Phytochrome-regulated expansion of mustard (Sinapis alba L.) cotyledons

Abstract

In developing mustard (Sinapis alba L.) seedlings continuous red light acting via the agency of phytochrome stimulates the rate of expansion of cotyledons. Although phytochrome action on cotyledon expansion is evident only after 36 h from sowing, the photoresponse escapes from reversibility at about 15 h from sowing. The time lag of 21 h between loss of photoreversibility and the onset of photoregulated cotyledon expansion indicates the existence of long-lived components in the phytochrome-triggered signal chain. Phytochrome-regulated cotyledon expansion does not require the involvement of photosynthesis, as application of SAN 9789, an inhibitor of chloroplast biogenesis, did not affect cotyledon expansion. The role of turgor pressure-related cellular parameters such as osmotic potential (π) cell wall extensibility (m), hydraulic conductivity (L) and yield threshold (Y) for cell expansion were examined during photoregulated cotyledon expansion. Using the general equation of cell growth dv/dt = [Lm/(L+m)](Δπ - Y), where dv/dt is the rate of volumetric growth, it was demonstrated that the light-mediated cotyledon expansion results from an increase in cell wall extensibility (m). These results are discussed in relation to the photoregulation of cotyledon expansion. © 1988 Oxford University Press.