Struthanthus longipetiolatus Mald.-Borja & Cerros, 2025

Maldonado-Borja, María Guadalupe & Cerros-Tlatilpa, Rosa, 2025, A new and endemic species of Struthanthus (Loranthaceae) from Guerrero, Mexico, Plant Ecology and Evolution 158 (2), pp. 195-204 : 195-204

Struthanthus longipetiolatus Mald.-Borja & Cerros sp. nov.

Figs 1 , 2 , 3 , 4 , 5 , Table 1

Type

MEXICO – Guerrero • Leonardo Bravo, Los Morros ; 17°41’27.06”N, 99°48’8.93”W; 2098 m; 22 Apr. 2023; ♂ fl.; Maldonado-Borja 87; holotype: HUMO [40027] .

Diagnosis

Struthanthus longipetiolatus is similar to S. ibe-dzi , S. racemosus , and S. ramiro-cruzii . Plants scandent and pendant with inflorescences in racemes and triads peduncled, the central flower sessile, and the lateral ones pedicellate as in S. ibe-dzi and S. ramiro-cruzii . However, the new taxon is distinguished by its olive green yellowish to pink-reddish stems, petioles, leaves, inflorescence, petals, and style of the pistillate flower and by the long petioles reaching up to 3.2 cm.

Description

Shrubs woody, scandent, pendant, 0.8–4 (– 6.5) m long, perennial, hemiparasitic, dioecious. Haustorium terete, with several epicortical roots at the base of the plant; epicortical roots produce secondary haustoria that attach to the host. Stems 1–4.5 mm diameter, voluble, terete to subterete, glabrous, olive green to pinkish, and brown greenish when dried, with lenticels when mature; with epicortical roots. Nodes subterete to complanate, bicarinate mainly when young; stem internodes 3.5–8.6 (– 10) cm long. Leaves opposite to subopposite; petioles 1.2–3.2 cm long, channelled near the leaf blade base, resupinate; blades lanceolate, ovate, or narrowly ovate, 1.2–5.0 × 6.5–16.5 cm, apex long acuminate to acute mucronate, base attenuate to oblique, olive green to pinkish-reddish, margin entire, repand when mature, hyaline, venation pinnate, glabrous, papyraceous. Inflorescences indeterminate, axillary, 1 per axil, racemes of triads, rarely paniculated; rachis subterete to applanate, nodes plane, triads opposite to subopposite, decussate; central flower from triad sessile, lateral flowers pedicellate 0.22–0.65 mm long. Bracts and bracteoles caducous, cupuliform. Pistillate inflorescences 3.5–7.6 cm long, peduncles 0.5–1.35 cm long, 10–14 (– 16) triads, triad peduncle 0.5–1.28 mm long. Staminate inflorescences 3.6–10.8 (– 12) cm long, peduncle 0.45–1.6 (– 2) cm long, 12–16 (– 18) trads, triad peduncle 0.6–1 mm long. Pistillate flowers hexamerous. Flower buds cylindrical, rounded at the apex; mature flowers 1.25–2.2 × 4.7–6 mm, calyculus irregular, 0.1–0.2 mm, petals linear 0.5–0.75 × 2.8–3.65 mm, olive green yellowish to pink-reddish, staminodes one series; nectary globose, hexagonal, surrounding the style; ovary inferior 1–1.2 × 2–2.6 mm, green yellowish; style strongly convolute, 3.2–4 mm long, stigma capitate, pink-reddish when ripe. Staminate flowers hexamerous, central flower of triad sessile, pedicels of lateral flowers 1.2–2.2 mm long. Flower buds claviform with rounded apex; mature flowers 2.4–4.8 × 6.2–10.2 mm, petals linear, 0.8–1.25 × 5–8.5 mm, olive green yellowish to pink-reddish, without trichomes at the insertion of the petal and the filament; anthers basifixed, dimorphic, and elliptical, 0.9–1.18 × 2.2–3.2 mm, fully attached to petals, asymmetrical theca, apicular connective; calyculus irregular, green yellowish 0.2–0.5 mm, vestigial ovary 1.2–2.25 × 1–2 mm, nectary globose, hexagonal, surrounding the stylodium base; stylodium straight to slightly wavy, 1.4–3 mm long, stigma undifferentiated. Fruit a berry, ovoid, green when immature, orange to vermillion when ripe, bluish-black when dry, glaucous, 3.45–6.65 × 8.2–10 mm. Seed ovoid, 2.2–4.8 × 4.2–5.2 mm.

