Phytochrome-regulated Expansion of Mustard (Sinapis alba L.) Cotyledons

In developing mustard (Sinapis alba L.) seedlings continuousred light acting via the agency of phytochrome stimulates therate of expansion of cotyledons. Although phytochrome actionon cotyledon expansion is evident only after 36 h from sowing,the photoresponse escapes from reversibility at about 15 h fromsowing. The time lag of 21 h between loss of photoreversibilityand the onset of photoregulated cotyledon expansion indicatesthe existence of long-lived components in the phytochrome-triggeredsignal chain. Phytochrome-regulated cotyledon expansion doesnot require the involvement of photosynthesis, as applicationof SAN 9789, an inhibitor of chloroplast biogenesis, did notaffect cotyledon expansion. The role of turgor pressure-relatedcellular parameters such as osmotic potential () cell wall extensibility(m), hydraulic conductivity (L) and yield threshold (Y) forcell expansion were examined during photoregulated cotyledonexpansion. Using the general equation of cell growth dv/dt =[Lm/(L+m)]( - Y), where dv/dt is the rate of volumetric growth,it was demonstrated that the light-mediated cotyledon expansionresults from an increase in cell wall extensibility (m). Theseresults are discussed in relation to the photoregulation ofcotyledon expansion. Key words: Cell wall extensibility, growth, Sinapis alba L., phytochrome