The optical, absorptive and chlorophyll fluorescence properties of young stems of five woody species

Abstract

To reveal the roles of stems in responding to the surrounding light environment, the optical properties of stem tissues of five woody species and their effect on bark, xylem and pith photosynthesis were investigated. Species-specific bark morphology and the distribution of pigments along the stem cross-section altered the radial conduction of light to a different degree, but in all investigated species light gradients within stems were markedly steep and visible light was largely depleted within the outer bark and cortex of the stems. Thereby, optical properties of the outer and inner bark resulted in an internal light environment, which is very different from outside according to intensity and spectral composition. Among species PAR-transmittance of outer bark or periderm varied in quantity between mean values of 8.5 and 42% and PAR-transmittance of total bark between 2.2 and 6.2%. The blue-band of the spectrum was totally absorbed by the outer and inner bark tissues. In the red-band total bark transmittance varied between 2 and 11%. Thus, blue light can only trigger photosynthetic electron transport in the cell layers of the cortex near the stem surface, whereas in the underlying cell layers of the xylem or pith the energy for photosynthesis can only be provided by longer wavelengths (green and particularly red light). In all species, the internal light gradient matched well with the chlorophyll fluorescence properties of the respective stem tissues. The maximum and effective quantum efficiencies of PSII clearly decreased from the cortex toward the innermost stem tissues (xylem, pith), which showed also a lower capability of photosynthetic electron transport of PSII. Our results indicate not only that light signals can enter the interior of stems and are modified in quantity and quality by its path through the stem. There is evidence to suggest that the light regime within stems has also profound effects on plastid differentiation and thus the photosynthetic properties of the respective stem tissues.