Growth, Rubber Storage, and Seed Production by Guayule as Affected by Boron Supply

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Abstract
1. Plants of guayule were grown from early seedling stage in gravel culture for a period of 16 months, during which time they were supplied with nutrient solutions containing 0.0-10.0 p.p.m. of boron. In other experiments, somewhat older seedlings were transplanted from the field and grown in sand or gravel cultures to determine the effects of deficient or excessive amounts of boron (0.0-40.0 p.p.m.) when these were supplied by means of nutrient solutions. 2. The plants showed marked boron-deficiency symptoms when supplied with a boron-free nutrient, but they subsequently grew vigorously and produced seeds when supplied with a nutrient solution containing 4 p.p.m. 3. According to results based on vegetative growth in gravel cultures, guayule has a tolerance range of from 0.1 to 2.0 p.p.m. of nutrient solution, a range somewhat wider than that reported for most crop plants of a mesophytic type. 4. Data on percentage of rubber in gravel-culture plants show an optimum boron requirement of 0.1-2.0 p.p.m., a concentration range similar to that required for maximum vegetative growth and seed production. The concentration of rubber in the stems and roots of boron-deficient plants was lower than that of similar parts of plants grown with an adequate supply. 5. Since boron deficiency was associated with both reduced vegetative growth and lower percentage of rubber, the total rubber output of boron-deficient plants was relatively small. An excessive amount of boron (10 p.p.m. in the nutrient) was also associated with a decrease in rubber output. Over the range studied, the amount of boron supplied to the plants had only a very slight effect on the concentration of resins in their stems and roots. 6. The stem diameters of gravel-culture plants showed that cambial activity was somewhat reduced in the plants receiving nutrient containing 10.0 p.p.m. of boron, and also in the stems of plants that received a deficient amount. The proportions of pith, xylem, bark, and bark fiber in the basal region of the stem were not altered as the result of supplying the plants with nutrient containing various levels of boron (0.01-10.0 p.p.m.). In stems of plants grown with a limited supply (0.004 p.p.m.) the area of bark was greater, in proportion to the amount of wood produced, than in comparable segments of stem from plants grown with an adequate boron supply. 7. Boron-deficient plants produced fewer seeds than did plants supplied with an adequate amount. Seeds produced by boron-deficient plants were smaller and their germination was lower than those from plants grown with an adequate supply. With respect to size, number, and percentage germination, the quality of seeds produced by plants supplied with nutrient containing an excessive amount of boron (10 p.p.m.) was equal to that produced by plants grown with a moderate supply (0.1 p.p.m.). Boron accumulated in the seeds (and also leaves) of the plants grown with an adequate boron supply.