Anatomy and ultrastructure adaptations to soil flooding of two full-sib poplar clones differing in flood-tolerance

- Stable cell structure in leaves and roots contribute to flood tolerance of poplar.
- Roots suffered more severe cell structure damages than leaves under soil flooding.
- Flood-tolerant clone kept stable root aerenchyma and ultrastructure under hypoxia.
- Flood-sensitive clone was prone to lost normal cross-section anatomy under hypoxia.

Flooding stress always depressed plants survival and growth in flood-affected areas. To explore anatomy and ultrastructure adaptations to hypoxia stress, two full-sib poplar clones differing in flood-tolerance LS1 (flood-tolerant) and LS2 (flood-susceptible) were compared for waterlogging effects on them. The two clones LS1 and LS2 originated from Populus deltoides cv. Lux ex. I-69/55 × P. simonii. Morphological, ecophysiological and growth parameters, as well as anatomy and ultrastructure characteristics of their seedlings were subjected for 15 days to flooding treatment, followed by a three-day drainage and recovery stage. Results showed that flooding stress adversely influenced all characteristics aforementioned in all flooded plants. Compared with LS1, LS2 suffered clearly more severe flood injury during hypoxia and slower recovery ability after drainage, illustrated by morphology, biomass accumulation, gas exchange, chlorophyll fluorescence, root metabolism, and relative membrane permeability and malonaldehyde content of roots. Correspondingly, more pronounced anatomy and ultrastructure damages in leaves and roots were found in flooded LS2 as well, including palisade cell deformation in leaves, as well as serious lysis of cortical parenchyma cells and decompositions of nucleus and organelles in roots. Our results showed that morphological, ecophysiological and growth responses to soil flooding paralleled their anatomy and ultrastructure adaptations in leaves and roots. Stable intercellular and intracellular structures in leaves and roots, especially in the latter, helped the flood-tolerant clone behaved better than the flood-susceptible clone. Roots suffered more severe anatomy and ultrastructure injury than leaves under hypoxia stress. The flood-tolerant clone kept a stable cross-section anatomy with normal aerenchyma and ultrastructure in roots, which enable plant-internal aeration so that maintain aerobic respiration and basic root activities under flooding condition. The flood-susceptible clone was prone to lost normal cross-section anatomy under soil flooding caused by serious cortical parenchyma cell lysis and ultrastructure destruction, which resulted in root disorganization and dysfunction.

Cross-sections and ultrastructure of leaves and roots in watered and flooded LS1 and LS2. (1) In leaf anatomy: PP-palisade parenchyma; SP-spongy parenchyma; TVB-transcurrent vascular bundle; Hollow arrow indicated air cavities caused by enlarged interspaces in PP. Scale bars = 100 μm; (2) In leaf ultrastructure: Chl-chloroplast; CW-cell wall; M-mitochondria; N-nucleus; S- starch granule; V-vacuole; O-osmophilic globule. Scale bars = 1 μm, except 15-day flooded LS2 (2um); (3) In root anatomy: CPC- cortical parenchyma cell; ST-stele; Arrows indicated large cells interspaces or air cavity; Scale bars = 200 μm. (4) In root ultrastructure: CW- cell wall; M- mitochondria; N-nucleus; S-starch granule; V-vacuole; ER- endoplasmic reticulum; O-osmophilic globule; Scale bars = 2um, except watered LS1 (1um).230