Fig. 1.Lindernia dubia (L.) Pennell from Georgia: A – group of plants collected for herbarium vouchers from the quarry Aug 5, 2023, note veins clearly visible from the abaxial side of leaves; B – plants from the Chorokhi channel observed Sept 21, 2023, note toothed upper leaves and short axillary pedicels.

Glabrous plants with mostly erect stems rooting at lower nodes, 3−30 cm in height. Leaves opposite, sessile, 5 veined; ovate to ovate-linear, with slightly dentate margins, 9−20 × 5−10 mm. Flowers solitary, axillary; pedicels shorter, equal or slightly longer than the subtending leaves. Calyx deeply 5-lobed; sepals narrow, linear, 3.5−5 mm. Corolla zygomorphic, tubular 7−8.2 mm long, white in color, with blue to violet tint; lower lip distinctly 3-lobed. Stamens in two pairs, one posterior fertile and one anterior rudimentary. Capsules ovate, tapering towards the apex, 4−5.5 mm long, containing numerous, dust-like, yellowish seeds less than 0.5 mm long.

The morphology of the collected plants closely matched published descriptions (Casper and Krausch 1981, Lewis 2000, Aybeke 2016). L. dubia differs from the reported L. procumbens by having mostly chasmogamous flowers, a rudimentary pair of stamens, a longer corolla, shorter pedicels, and toothed leaves (at least the upper ones), which have 5 veins (Škondrić et al. 2023). In the absence of flowers, the species can be distinguished by the morphology of the capsules (Molnár et al. 2000). Additionally, L. dubia typically branches in the middle and upper parts, unlike L. procumbens, which branches at the base (Zelenskaya 2020).

The Lindernia population was found in a shallow quarry, created a year earlier on the left bank of the Chorokhi River, which is no longer in use. The locality was revisited multiple times since spring (May), when the quarry was completely filled with rainwater, and pioneer communities of charophytes and pondweeds had developed. The Lindernia plants were first observed in early August as the water receded. By that time, the species had formed the lower tier of a sparse Schoenoplectus triqueter (L.) Palla community in the shallowest parts. By September, the quarry had dried up and was fully occupied by the Lindernia community, which also included various wetland annuals, with a prominent role being taken by Cyperaceae species: Cyperus difformis L., C. flavescens L., and Fimbristylis bisumbellata (Forssk.) Bubani. A few isolated plants were also observed along the bank of the Chorokhi channel below the quarry in September 2023: https://www.inaturalist.org/observations/184639702.

This is the first record of the species in Georgia and the South Caucasus. The nearest known localities are in the Krasnodar Territory, Russia, almost 500 km northwest of Adjara, where the species has become a common weed in rice fields of the Kuban River valley since 2017 (Zelenskaya 2020). Other nearby localities belong to southwest Romania (Neacșu et al. 2021), northwest Turkey (Aybeke 2016), and India (Krishnasamy and Arumugam 2015). It was demonstrated that the recent range expansion of this species is linked to human activity, particularly fish farming and shipping, as well as to hydrochory and avichory (Tzonev and Šumberová 2004, Šumberová et al. 2012, Schmotzer 2015). In the case of the Georgian locality, the exact mode of introduction remains unclear. It is possible that the species invaded the region only recently, as its distribution in the Chorokhi floodplain appears limited. However, due to the species’ known invasive potential and high likelihood of further spread from the already invaded area, monitoring of the L. dubia population is essential. The quarry is frequented by humans and livestock, and L. dubia seeds can easily be transported by mud particles on rubber boots and hooves. Spring floods of the Chorokhi River may also facilitate seed dispersal.

The new locality is part of the region dominated by Colchis flora, which includes several endemic species restricted to wetland habitats. Among them, Rhamphicarpa medwedewii Albov was found growing alongside L. dubia, though it was represented by only a few scattered plants (IBIW). The invasion of L. dubia could pose a threat to this rare, species of low abundance.

Herbarium search confirmed that the genus Lindernia was previously represented in the South Caucasus by a single species, L. procumbens. Notably, the most recent collection from the region dates back to 1917 (for Azerbaijani specimens). In Georgia, the species was last collected in 1910: Russian Lazistan, swampy bank of the Malyi Chorokh, coll.: G. Woronow no. 2679, Aug 27, 1910, TGM 40879!, TGM 40885!. Woronow’s plants originate from the same area as L. dubia, i.e., south of the Chorokhi River delta in the Kakhaberi Lowland. However, no L. procumbens plants were found in this area in 2023. One possible explanation is that the species has gone unobserved for a long time or may be extinct. A decline in localities of this species has been observed in Europe, making it locally threatened. The invasion of L. dubia also presents a competitive disadvantage for the species (Šumberová et al. 2012, Schmotzer 2015). Another possibility is that L. procumbens only emerges from the soil seed bank during favorable conditions that occur intermittently (cf. Kaplan et al. 2016).

The findings of this study highlight the need for targeted searches for new localities of both Lindernia species in order to resolve the issue of L. dubia invasiveness and the current status of L. procumbens in the South Caucasus.