JXB Advance Access published online on April 23, 2008
Journal of Experimental Botany, doi:10.1093/jxb/ern105
RESEARCH PAPER |
The influence of epidermal windows on the light environment within the leaves of six succulents
1Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas 66045, USA
2Department of Plant Biology, University of Vermont, Burlington, Vermont 05405, USA
* To whom correspondence should be addressed. E-mail: ecophys{at}ku.edu
An omni-directional fibre optic microprobe was used to measure the quantity and quality of light within the leaves of six succulents having epidermal windows, three species having a subterranean growth habit (Haworthia truncata, Lithops olivacea, and Opthalmophyllum longum) and three growing above ground (Peperomia dolabriformis, P. graveolens, and the sprawling vine Senecio rowleyanus). Although light levels at most locations inside the leaves of all species were high, near those incident on the window surfaces, light levels inside the leaves of the two species of Peperomia often greatly exceeded incident light levels, indicating considerable light scattering and focusing by the leaf tissue. The spectral quality of light inside the leaves of all taxa reflected the absorption properties of chlorophyll, with most of the photons in the green wavelengths. Light quality and quantity inside the leaves did not correlate with the growth habit of the plants, the size of the window (as a proportion of the total leaf area), or location inside the leaf, although light levels generally declined and wavelengths increased deeper in the leaves. Application of reflective tape to the windows reduced internal light levels in L. olivacea and S. rowleyanus, although reductions were not always statistically significant. Although light levels throughout the leaves of P. graveolens were substantially and significantly reduced as a result of the application of reflective tape to its windows, the light levels even at the basal chlorenchyma on the abaxial side of the leaf remained high. In all species investigated, the levels of near-infrared radiation inside the leaves were surprisingly high, yet also declined deeper inside the succulent leaves. This near-infrared radiation may add to the heat load of these plants. Furthermore, application of reflective tape to the windows also reduced the amount of near-infrared radiation inside the leaves of the three succulents examined. These results led to a novel, testable hypothesis that may help to explain previous findings that application of reflective tape to the windows of the leaves of these succulents did not effect a reduction in photosynthetic activity.
Key words: Chlorenchyma, Haworthia truncata, hydrenchyma, infrared radiation, internal light environment, leaf windows, light focusing and scattering, Lithops olivacea, omni-directional fibre optic microprobe, Peperomia dolabriformis, Peperomia graveolens, Opthalmophyllum longum, photon flux density, Senecio rowleyanus, succulents, visible radiation
Received 12 January 2008; Revised 9 March 2008 Accepted 17 March 2008