Phytochrome changes correlated to mesocotyl inhibition in etiolated Avena seedlings.

The elongation of etiolated Avena mesocotyls is inhibited by red light (660 mmu). Immediately after exposing mesocotyl sections to varying doses of red light the ensuing concentrations of phytochrome in the far-red absorbing form (P(730)) were measured. The extent of mesocotyl inhibition observed 5 days later is proportional to the logarithm of P(730) concentration in mesocotyl tissue at the time of red light exposure.The inhibition of mesocotyl growth by red light can be reversed partially by subsequent exposure to far-red light (730 mmu). Increasing doses of far-red light result in decreasing concentrations of P(730) as compared with the original P(730) level due to the preceding red light exposure. The reduced mesocotyl inhibition of seedings which had been exposed to red and far-red light is proportional to the logarithm of P(730) concentration remaining in the tissue at the end of the two light exposures.This indicates that the same correlation exists between P(730) concentration and growth response whether the seedlings had been exposed to red light only or to red followed by far-red light.