Effects of &lt;em&gt;Pinus taeda&lt;/em&gt; leaf anatomy on vascular and extravascular leaf hydraulic conductance as influenced by N-fertilization and elevated CO&lt;sub&gt;2 &lt;/sub&gt;

Jean-Christophe Domec,Sari Palmroth,Ram Oren

doi:10.20870/jph.2016.e007

Abstract

Silvicultural practices (e.g., nitrogen addition through fertilization) and environmental changes (e.g., elevated [CO2]) may alter needle structure, impacting mass and energy exchange between the biosphere and atmosphere through alteration of stomatal function. Hydraulic resistances in leaves, controlling the mass and energy exchanges, occur both in the xylem and in the flow paths across the mesophyll to evaporation sites, and therefore largely depends on the structure of the leaf. We used the Free-Air Carbon dioxide Enrichment (FACE) experiment, providing a unique setting for assessing the interaction effects of [CO2] and nitrogen (N) supply to examine how leaf morphological and anatomical characteristics control leaf hydraulic conductance (Kleaf) of loblolly pine (Pinus taeda L.) trees subjected to ambient or elevated (+200 ppmv) CO2 concentrations (CO2a and CO2e, respectively) and to soil nitrogen amendment (N). Our study revealed that CO2e decreased the number of tracheids per needle, and increased the distance from the xylem vascular bundle to the stomata cavities, perturbing the leaf hydraulic system. Both treatments induced a decrease in Kleaf, and CO2e also decreased leaf extravascular conductance (Kextravascular), the conductance to water flow from the xylem to the leaf-internal air space. Decline in Kleaf under CO2e was driven by the decline in Kextravascular, potentially due to longer path for water movement through the mesophyll, explaining the decline in stomatal conductance (gs) observed under CO2e. This suggests that the distance from vascular conduits to stomata sub-cavity was a major constraint of leaf water transport. Across treatments our results showed that needle vein conductivity was slightly more limited by the lumen than by the bordered-pits, the latter accounting for 30-45% of vein resistance. CO2e-induced reduction in Kleaf was also consistent with an increased resistance to xylem collapse due to thicker cell wall. In addition, stomatal closure corresponded to the water potential inducing a reduction in 50% of leaf vascular conductance (Kvascular) via xylem wall rupture. The water potential that was estimated to induce complete xylem wall collapse was related to the water potential at turgor loss. Our study provided a framework for understanding the interaction between CO2e and N availability in affecting leaf anatomy, and the mechanisms for the response of Kleaf to the treatments. These mechanisms can be incorporated into predictive models of gs, critical for estimating forest productivity in water limited environments in current and future climates and a landscape composed of sites of a range in soil N fertility.

Highlights

Water flow in the soil-plant-atmosphere continuum is determined by the whole-plant hydraulic conductance from soil through plant tissues (Kplant), characterizing the structural capacity of the plant for water flow (Zimmermann 1983)
As expected (Chandler and Dale 1996), N-Fertilization increased needle length by 9% and fascicle width by 6%, which translated into a significant increase in total needle surface area (Table 1)
Transverse section of pine needles revealed that the single central vein had two strands of phloem and xylem embedded in transfusion tissue that represented around 15% of the whole surface area across treatments (Table 1, Supplementary Figure 1)

Summary

Introduction

Water flow in the soil-plant-atmosphere continuum is determined by the whole-plant hydraulic conductance from soil through plant tissues (Kplant), characterizing the structural capacity of the plant for water flow (Zimmermann 1983). In Pinus taeda, leaf hydraulic conductance (Kleaf) was strongly correlated with Kplant among trees exposed to a range in resource availabilities imposed by soil nitrogen amendment (N) and CO2e Because leaves represented a substantial component of Kplant, reduction in Kleaf under CO2e appeared to impose a decline in the efficiency of water conduction through the plant, affecting leaf gas exchange and tree transpiration over the long term. The differences in the CO2e x fertilized response of Kleaf may potentially be related to the characteristic differences in the development of conducting tissue effects of the treatments on plant ontogeny (Pritchard et al 1999, Lin et al 2001). The responses of leaf anatomy to rising CO2e and to fertilization has already been quantified, but most studies have focused on stomatal structure and density (Radoglou and Jarvis 1990, Woodward and Kelly 1995, Reid et al 2007), and no data are available on anatomical characteristics of the leaf xylem of trees growing under future CO2 conditions, especially in relation to Kleaf

Methods

Results

Conclusion