Acanthus ilicifolius

Overview

The 2024 taxonomic revision narrowed the circumscription of Acanthus ilicifolius. Mangrove, physiological, and salinity records under the former broad name concern different materials whose identity cannot be established from the old name alone. [S1, S8, S9]

Names and classification

Feng et al. (2024) separated the eastern tetraploid lineage of the former broad A. ilicifolius as A. tetraploideus H. Feng & Y. L. Huang. Narrowly defined A. ilicifolius refers mainly to diploid populations west of the Thai-Malay Peninsula, including the Andaman region, India and Sri Lanka. The Chinese flora and older databases retain the broader name, so those records cannot be assigned to the current narrow range by name alone. The placement of xiamenensis material requires voucher review. [S1, S8, S9, S11]

The 2024 revision separated the eastern tetraploid lineage of broad A. ilicifolius as A. tetraploideus; narrow A. ilicifolius is diploid and western. Ecological and experimental records are interpreted by the material names and localities given in their sources. [S1, S8, S9, S11]

Morphology and identification

A mangrove shrub with relatively thick, spiny-toothed leaves, bilaterally symmetrical flowers with colored bands, and capsule fruit. The former broad concept included the eastern tetraploid lineage now treated as A. tetraploideus. Identification under the narrowed A. ilicifolius concept should consider distribution, morphology, and the taxonomic revision rather than an older database name alone. [S1, S8, S9, S10]

Distribution, habitat and phenology

Feng et al.'s 2024 field-sampling table records the narrowed A. ilicifolius in natural mangroves at Phuket, Kerala, Katka Sundarban, and Chilaw. Ragavan et al. (2015) independently documented flowering and fruiting plants in Andaman–Nicobar mangroves near the landward high-tide zone and in the understory, with Shoal Bay Creek vouchers PBL30965 and PBL30966. The large flowers, dark corolla bands, and western-range locality are consistent with the narrowed concept, but the vouchers were not re-examined for ploidy. [S1, S8, S9, S11]

Ragavan et al. report flowering in February–March and fruiting in April–May for Andaman and Nicobar populations; these are local observations. [S9, S11]

Salinity in natural habitats

Sampling sites assigned to narrowly defined A. ilicifolius by Feng et al. (2024) fall within its western diploid range. East Asian records under the former broad name require voucher review before they can inform this species’ ecology. The available natural records describe mangrove habitat but do not give soil salinity measured alongside the plants. [S1, S8, S9]

Salinity studies

Healthy 20–30 cm stem cuttings were collected in Kadalundi Vallikkunnu Community Reserve, Kerala, India, treated with 15 μM IBA for 2 h to induce rooting, then grown for 35 days in half-strength modified Hoagland solution in a greenhouse until two pairs of new leaves had developed. They were then exposed hydroponically to 0, 200, 400, or 600 mM NaCl for 15 days. Antioxidant responses were more pronounced at 400 mM; at 600 mM, antioxidant-enzyme activity declined over time while antioxidant molecules and MDA accumulated. The study measured physiological variables, not sustained growth or reproduction. The material was reported under the broad name used before the 2024 split, with no voucher or ploidy reassessment. [S3]

An accessible article abstract reports biomass at twice the control at soil ECe of 8.30 dS/m. Osmotic potential was unchanged through ECe 13.20 dS/m and increased above that level. The abstract does not specify salt composition, plant provenance, treatment duration, cultivation method, or how ECe was measured; it uses the broad A. ilicifolius name from before the 2024 split. [S2]

The GRHC mangrove-soil entry reports soil salinity as “>0.6%” but gives no percentage basis, measurement method, or specific site. The entry uses the broad A. ilicifolius name from before the 2024 split. [S7]

CNHP_1320 lists 500 mM in a field labelled “seawater salinity” and gives a separate database value of 400 mM. The entry cites no underlying experiment or treatment conditions, and the two fields are not verified measurements from the same assay. [S5]

Reported mechanisms and ecological adaptations

The short NaCl hydroponic experiment measured antioxidant enzymes, MDA, antioxidants, and osmotic responses, but not sustained growth. Older soil-ECe work used a different salt system and was published under the broader taxonomic concept. [S1, S2, S3, S9, S10]

Conclusions and evidence gaps

Narrowly defined A. ilicifolius is recorded from western diploid mangrove populations; East Asian material published under the former broad name cannot be mapped to the current range without review. The short-term NaCl study measured antioxidant and osmotic variables, not sustained growth. Older soil-ECe data, the 500 mM “seawater” field and duplicate GRHC entries each have methodological or taxonomic limitations. [S1, S2, S3, S4, S5, S7, S8]