Citrus plants are sensitive to waterlogging because it frequently causes root zone hypoxia (RZH), a serious abiotic stress to its growth. As root is the first organ to be affected by hypoxia stress, its ability to sense and respond to hypoxia stress is crucial to survive. In this study, the response of citrus plant to RZH stress was investigated at morphological, physiological and molecular levels. Our data showed that RZH stress significantly decreased root length, dry weight, and the number of root tips, and not obviously affected shoot growth. Under RZH stress, the malondialdehyde (MDA) contents in leaf and root increased, and the enzyme activities of SOD, POD and APX significantly enhanced in root, while in leaf just POD activity increased with an opposite trend to CAT. RZH stress increased IAA and decreased ABA contents in root, and in leaf all hormones including IAA, ABA and GAs were increased. Despite the leaf chlorophyll content unchanged, several photosynthesis-related parameters were significantly affected by RZH stress, including ABS/RC, TRo/RC and DIo/RC etc. The root soluble sugar and lignin contents were remarkably decreased and increased by RZH stress, respectively. Besides these, RZH stress also reduced nutrient uptake, reflecting in the root decreased Fe, Mn and Zn contents. RNA-Seq analysis revealed that most of the genes were up-regulated by RZH stress, and the differentially expressed genes (DEGs) mainly linked to carbon metabolism, reactive oxygen species scavenging, nutrient uptake and transport, and hormone synthesis/signaling. Taken together, our data obtained in this study will be helpful in further studies regarding hypoxia tolerance in citrus or other crops.

柑橘类植物对涝渍很敏感，因为它经常导致根区缺氧 (RZH)，这是对其生长的严重非生物胁迫。由于根是第一个受到缺氧胁迫影响的器官，其感知和应对缺氧胁迫的能力对于生存至关重要。本研究从形态、生理和分子水平研究柑橘植物对 RZH 胁迫的响应。我们的数据表明，RZH 胁迫显着降低了根长、干重和根尖数量，但对枝条生长没有明显影响。RZH胁迫下，叶片和根中丙二醛（MDA）含量增加，根中SOD、POD和APX酶活性显着增强，而叶片中仅POD活性增加，与CAT相反。RZH 胁迫增加了 IAA 并降低了根中 ABA 的含量，叶中所有激素包括IAA、ABA和GAs均增加。尽管叶片叶绿素含量没有变化，但几个光合作用相关参数受到 RZH 胁迫的显着影响，包括 ABS/RC、TRo/RC 和 DIo/RC 等。 RZH 胁迫分别显着降低和增加根可溶性糖和木质素含量. 除此之外，RZH 胁迫还降低了养分吸收，反映在根部降低了 Fe、Mn 和 Zn 含量。RNA-Seq分析表明，大多数基因被RZH胁迫上调，差异表达基因（DEGs）主要与碳代谢、活性氧清除、养分吸收和运输以及激素合成/信号传导有关。综合起来，