Abstract
Metasequoia glyptostroboides (Cupressaceae) is a rare deciduous conifer which grows successfully in both aquatic and terrestrial environments. This tree has a narrow natural distribution in central China but is cultivated worldwide. Using histochemical staining and microscopy (both brightfield and epifluorescent), we investigated whether the phenotypic anatomical and histochemical plasticity in the fine adventitious roots of M. glyptostroboides has promoted the adaptation of this plant to aquatic and terrestrial environments. The fine root development and cortex sloughing of M. glyptostroboides occurs later in aquatic habitats than in terrestrial habitats. Anatomical and histochemical analyses have revealed that the apoplastic barriers in the primary growth of the fine roots consist of the endodermis and exodermis with Casparian bands, suberin lamellae, and secondarily lignified cell walls. There were also lignified phi (Φ) thickenings in the cortex. In both aquatic and terrestrial roots, secondary growth was observed in the vascular cambium, which produced secondary xylem and phloem, as well as in the phellogen, which produced cork. As compared to terrestrial adventitious roots, aquatic adventitious roots had multiple lignified Φ thickenings throughout the cortex, larger air spaces, dilated parenchyma, and dense suberin and lignin depositions in the exodermis. Our results thus indicate that phenotypic plasticity in the anatomical features of the fine adventitious roots, including apoplastic barriers, air spaces, and lignified Φ thickenings, might support the adaptation of M. glyptostroboides to both aquatic and terrestrial environments.
Highlights
Metasequoia glyptostroboides (Cupressaceae) is a rare deciduous conifer, with a natural distribution limited to small areas of central China, including western Hubei, northern Hunan and eastern Chongqing [1,2], this species is commonly cultivated worldwide [3]
In the fine adventitious roots of M. glyptostroboides from terrestrial habitats, the cortex was observed to be closer to the root tip than in the roots from aquatic habitats (Table 1)
In the secondary growth stage, the suberin and lignin of the exodermis were densely located in the roots from the aquatic habitats, as compared to the roots from the terrestrial habitats (Figure 1C–F, Figure 2A, Figure 3A–C,E–G; Table 1)
Summary
Metasequoia glyptostroboides (Cupressaceae) is a rare deciduous conifer, with a natural distribution limited to small areas of central China, including western Hubei, northern Hunan and eastern Chongqing [1,2], this species is commonly cultivated worldwide [3]. Lignified phi Plants 2019, 8, 501; doi:10.3390/plants8110501 www.mdpi.com/journal/plants (Φ) thickenings have been recorded in the inner cortex adjacent to the endodermis in M. milleri [4] and are considered similar to the endodermis in the roots of C. lanceolata, M. glyptostroboides, and other gymnosperm species [9,14,15]. Lignified Φ thickenings have been reported in the root cortices of several species, including the gymnospermous Ginkgo biloba [13] and C. lanceolata [14], and the angiospermous Pyrus malus [16], Brassica napus, B. oleracea [17,18], Myrica rubra [19], and Cardamine hupingshanensis [20]
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