Anatomical adjustment of mature leaves of sycamore maple (Acer pseudoplatanus L.) to increased irradiance

Tomasz P Wyka,Jacek Oleksyn,Piotr Robakowski,Roma Żytkowiak

doi:10.1007/s11120-022-00898-8

Abstract

Trees regenerating in the understory respond to increased availability of light caused by gap formation by undergoing a range of morphological and physiological adjustments. These adjustments include the production of thick, sun-type leaves containing thicker mesophyll and longer palisade cells than in shade-type leaves. We asked whether in the shade-regenerating tree Acer pseudoplatanus, the increase in leaf thickness and expansion of leaf tissues are possible also in leaves that had been fully formed prior to the increase in irradiance, a response reported so far only for a handful of species. We acclimated potted seedlings to eight levels (from 1 to 100%) of solar irradiance and, in late summer, transferred a subset of them to full sunlight. Within 30 days, the shaded leaves increased leaf mass per area and became thicker mostly due to elongation of palisade cells, except for the most shaded individuals which suffered irreversible photo-oxidative damage. This anatomical acclimation was accompanied by partial degradation of chlorophyll and a transient decline in photosynthetic efficiency of PSII (Fv/FM). These effects were related to the degree of pre-shading. The Fv/FM recovered substantially within the re-acclimation period. However, leaves of transferred plants were shed significantly earlier in the fall, indicating that the acclimation was not fully effective. These results show that A. pseudoplatanus is one of the few known species in which mature leaves may re-acclimate anatomically to increased irradiance. This may be a potentially important mechanism enhancing utilization of gaps created during the growing season.

Highlights

Survival and growth of trees regenerating in the understory is made possible by adjustments of plant architecture and leaf anatomy, and physiological regulation, together increasing the efficiency of light harvesting and improving the whole plant’s energy budget (Givnish 1988; Valladares and Niinemets 2008)
Analysis of variance showed that leaf mass per area (LMA) and all leaf tissues adjusted to growth irradiance (Table 1)
When plants maintained under the same shading intensities were resampled after 30 days, the LMA value was significantly higher while retaining the positive relationship with irradiance

Summary

Introduction

Survival and growth of trees regenerating in the understory is made possible by adjustments of plant architecture and leaf anatomy, and physiological regulation, together increasing the efficiency of light harvesting and improving the whole plant’s energy budget (Givnish 1988; Valladares and Niinemets 2008). With an early stage of area expansion and tissue formation lasting usually up to 6 weeks in temperate trees (Niinemets et al 2004; Ding et al 2014) followed by a developmental plateau (Pantin et al 2012) According to this accepted view, formation of the final leaf shape, thickness and tissue structure takes place during its early stage of growth. Gap formation is required by many shade-tolerant trees for growth into the canopy layer Such sudden increases in irradiance may cause photo-oxidative stress in unacclimated leaves (Oguchi et al 2006), setting off complex defensive and acclimative responses (Tognetti et al 1998; Campa et al 2017; Oguchi et al 2017) or causing premature leaf death. In pre-existing leaves the extent of physiological, photosynthetic, re-acclimation varies among species (Yamashita et al 2000; Calzavara et al 2017; Martinez and Friedley 2017; Oguchi et al 2017) and anatomical reacclimation is considered rare (Oguchi et al 2018)

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