2019 ASHS Annual Conference

Plants use the photoreceptor phytochrome to sense and respond to shade. The ratio of active phytochrome to total phytochrome (also known as phytochrome photo-equilibrium or PPE) has been predicted by measurements of spectral distribution and weighting factors published from multiple sources. This estimation is often well correlated with plant morphology under a spectral distribution that only changes in quantity of far-red photons. However, it is unreliable when changing ratios of colors below 700 nm. This may be the result of several factors: 1) Variation in PPE weighting factors among studies. 2) The values are primarily for phytochrome-A in oats. 3) There are interactions among photoreceptors. 4) Weighting factors come from estimations of phytochrome conversions in etiolated tissue. The dominating effect of chlorophyll in leaf tissue means that phytochrome is not exposed to the incident radiation above the leaf. Instead, phytochrome perceives a spectral distribution enriched in green and far-red. Our data indicate that the estimation of PPE in green leaves is improved by multiplying the spectral distribution by leaf transmission, then applying the weighting factors to the transmitted spectra.