Chrono-spatial reproductive phenology of Swertia bimaculata Hook.f. & Thomson ex C.B.Clarke reveals suitability as climate-changing bio-indicator along the Sikkim Himalayan altitude

Genus Swertia under the family Gentianaceae is morphologically and taxonomically distinct with about 14 species from Sikkim Himalayan region. A Chrono-spatial floral phenology study was conducted on Swertia bimaculata from 1580 to 2400 m altitude in West Sikkim Himalayan region. The floral phenology was studied in a field nursery at 1200 m altitude and spatial floral phenology was studied over a 1580-2400 m altitudinal range in the West Sikkim Himalayan region. Detailed study on flowering pattern and seed formation in S. bimaculata revealed late flowering at 1200 m compared to 2400 m. Plants at the lower altitude showed prolonged and variable flowering from September to November and seed formation from November to December. During September to mid-October, pentamerous and bisexual flowers were observed, followed by 9-16 % tetramerous until November; and up to 24 % trimerous and 10 % bimerous until the end of flowering season in December. By the end of December, 43 % of flowers become stamen-less (monoclinous). Less than 2 % seed formation was observed in stamen-less flowers. The temporal phenological behavior reveals a successful reproductive adaptive strategy, depicting pentamerous flowers during favorable conditions, reducing to tetramerous, followed by tri- & bimerous with female (monoclinous) flowers until the end of flowering with stressful conditions, suggesting its suitability as a stress indicator to the changing climatic conditions. Along the altitudinal range floral anomalies from the usually observed pentamerous (and very rare hexamerous) flowers in favorable conditions change to tetramerous and/or trimerous, as the environmental conditions become more stressful, suggesting that the changes in this species could be used as an indicator of stress in changing climatic conditions.