Phloem Girdling of Norway Spruce Alters Quantity and Quality of Wood Formation in Roots Particularly Under Drought.

Gina Rainer-Lethaus,Walter Oberhuber

doi:10.3389/fpls.2018.00392

Abstract

Carbon (C) availability plays an essential role in tree growth and wood formation. We evaluated the hypothesis that a decrease in C availability (i) triggers mobilization of C reserves in the coarse roots of Picea abies to maintain growth and (ii) causes modification of wood structure notably under drought. The 6-year-old saplings were subjected to two levels of soil moisture (watered versus drought conditions) and root C status was manipulated by physically blocking phloem transport in the stem at three girdling dates (GDs). Stem girdling was done before the onset of bud break [day of the year (doy) 77], during vigorous aboveground shoot and radial stem growth (GD doy 138), and after cessation of shoot growth (GD doy 190). The effect of blockage of C transport on root growth, root phenology, and wood anatomical traits [cell lumen diameter (CLD) and cell wall thickness (CWT)] in earlywood (EW) and latewood (LW) was determined. To evaluate changes in belowground C status caused by girdling, non-structural carbohydrates (soluble sugars and starch) in coarse roots were determined at the time of girdling and after the growing season. Although fine root mass significantly decreased in response to blockage of phloem C transport, the phenology of root elongation growth was not affected. Surprisingly, radial root growth and CLD of EW tracheids in coarse roots were strikingly increased in drought-stressed trees, when girdling occurred before bud break or during aboveground stem growth. In watered trees, the growth response to girdling was less distinct, but the CWT of EW significantly increased. Starch reserves in the roots of girdled trees significantly decreased in both soil moisture treatments and at all GDs. We conclude that (i) radial growth and wood development in coarse roots of P. abies saplings are not only dependent on current photosynthates, and (ii) blockage of phloem transport induces physiological changes that outweigh drought effects imposed on root cambial activity and cell differentiation.

Highlights

Norway spruce [Picea abies (L.) Karst.] is the dominant coniferous tree species in the Central European Alps
The fine root biomass (≤ 2 mm) in both soil moisture treatments was significantly reduced compared to controls irrespective of girdling dates (GDs), except in drought-stressed trees at GD doy 190 (Figure 2)
All drought-stressed trees girdled in mid-May (GD doy 138) had died by the end of September, watered trees girdled on the same date were less affected

Summary

Introduction

Norway spruce [Picea abies (L.) Karst.] is the dominant coniferous tree species in the Central European Alps. Girdling Alters Wood Formation in Roots in the upper soil horizon (Puhe, 2003), P. abies shows high sensitivity to drought stress, leading to reduced growth, a shortened growing season, and early culmination of radial stem growth in late spring (Mäkinen et al, 2001; Alavi, 2002; Pichler and Oberhuber, 2007; Lévesque et al, 2013; Swidrak et al, 2014). A significant increase in the root-shoot ratio in response to periodic drought treatment was reported for P. abies seedlings (Sonesson and Eriksson, 2003), the degree of change in the rootshoot ratio is affected by the intensity of soil drought (Schall et al, 2012), because suppressed root growth and increase in fine root mortality in response to drought stress is a frequently reported phenomenon (e.g., Gaul et al, 2008; Brunner et al, 2009; Rytter, 2013; for a review see Brunner et al, 2015)

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