Evidence for light-independent and steeply decreasing PSII efficiency along twig depth in four tree species

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Recent reports have indicated a considerably inactivated PSII in twig cortices, in spite of the low light transmittance of overlying periderms. Corresponding information for more deeply located and less illuminated tissues like xylem rays and pith are lacking. In this investigation we aimed to characterize the efficiency of PSII and its light sensitivity along twig depth, in conjunction with the prevailing light quantity and quality. To that aim, optical methods (spectral reflectance and transmittance, chlorophyll fluorescence imaging, low temperature fluorescence spectra) and photoinhibitory treatments were applied in cut twig sections of four tree species, while corresponding leaves served as controls. Compared to leaves, twig tissues displayed lower chlorophyll (Chl) levels and dark-adapted PSII efficiency, with strong decreasing gradients towards the twig center. The low PSII efficiencies in the inner stem were not an artifact due to an actinic effect of measuring beam or to an enhanced contribution of PSI fluorescence. In fact, the PSII/PSI ratios in cortices were higher and those in the xylem rays similar to that of leaves. Inner twig tissues were quite resistant to photoinhibitory treatments, tolerating irradiation levels several-fold higher than those encountered in their microenvironment. Moreover, the extent of high light tolerance was similar in naturally exposed and shaded twig sides. The results indicate an increasing, inherent and light-independent inactivation of PSII along twig depth. The findings are discussed on the basis of a recently proposed model for photosynthetic electron flow in twigs, taking into account the specific atmospheric and light microenvironment as well as the possible metabolic needs of such bulky organs.

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