[Investigations on the energetics of phytochrome mediated photomorphogenesis in mustard seedlings (Sinapis alba L.)].

To characterise the energetics of whole seedlings of Sinapis alba L. during photomorphogenesis under continuous far-red light, measurements were made (between 36 and 72 hours after sowing) of changes in dry weight, fat content, heat of combustion, O2 uptake and CO2 evolution in irradiated and dark treated control plants. Irradiation caused a delay in the time of maximum O2 uptake and CO2 evolution and led to an increase in the total gaseous metabolism. Likewise, the rates of breakdown of fats and release of energy were increased; this occurred even though the total synthesising capacity of the seedlings, as measured by fat free dry weight or by heat of combustion of fat free dry matter, was lower in the farred irradiated seedlings than in the dark controls. Thus, during the course of photomorphogenesis the yield of plant material, as measured by the increase in fat free dry matter divided by the decrease in fat, or by the increase in heat of combustion of fat free dry matter divided by the decrease in heat of combustion of fat, was markedly reduced (approximately 60%). Several possible explanations were put forward to account for this reduction, but lack of thermodynamic data allowed none to be favoured.Theoretical considerations showed that the experimental error associated with measurements of gaseous metabolism was always larger when the indirect manometric method rather than the direct method was used. This effect was more pronounced the higher the respiratory quotient was.