Effects of Elevated CO2 on Leaf Senescence, Leaf Nitrogen Resorption, and Late-Season Photosynthesis in Tilia americana L.

Xiaoke Wang,William J. Manning,Li Li

doi:10.3389/fpls.2019.01217

Abstract

We investigated the effects of elevated CO2 concentrations ([CO2]) on autumnal leaf falling, late-season photosynthesis, and leaf N resorption efficiency by exposing Tilia americana L. to three CO2 levels (800 ppm A8, 600 ppm A6, and ambient air A4) in nine continuous stirred tank reactors (CSTRs). All leaves were subdivided into the first (Z1), second (Z2), and third bud break (Z3) leaves. Whole plant photosynthesis (PNsat) was determined by summing the products of PNsat and total leaf area in Z1, Z2, and Z3, respectively. The results indicated that 1) the timing of leaf senescence in A8 treatments was 21 days in advance, while the senescence duration sustained 20 days longer than ambient treatment; 2) elevated [CO2] significantly induced the early formation of overwintering buds, with the number increased by 42 and 29% in A8 and A6 treatments, respectively; 3) Z3 leaf photosynthesis increases consistently until the end of the growing season, but Z2 leaves and whole plant showed acclimation when senescence happened; and 4) autumnal N resorption efficiency in A8 and A6 leaves were 25.5% and 22.7% higher than A4, respectively. In conclusion, autumnal senescence of T. americana was accelerated, while the leaf falling duration was extended by elevated [CO2]. The change in leaf phenology makes higher N resorption efficiency and earlier and more winter bud formation possible. Meanwhile, a different response of PNsat within different bud break leaves leads to the disparity between instantaneous measurements of leaf photosynthesis and whole plant photosynthesis in end season.

Highlights

Carbon dioxide (CO2) is essential for the photosynthesis of plants, and future elevated levels of CO2 concentrations have been predicted to increase (NOAA, 2019)
While most studies took once instantaneous measurements of upper wide-opened leaf photosynthesis in the mid-season, we found the disparity between leaf photosynthesis in different age and canopy photosynthesis during the end season
We found that elevated CO2 accelerates autumnal leaf senescence, but the senescence duration lasted longer

Summary

Introduction

Carbon dioxide (CO2) is essential for the photosynthesis of plants, and future elevated levels of CO2 concentrations have been predicted to increase (NOAA, 2019). From previous experiments which were performed with a great range of methods ranging from controlled growth chambers indoor to open-top chambers and free-air CO2 enrichment experiment in the field, the results indicated that there had been large variability of forest trees into the autumnal phenophase in response to elevated CO2, with advances (e.g., Sigurdsson, 2001; Asshoff et al, 2006; Mcconnaughay et al, 2010; Warren et al, 2011), delays (e.g., Asshoff et al, 2006; Rae et al, 2006; Taylor et al, 2008), or no effect (e.g., Jach and Ceulemans, 1999; Herrick and Thomas, 2003; Norby et al, 2003a, Norby et al, 2003b; Asshoff et al, 2006). Sink capability (Ainsworth et al, 2004) and nitrogen availability (Sigurdsson, 2001) were proposed as the possible explanations, great uncertainty for the tree autumnal phenology response to elevated CO2 still exists

Methods

Results

Discussion

Conclusion