. MATERIALS AND METHODS Leaves and seedlings from various strains of lob-lolly pine (Pinus taeda L.), whose drought resistance was well established by field experiment, were generously provided by the Texas Forest Genetics Institute. These strains, which were designated by the following symbols in decreasing order of drought resistance: BA-5, FA-2, AN-3, BA-2, LA-1, LA-2, TY-4, and NC, were shipped to the author's laboratory at different seasons of the year. From the beginning of the experiment with pine leaves it was realized that, in contrast to sugar cane, it would be practically impossible to obtain drip from pine leaves by any direct or indirect method. Attention was directed to other tests suggested directly or indirectly by the literature on the subject of drought hardiness (9, 6). Several attempts were made to correlate drought resistance with respiration intensity, with pentosan, calcium, protein, and ash content of the leaves. None of these studies gave results which were significantly correlated with drought hardiness. It was thought that, if the ill effect produced by drought on the plants could be reproduced in the laboratory on parts of the plants (leaves, for instance) under standardized conditions, it might be possible to measure critical changes and to use them as a test for drought resistance. The first change observed on plants suffering from drought was the wilting of the leaves and the gradual fading of the 1 Received revised manuscript January 30, 1958. green color. When the color change reached some critical point, recovery no longer was possible. An attempt was made to reproduce these changes by putting pine needles in large desiccators containing saturated solutions of various salts, placed at different temperatures. These test proved unsatisfactory, as (lid other subsequent tests with a desomatic dehumidi-fier and a constant temperature humidity chamber. Attention was then turned to pigment changes induced by heating which were found to correlate well with drought resistance. When pine needles were immersed in water and heated gradually in a temperature regulated water bath, a temperature was reachecd beyond which chlorophyll destrtuction proceeded rapidly. The critical temperature proved to be between 55 and 560 C. A 5-g representative sample of healthy unblem-ished pine needles was placed in a large tube one inch in diameter with 50 ml of distilled water and heated in a water bath at 560 + 1° C for exactly 30 minutes. At the end of this time the leaves were removed from the tubes and were ground in a Waring blendor for five minutes with 100 ml of 80 % acetone solution in water. The chlorophyll extract was filtered and the filtrate examined immediately for light absorption with a Klett photoelectric colorimeter using a no. 66 red filter. Pigment was extracted from a 5-g sample of unheated leaves and the light absorption was also measured. The difference between the two readings was defined as the "Chloro-phyll Stability Index" (C.S.I.). Other methods used in this study were: the determination of bound water as described by Nelson and Hulet (10) and the determination of sulfhydryl groups by the nitroprusside colorimetric procedure (1).