Distribution and habitat

Struthanthus longipetiolatus is endemic to Guerrero, Mexico, in cloud forests, oak-pine forests, and oak forests in the Sierra Madre del Sur Biogeographic Province ( Morrone et al. 2017), in the municipalities Chilpancingo de los Bravo, General Heliodoro Castillo, and Leonardo Bravo (Fig. 4 ). It is found at elevations between 2,332 and 2,577 m a. s. l.

Hosts

Struthanthus longipetiolatus mainly parasitizes Pinus spp. ( Pinaceae ), but also Ostrya virginiana (Mill.) K. Koch ( Betulaceae ), and Quercus spp. ( Fagaceae ). It was also found parasitizing cultivated fruit trees such as Crataegus mexicana Moc. & Sessé ex DC. , Prunus persica (L.) Batsch ( Rosaceae ), and Psidium guajava L. ( Myrtaceae ) (Fig. 5 ).

Phenology

Flowering from May to August and fruiting from December to February.

Etymology

The specific epithet refers to the characteristic long petioles in this taxon, unique among Mexican species.

Preliminary IUCN conservation assessment

Endangered: EN B 1 ab (iii) + 2 ab (iii). Struthanthus longipetiolatus is known only from the centre region of Guerrero in the Province of Sierra Madre del Sur. The extent of occurrence (EOO) is estimated at 293 km 2 (criterion B 1: EN) and the area of occupancy (AOO) is estimated at 52 km 2 (criterion B 2: EN). There are four locations (condition a) and there is a continuing decline in habitat quality (condition b (iii )), together indicating Endangered under criteria B 1 and B 2. The state of Guerrero experiences an annual deforestation rate averaging 13,311.8 hectares / year ( CONAFOR 2020). Additionally, in 2022 alone, forest fires impacted 108,597 hectares of forests ( ISNIF 2022). Likewise, the main hosts of S. longipetiolatus are pine trees, which are immoderately cut down. It is estimated that 98 % of the timber production in Guerrero is from Pinus spp. ( González et al. 2018). Press and Phoenix (2005) suggest that hosts are essential to keep parasitic plants (pp) in a location; therefore, local extinction of a host can cause the pp population to decline. Therefore, following the guidelines of the IUCN criteria ( IUCN 2022), S. longipetiolatus should be classified as Endangered: EN B 1 ab (iii) + 2 ab (iii).

Additional material examined

MEXICO – Guerrero • Chilpancingo de los Bravos, Llanos de Tepoxtepec ; 17°28’36.13”N, 99°31’36.35”W; 2331 m; 12 Jan. 2022; fr.; Maldonado-Borja 49; HUMO • ibid.; 12 Jan. 2022; fr.; Maldonado-Borja 50; HUMO • ibid.; 12 Jan. 2022; fr.; Maldonado-Borja 51; HUMO • ibid.; 18 Apr. 2022; fr.; Maldonado-Borja 54; HUMO • ibid.; 18 Apr. 2022; ♂ fl.; Maldonado-Borja 55; HUMO • ibid.; 13 May 2022; ♀ fl.; Maldonado-Borja 71; HUMO • ibid.; 26 Aug. 2023; ♂ fl.; Maldonado-Borja 80; HUMO • Km 7 hacia llanos de Tepoxtepec ; 17°28’44.11”N, 99°31’9.44”W; 2218 m; 18 Apr. 2022; ♀ fl.; Maldonado-Borja 53; HUMO • Llanos de Tepoxtepec; 17°28’42.99”N, 99°31’31.51”W; 2269 m; 18 Apr. 2022; ♂ fl.; Maldonado-Borja 56; HUMO • Camino hacia el parque de Llanos de Tepoxtepec ; 17°27’48.3”N, 99°32’49.6”W; 2577 m; 13 May 2022; ♀ fl.; Maldonado-Borja 68; HUMO • Camino hacia el parque de Llanos de Tepoxtepec ; 17°28’33.8”N, 99°31’50.4”W; 2399 m; 13 May 2022; ♂ fl.; Maldonado-Borja 69; HUMO • ibid.; 13 May 2022; ♂ fl.; Maldonado-Borja 70; HUMO • ibid.; 26 Aug. 2022; ♀ fl.; Maldonado-Borja 78; HUMO • Camino hacia el parque de Llanos de Tepoxtepec ; 17°28’32.79”N, 99°31’48.59”W; 2376 m; 26 Aug. 2022; ♀ fl.; Maldonado-Borja 79; HUMO • Llanos de Tepoxtepec; en bifurcación camino al Parque, a lado de Huerta de duraznos; 17°28’37.07”N, 99°31’26.32”W; 2383 m; 17 Jun. 2023; ♀ fl.; Maldonado-Borja 117; HUMO • ibid.; 17 Jun. 2023; ♂ fl.; Maldonado-Borja 118; HUMO • ibid.; 17 Jun. 2023; ♀ fl., fr.; Maldonado-Borja 119; HUMO • Llanos de Tepoxtepec, km 8 a 200 m de la telesecundaria ; 17°28’33.77”N, 99°31’50.29”W; 2265 m; 26 Aug. 2022; ♂ fl.; Maldonado-Borja 120; HUMO • ibid.; 26 Aug. 2022; ♀ fl.; Maldonado-Borja 121; HUMO • General Heliodoro Castillo, Carretera de Puerto del Varal a Verde Rico ; 17°42’32.75”N, 99°54’44.44”W; 2222 m; 24 Apr. 2023; ♂ fl.; Maldonado-Borja 96; HUMO • Leonardo Bravo, Filo de Caballos, Chichihualco ; [17.64897°N, - 99.82453°W]; 2240 m; 3 Jun. 1983; ♂ fl.; Martínez S. 3895; MEXU, ENCB • ½ km al E de Tres Caminos, Chichihualco ; [17.71117°N, - 99.77511°W]; 2420 m; 4 May 1987; ♀ fl.; Lorea 4083; XAL • Comunidad “Campo de Aviación” ; 17°39’9.51”N, 99°49’24.89”W; 2200 m; 22 Apr. 2023; ♂ fl.; Maldonado-Borja 88; HUMO .

