Spectral dependence of photosynthesis and light absorptance in single leaves and canopy in rose

Little is known about the effects of leaf pigmentation (related to leaf ontogeny), on the spectral dependence of photosynthesis and most observations have been limited so far to single leaves. This study aimed to investigate photosynthesis and the related optical properties of two types of rose leaves, young reddish leaves and middle age green leaves, and to quantify the spectral dependence of photosynthesis at the canopy level. Photosynthesis was measured with CO2/H2O gas analyzer on intact leaves of rose 'Akito' at narrow band light of 18 wavelengths. Subsequently, the optical properties (transmittance, absorptance and reflectance) were measured with spectrophotometer. A mechanistic crop model was used for up scaling measurements at the single leaf level to the crop level (crop with LAI = 3). The green and the reddish leaves had similar total PAR absorptance, even though absorptance around 550 nm was slightly lower in the green leaves. The maxima of photosynthesis efficiency were at 640-680 nm for quantum yield (per absorbed light unit) and at 660-680 nm for action spectrum (per incident light unit), regardless the colour of the leaf blade. In the range 500-580 nm, both the quantum yield and the action spectrum were lower in reddish than in green leaves. Differences in optical properties and photosynthetic behaviour were related to the higher content of anthocyanins in red leaves. The spectral dependence of light absorption and photosynthesis at the canopy level differed distinctly from that at leaf level. The spectral differences in absorption at the leaf level almost disappeared at the canopy level. Consequently, while the action spectrum of green light (520-570 nm) was only 67% of that of red light (680 nm) at the leaf level, it increased to 79% at the crop level. Young reddish leaves had higher absorptance but lower action spectrum and quantum yield at green light. Spectral differences in photosynthesis at the canopy level are much smaller than at the leaf level. Our short term measurements suggest that optimizing spectral output of LED lamps may increase photosynthesis up to 12% for a canopy with green leaves and up to 17% for a canopy with reddish leaves when compared to the spectrum of HPS lamps. (C) 2010 Elsevier B.V. All rights reserved.