Starch-filled leaves of plants which have been subjected to low dosages of naturally occurring photochemical oxidants, ozone, or peroxyacetyl nitrate hydrolyze their starch more slowly when placed in the dark. Delayed hydrolysis occurs irrespective of whether the oxidants were applied during the light or dark period. Occasionally this effect is evident only in the intervenal areas.

Parthenium argentatum Gray (guayule) and P. incanum (mariola) grow in close associations in their native habitat of Mexico. Variation in rubber concentration and morphology of guayule has been known for a long time. Studies of over 75 native guayule plants indicated the presence of at least three prominent forms of guayule plants. Group I guayule plants have oblanceolate leaves and leaf margins are entire to two‐toothed. T‐shaped leaf trichomes are with centrally attached stalk and cap cell with two blunt ends. Plants in this group are high in rubber, containing on the average 17% rubber. Group II guayule plants have narrow elliptic leaves and leaf margins are entire to four‐toothed. T‐shaped trichomes have an acentrally attached stalk and a cap cell with short end blunt, long end pointed and straight. Rubber content in these plants averages 10%. Group III guayule plants have ovate leaves and leaf margins are four‐ to eight‐toothed. T‐trichomes have an acentrally attached stalk and a cap cell with short end blunt, long end pointed and wavy or curved. These plants on the average contain 6% rubber. Morphological as well as biochemical data indicate the presence of mariola genes in the last two groups of plants and this has resulted in an increase in trichome length and a decrease in the rubber content. Group II guayule plants are of more common occurrence than the others. Based on the data presented here, high rubber bearing guayule plants in native stands can be easily selected by analyzing trichome morphology.

Guayule (Parthenium argentatum Gray), a xerophytic shrub native to northeastern Mexico and southern Texas, is receiving renewed attention as a potential source of natural rubber in the semiarid regions of the world. A major problem in propagating guayule is seed dormancy. The objectives of this investigation were to evaluate and modify old seed treatment methods and to develop an improved standard procedure for quick and complete germination of guayule seeds to enhance breeding and improvement of the crop. The optimum concentration of sodium hypochlorite (NaOCl) needed for breaking seed dormancy was higher for younger than older seeds. Seeds should be washed and soaked in distilled water for 8 hours followed by a 2‐hour treatment with a solution of equal parts of gibberellic acid (200 ppm) and NaOCl (1.0% for fresh seeds, and 0.25% for 1‐year‐old seeds). With this procedure freshly harvested seeds germinated 100% under light and over 70% in complete darkness. Emergence of seedlings from soil depended primarily upon seed size. Large guayule seeds emerged better at all depths to 18 mm after standard treatment than did medium or small seeds.