Winter Hardiness in Guayule

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Abstract
Woody outdoor seedlings of guayule not yet unacclimated to low temps. survived only slightly lower temps. than did very succulent greenhouse plants of the same age. The rate of cooling, preceding an extended exposure to low temps., did not appreciably alter the resistance of the plant to frost injury. Unhardened seedlings of 4 vars. withstood prolonged exposure to 25[degree] but were severely injured as the result of an 8- to 10-hr. exposure to 22[degree]F. In most instances, the roots were severely injured by prolonged exposure to soil temps. of 26-28[degree]F. As an exception, roots of plants of selection A-5058 survived a short exposure (< 3 hrs.) to a soil temp. of 16[degree]F. Stems and leaves of transplants were readily acclimated by means of low temp. hardening treatments, so that they withstood repeated exposure to temps. ranging 5[degree]-10[degree] above zero. Stems of hardened plants of selection A-5058 withstood a 3-hr. exposure to[long dash]5[degree]. The roots of these plants were slightly injured but the plants survived. Based on a limited study of 4 selections, plants of A-5058 were most resistant to frost injury. Exptl. field plantings can readily be protected from temps. as low as 4[degree]F, and probably lower, by means of an adequate mulch. The rubber content of plants severely injured by frost was not significantly different from that of comparable uninjured plants during a period of 1-2 months after treatment.

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