Response of Guayule to Alternating Periods of Low and High Moisture Stresses

Abstract

1. Guayule nursery-transplants were grown for 14 months in alternating periods of low and high moisture stress of different durations. The dry weights and the carbohydrate, resin, and rubber contents of the plants were determined at the end of each period of moisture stress. During periods of high moisture stress, the rate of increase in dry weight of the entire plant was depressed, but most of this could be accounted for by the effect on leaf weight. The percentages of levulins, in roots and stems especially, during these periods increased markedly, while the percentages of the other carbohydrates-sugars, inulin, and pentosans-remained fairly constant. Except for the period from November 6 to March 6, the percentages of rubber increased during periods of high stress at a more rapid rate than during periods of low stress. 2. During periods of low stress following high stress, the percentage and absolute content of levulins per plant decreased. This indicated that the levulins were serving as the principal reserve compounds, inasmuch as the amount of the other carbohydrates did not change greatly. 3. The rubber content on a grams-perplant basis generally increased throughout the experiment, except for the period November 6-March 6, when it did not change. With this exception, the increase was greater during periods of high moisture stress than during low moisture stress. The absolute rubber content perplant was further increased, above that of plants continuously on low moisture stress, after each successive period of high moisture stress. 4. The results indicate that rubber accumulation can be forced in guayule plants by alternating periods of low and high moisture stress but that the periods of high stress must not be too short and, to induce maximum yields, should occur during seasons of the year other than the winter months.