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

Abstract

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.

Analysis of fast chlorophyll fluorescence rise (O-K-J-I-P) curves in green fruits indicates electron flow limitations at the donor side of PSII and the acceptor sides of both photosystems

“…Seeking for an additional significance for fruit photosynthesis, the apparent similarities in photosystem attributes between fruits and twigs cannot pass unnoticed. As in fruits (this investigation), trapping efficiency in twigs is lower than leaves (Filippou et al 2007, Manetas and Pfanz 2005, Tausz et al 2005) and this is not the result of sustained photoinhibition (Yiotis et al 2009). Moreover, a high population of non‐ Q b ‐reducing centers is evident in twigs (Kotakis et al 2006) and the same can be deduced for fruits from the low probability for a trapped exciton to move an electron further than Q A (Table 3).…”

The existence of C4-bundle-sheath-like photosynthesis in the mid-vein of C3 rice

“…These properties may require qualitative and quantitative adjustments of the photosynthetic attributes in such organs for both light and biochemical reactions (Yiotis and Manetas 2010 ). As observed in NADP-ME type C 4 plants, chloroplasts in PCSVS of C 3 plants deficient in linear but sufficient in cyclic electron flow would act as electron valves restoring the ATP/NADPH ratio (Yiotis et al 2009 ). For example, Kotakis et al ( 2006 ) show that twigs of Eleagnus angustifolius display low dark-adapted PSII photochemical efficiency and linear electron transport rates.…”

“…Because of fewer stomata and intercellular air spaces, but more layers of surrounding cells, the vascular tissue has been previously noted to reduce solubility and diffusivity of oxygen (Raven 1991 ; Hibberd and Quick 2002 ). Those further suppress mitochondrial respiration and result in ATP losses (Geigenberger 2003 ) in conjunction with the NADP + requirement for high C 4 -acid decarboxylating activity (Yiotis et al 2009 ). Therefore, the compensatory metabolic demands of higher ATP/NADPH ratios in mid-veins are comparable to that found in BS cells of NADP-ME type C 4 plants.…”

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

“…First the light has to pass the outer bark or periderm. Below the outer bark most stems of woody plants possess light absorbing, greenish inner bark or cortex parenchyma (Foote and Schaedle, 1976;Wittmann et al, 2001Wittmann et al, , 2006Pfanz et al, 2002;Pfanz, 2008, 2014), but chloroplasts can also be found in internal tissues isolated from light scources like xylem, xylem rays and pith (Pfanz et al, 2002;Dima et al, 2006;Berveiller et al, 2007;Yiotis et al, 2009). In young stems these chlorophyllcontaining tissues are able to use the transmitted light and the stem internal CO 2 for photosynthesis (e.g., Wittmann et al, 2001Wittmann et al, , 2006Pfanz et al, 2002;Wittmann and Pfanz, 2007).…”