Notes

A single individual can parasitize different tree strata, forming the main haustorium on the top branches and descending to the lower branches, measuring up to 6.5 m in length. This new species is characterized by its long petioles up to 3.2 cm. It can also have olive green yellowish to pink-reddish tones in stems, leaves, flower peduncles, petals, and even in the styles. Unlike other species of the genus, S. longipetiolatus has a well-marked sexual dimorphism, the staminate flowers being larger compared to the pistillate flowers (Fig. 3 G ). Struthanthus longipetiolatus exhibits autoparasitism on its stems and leaves.

Figures

Figure 1. Holotype of Struthanthus longipetiolatus (HUMO 40027).

Figure 2. Struthanthus longipetiolatus. A. Habit, scandent and pendant shrub. B. Staminate inflorescence. C. Lateral view of a staminate flower with two petals removed to show the anthers. C 1. Front and side view of anther basifixed, dimorphic, and elliptical with an apicular connective. C 2. Dissected staminate flower with two alternating stamens, stylodium straight to slightly wavy, and undifferentiated stigma. D. Lateral view of a pistillate flower with two petals removed to show the contour of the convolute style. D 1. Dissected pistillate flower with staminodes, a convolute style, and a capitate stigma. E. Mature leaf. F. Infructescence. Male inflorescence from Maldonado-Borja 87 (A, B, C, C 1, C 2); female inflorescence from Maldonado-Borja 119 (D, E) and Maldonado-Borja 54 (F). Illustration by Eduardo Santaella.

Figure 3. Struthanthus longipetiolatus. A. Pistillate inflorescence with reddish styles. B. Dissected pistillate flower with staminodes (st), a convolute style, and a capitate stigma (s) above the ovary (o). C. Hexamerous nectary (ne), irregular calyculus (ca). D. Staminate inflorescence with pendant petioles (p), pink-reddish leaves. E. Dissected staminate flower showing two series of stamens (S 1, S 2), basifixed, dimorphic, with elliptical anthers and apicular connective; stylodium slightly wavy, and undifferentiated stigma. F. Hexamerous nectary (ne), irregular calyculus (ca). G. Sexual dimorphism, female (left) and male (right) flowers, two petals were removed. H. Immature fruits. I. Mature orange and vermillion fruits. Photos by Rosa Cerros-Tlatilpa (A, D, H, I) and Maria Guadalupe Maldonado-Borja (B, C, E, F, G).

Figure 4. Distribution of Struthanthus longipetiolatus and congeners.

Figure 5. Habit of Struthanthus longipetiolatus. A. Pendant plants in the type locality. B. Mistletoe on Pinus spp. C. Haustorium and epicortical roots with red lenticels (l). Photos by Maria Guadalupe Maldonado-Borja (A) and Rosa Cerros-Tlatilpa (B, C).