Coleoptile Growth: Variation in Elongation Patterns of Individual Coleoptiles

Etiolated coleoptiles of barley differ in rate of growth and the time during germination when most rapid growth occurs. The final heights of coleoptiles fall into these classes: (1) short, ^ 10 mm, (2) medium, ^ 20 mm, and (3) tall, > 30 mm. Tall coleoptiles are produced by coleoptiles that show rapid growth, ^ 800 /z/h, in one of three periods, i.e. around 33 h, or 45 h or 70 h. The time when rapid growth occurs is, therefore, a critical characteristic by which we can distinguish three types of coleoptiles. Medium-height coleoptiles result from fairly rapid growth, ^ 400 /z/h, around either 45 h or 70 h . Small coleoptiles form a class that grows very slowly throughout the whole period of germination. Coleoptiles of uniform height at any time do not have identical growth-rates; we suggest they are in different physiological states. Variation in growth-rate parallels the variation in response of coleoptile segments to IAA. When coleoptiles of similar growth-rate and height are selected, segments show a repeatable stimulation of elongation when treated with io"*M IAA. Two separate mechanisms that initiate elongation appear to control coleoptile growth; one produces the initial growth seen in all coleoptiles and the second initiates rapid growth and operates at different times in different coleoptiles.