Effect of Light Intensity and Nutrient Supply on Growth and Production of Rubber and Seeds by Guayule

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Abstract
1. Potted plants of guayule were grown for a period of 5 months during the spring, summer, and fall in natural light of full intensity, and other plants were subjected to lower intensities obtained by means of cloth filters. Half the total number of plants were supplied with nutrient at intervals to maintain soil fertility, while the remainder were grown in relatively infertile soil. During the winter the experiment was transferred to a greenhouse and continued under controlled temperature conditions. 2. A 25% reduction in light intensity was associated with a 36% reduction in the amount of rubber produced by plants grown in relatively fertile soil. Further reduction of light was associated with a corresponding decrease in the percentage and absolute amount of rubber stored. The percentage and absolute amounts of rubber produced by plants grown in infertile soil were relatively low and not greatly affected by shading. 3. A reduction of 25% in light intensity reduced the dry weight of stems and roots of plants grown in fertile soil by 14.5%; and more extreme reduction in light further retarded vegetative growth. Of plants grown in infertile soil, the greatest leaf weight was produced by those exposed to 75% of full sunlight, but moderate shading had no appreciable effect on the weight of stem and roots. 4. Two-thirds of the leaves of plants grown in direct light remained green throughout the winter, but only one-third of those on deeply shaded plants remained green and functioning. 5. Reduced light intensity (44%-75% of unscreened light) favored seed production during June, July, and August. Shading was less effective in stimulating seed production during September. Decrease in light intensity was associated with decrease in the proportion of large seeds produced. Withholding inorganic nutrients was associated with an increase in the proportion of small and a decrease in the proportion of large seeds. Reduction in light intensity was not associated with an appreciable change in seed quality. 6. The cross-sectional area of stems was not significantly affected by a 25% reduction in intensity; but further reduction was associated with marked decrease in stem thickness, cross-sectional area of pith, xylem, bark, and fiber tissue in the bark. 7. Percentage areas of the various stem tissues were essentially constant both in the different light intensities and with the two fertilizer treatments. The percentage of bark was not correlated with changes in percentage of rubber. 8. Under full light intensity, the percentage of resin in plants grown with a limited nutrient supply was equal to that of plants to which nutrients were supplied; and with shading those subjected to a limited nutrient supply contained a higher percentage of resin than did those afforded nutrients.

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