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Phytochrome was assayed in vivo in the rhizomes of 12 to 14-week old johnsongrass [Sorghum halapense(L.) Pers.] plants. The phytochrome levels decreased basipetally from the apex through the eighth node. The decrease was logarithmic on a dry weight basis, but less rapid on a per segment basis. On a dry weight basis, higher phytochrome measurements were obtained with samples from the interior of the rhizome than from the external regions. The far-red absorbing form of phytochrome (Pfr) was shown to be very labile at 25 C in johnsongrass rhizomes. These results provide a basis for examining photomorphogenic responses of rhizomes to light and for correlating these responses to phytochrome photophysiology.

Various levels of intraspecific competition influenced johnsongrass [Sorghum halepense(L.) Pers.] reproduction. Increases in density, from one to eight plants per 20-cm pot, delayed for formation of rhizomes and panicles and reduced the final dry weight accumulation in these reproductive structures under greenhouse conditions. Tillering was delayed by medium density (4 plants per pot) and inhibited by the highest density (8 plants per pot). At all densities, the dry weight accumulation in reproductive structures was associated with a decrease in dry weight of the shoot material. Most of the reproductive effort was in the formation of rhizomes rather than panicles. Crowding of johnsongrass plants delayed the formation of the reproductive structures and reduced their final dry weight.