Osmotic Adjustment and Antioxidant System Regulated by Nitrogen Deposition Improve Photosynthetic and Growth Performance and Alleviate Oxidative Damage in Dwarf Bamboo Under Drought Stress

Abstract

Dwarf bamboo (Fargesia denudata) is a staple food for the endangered giant pandas and plays a critical role in the sub-alpine ecosystem. Characterized by shallow roots and expeditious growth, it is exceedingly susceptible to drought stress and nitrogen (N) deposition in the context of a changing global environment. However, a comprehensive picture about the interactive response mechanism of dwarf bamboo to the two factors, water regime and N deposition, is far from being given. Therefore, a completely randomized design with two factors of water regimes (well-watered and water-stressed) and N deposition levels (with and without N addition) of F. denudata was conducted. In view of the obtained results, drought stress had an adverse impact on F. denudata, showing that it destroyed ultrastructure integrity and induced oxidative damage and restricted water status in leaves and roots, as well as declined photosynthetic efficiency in leaves, especially in N non-deposition plants. Nevertheless, F. denudata significantly increased heat dissipation in leaves, regulated antioxidant enzymes activities, antioxidants contents, and osmoregulation substances concentrations in leaves and roots, as well as shifted biomass partitioning in response to drought stress. However, regardless of water availability, N deposition maintained better ultrastructure in leaves and roots, resulting in superior photosynthesis and growth of F. denudata. Additionally, although N deposition did not cause oxidative damage in well-watered plants, ameliorated the effects of drought stress on F. denudata through co-deploying heat dissipation in leaves, the antioxidant system in roots as well as osmotic adjustment in leaves and roots. Noticeably, the leaves and roots of F. denudata expressed quite distinct acclimation responses to drought resistance under N deposition.

Keywords: Fargesia denudata; antioxidative defense system; drought stress; nitrogen deposition; osmotic adjustment.

Figure 1

Leaf relative water content of Fargesia denudata Yi plants for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. F_W, water effect; F_N, N deposition effect; and F_W × F_N, interactive effect of water and N deposition. Values with different letters are significantly different at p < 0.05. Vertical bars show ± S.E. Significant levels: ^***p < 0.001; ^**p < 0.01; ns (non-significant) p > 0.05.

Figure 2

Gas exchange and chlorophyll fluorescence parameters of Fargesia denudata leaves for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. F_W, water effect; F_N, N deposition effect; and F_W × F_N, interactive effect of water and N deposition. Values with different letters are significantly different at p < 0.05. Vertical bars show ± S.E. Significant levels: ^***p < 0.001; ^**p < 0.01; ^*p < 0.05; ns (non-significant) p > 0.05.

Figure 3

Antioxidant enzymes of Fargesia denudata leaves and roots for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. F_W, water effect; F_N, N deposition effect; and F_W × F_N, interactive effect of water and N deposition. SOD, superoxide dismutase; CAT, catalase; APX, ascorbate peroxidase; MDHAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase. Values with different letters are significantly different at p < 0.05. Vertical bars show ± S.E. Significant levels: ^***p < 0.001; ^**p < 0.01; ^*p < 0.05; ns (non-significant) p > 0.05.

Figure 4

Osmotic adjustment substances of Fargesia denudata leaves and roots for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. F_W, water effect; F_N, N deposition effect; and F_W × F_N, interactive effect of water and N deposition. Values with different letters are significantly different at p < 0.05. Vertical bars show ± S.E. Significant levels: ^***p < 0.001; ^**p < 0.01; ^*p < 0.05; ns (non-significant) p > 0.05.

Figure 5

In situ detection of reactive oxygen species (ROS) in leaves, and quantitative measurements of ROS in leaves and roots of Fargesia denudata for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. (A) hydrogen peroxide (H₂O₂) accumulation, (B) producing rate of superoxide anion ( ${\mathrm{O}}_2^{·-}$ ). F_W, water effect; F_N, N deposition effect; and F_W × F_N, interactive effect of water and N deposition. Values with different letters are significantly different at p < 0.05. Vertical bars show ± S.E. Significant levels: ^***p < 0.001; ^**p < 0.01; ^*p < 0.05; ns (non-significant) p > 0.05.

Figure 6

Lipid peroxidation (MDA) of Fargesia denudata leaves and roots for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. F_W, water effect; F_N, N deposition effect; and F_W × F_N, interactive effect of water and N deposition. Values with different letters are significantly different at p < 0.05. Vertical bars show ± S.E. Significant levels: ^***p < 0.001; ^**p < 0.01; ns (non-significant) p > 0.05.

Figure 7

Ultrastructural observations of leaves and roots of Fargesia denudata plants for N non-deposition (−N) and N deposition (+N) treatments with and without drought stress. The bars shown are 1 μm of leaf and 2 μm of root. C, chloroplast; CW, cell wall; G, granum; M, mitochondria; P, plastoglobulus; PM, plasma membrane; S, starch granule.