AUTHOR=Li Yiyong , Wang Zhaocheng , Liu Huihui , Zhang Cheng , Fu Songling , Fang Xiong 

TITLE=Responses in Growth and Anatomical Traits of Two Subtropical Tree Species to Nitrogen Addition, Drought, and Their Interactions

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.709510

DOI=10.3389/fpls.2021.709510

ISSN=1664-462X

ABSTRACT=<p>Nitrogen (N) deposition and drought are two major stressors that influence tree growth and propagation. However, few studies have investigated their interactions. In this study, saplings of the two co-occurring species <italic>Ormosia pinnata</italic> (leguminous) and <italic>Schima superba</italic> (non-leguminous) were cultivated under two N addition rates (0 and 80 kg N ha<sup>–1</sup> year<sup>–1</sup>) with well-watered (WW, 80% of field capacity), moderate drought (MD, 60% of field capacity), and severe drought conditions (SD, 40% of field capacity). We examined their growth, as well as multiple anatomical and non-structural carbohydrate (NSC) responses, after 2 years. Results revealed that N addition significantly promoted the growth of MD-stressed <italic>S. superba</italic>, whereas no significant effect was detected in <italic>O. pinnata</italic>. Decreased leaf water potential (both Ψ<sub>md</sub> and Ψ<sub>pd</sub>) was also observed with N addition for both species under MD, but not under SD. Furthermore, the application of N positively impacted drought adaptive responses in the stem xylem of <italic>S. superba</italic>, showing decreased stem xylem vessel diameter (<italic>D</italic><sub>H</sub>), theoretical hydraulic conductivity (<italic>K</italic><sub>th</sub>), and increased vessel frequency (<italic>VF</italic>) upon drought under N addition; such impacts were not observed in <italic>O. pinnata</italic>. Regarding leaf anatomy, N addition also caused drought-stressed <italic>S. superba</italic> to generate leaves with a lower density of veins (<italic>VD</italic>) and stomata (<italic>SD</italic>), which potentially contributed to an enhanced acclimation to drought. However, the same factors led to a decrease in the palisade mesophyll thickness (<italic>PMT</italic>) of SD<italic>-</italic>stressed <italic>O. pinnata</italic>. Moreover, N addition increased the xylem soluble sugar and starch of MD-stressed <italic>O. pinnata</italic>, and decreased the xylem soluble sugar under SD for both species. The results suggest that N addition does not consistently modify tree growth and anatomical traits under variable water availability. <italic>S. superba</italic> appeared to have a greater capacity to be more adaptable under the future interactive effects of N addition and drought due to major modifications in its anatomical traits.</p>