Phytochrome-controlled extension growth of Avena sativa L. seedlings : II. Fluence rate response relationships and action spectra of mesocotyl and coleoptile responses.

Fluence rate response curves for light-induced inhibition of mesocotyl growth and promotion of coleoptile growth in etiolated Avena sativa L. (cv. Victory) were developed. The irradiation time was 24 h. Fluence rates between 10(-6) and 10(5) nmol m(-2)s(-1) and 30 wavelengths between 563 and 1,093 nm were used. The main conclusions are as follows: 1. Both organs exhibit a low fluence rate response as well as a high fluence rate response. 2. The mesocotyl response is more sensitive to light than the coleoptile response. 3. The low fluence rate response of the mesocotyl shows a threshold of sensitivity at about 10(-7) nmol m(-2)s(-1) (i.e., total fluence of 5·10(-2) nmol m(-2) during the experiment) in the red and a saturation (about 70% inhibition of growth) at 10(-4) nmol m(-2)s(-1) (50 nmol m(-2)). 4. The action spectrum for the low fluence rate response parallels the Pr absorption spectrum. Alterations induced by screening are discussed. 5. The action spectrum demonstrates an exponential decrease in apparent photoconversion cross-section (Pr→Pfr) up to about 800 nm. Between 800 and 1,093 nm the photoconversion cross-section is only weakly dependent on wavelength. 6. The action spectrum for the high fluence rate response shows a broad peak in the red, a trough at 723 nm, and a sharp peak at 740-750 nm.