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<p>Nitrogen is essential for sugar beet growth, but excessive nitrate (NO<sub>3</sub><sup>−</sup>) induces stress and inhibits early development. This study investigated the responses of hydroponic sugar beet seedlings to nitrate gradients (5, 10, 15, and 20&#xa0;mmol·L<sup>−1</sup>; N5-N20). Growth increased from N5 (sub-optimal) to N15 (optimal), but was severely inhibited at N20 (excessive), reducing height of the plant, root length, root surface area, stem diameter, and fresh/dry weight by 40.13%-80.77% relative to N15. Most photosynthetic parameters improved from N5 to N15 but were suppressed under N20, where Fv/Fm, ΦPSII and qP declined by 20.59%, 36.20% and 10.96%, respectively, alongside an 85.00% rise in qN (vs. N15). SOD activity declined from N5 to N20. Leaf POD activity followed N15 &gt; N10 &gt; N5 &gt; N20, while root POD activity was highest at N5. Leaf CAT activity was elevated at N15/N20, and root CAT activity ranked N20 &gt; N5 &gt; N15 &gt; N10. MDA decreased until N15 then rose at N20. Proline and soluble sugar content decreased after N5 but accumulated at N20 in leaves, while remaining low in roots until N15. Epidermal thickness increased with nitrate supply. Compared to N15, other treatments showed greater spongy tissue thickness, a lower palisade-to-spongy tissue ratio, and reduced root diameter, xylem thickness, parenchyma thickness, and cambium thickness, most markedly under N20. Correlation analysis linked ΦPSII positively with height of the plant, stem diameter and palisade tissue thickness, and negatively with qN and leaf proline content; leaf peroxidase activity was positively correlated with stem/root diameter and xylem/parenchyma thickness; leaf proline/soluble sugar were negatively correlated with palisade thickness, and root proline/soluble sugar with cambium thickness (all <i>p</i> &lt; 0.05 or 0.01). These findings demonstrate that excessive nitrate inhibits growth through a cascade involving redox disruption, photosynthetic impairment, osmotic adjustment, and structural degradation, highlighting a resource-allocation trade-off under nitrate stress.</p>

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Integrated Physiological and Anatomical Analysis Reveals Growth Inhibition of Sugar Beet Seedlings Under Excessive Nitrate Supply

  • 期刊论文

  • 发表日期：

    2026年2月21日

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  • Kehua Chen,
  • Jiaxuan Li,
  • Qing Bai,
  • Lingqing Xu,
  • Mingyue Chu,
  • Hao Wang,
  • Dali Liu,
  • Wang Xing

摘要

Nitrogen is essential for sugar beet growth, but excessive nitrate (NO3−) induces stress and inhibits early development. This study investigated the responses of hydroponic sugar beet seedlings to nitrate gradients (5, 10, 15, and 20 mmol·L−1; N5-N20). Growth increased from N5 (sub-optimal) to N15 (optimal), but was severely inhibited at N20 (excessive), reducing height of the plant, root length, root surface area, stem diameter, and fresh/dry weight by 40.13%-80.77% relative to N15. Most photosynthetic parameters improved from N5 to N15 but were suppressed under N20, where Fv/Fm, ΦPSII and qP declined by 20.59%, 36.20% and 10.96%, respectively, alongside an 85.00% rise in qN (vs. N15). SOD activity declined from N5 to N20. Leaf POD activity followed N15 > N10 > N5 > N20, while root POD activity was highest at N5. Leaf CAT activity was elevated at N15/N20, and root CAT activity ranked N20 > N5 > N15 > N10. MDA decreased until N15 then rose at N20. Proline and soluble sugar content decreased after N5 but accumulated at N20 in leaves, while remaining low in roots until N15. Epidermal thickness increased with nitrate supply. Compared to N15, other treatments showed greater spongy tissue thickness, a lower palisade-to-spongy tissue ratio, and reduced root diameter, xylem thickness, parenchyma thickness, and cambium thickness, most markedly under N20. Correlation analysis linked ΦPSII positively with height of the plant, stem diameter and palisade tissue thickness, and negatively with qN and leaf proline content; leaf peroxidase activity was positively correlated with stem/root diameter and xylem/parenchyma thickness; leaf proline/soluble sugar were negatively correlated with palisade thickness, and root proline/soluble sugar with cambium thickness (all p < 0.05 or 0.01). These findings demonstrate that excessive nitrate inhibits growth through a cascade involving redox disruption, photosynthetic impairment, osmotic adjustment, and structural degradation, highlighting a resource-allocation trade-off under nitrate stress.