Pollinators preferences in sympatric Ixora chinensis and Ixora coccinea; an integrative analysis of geometric morphometrics, floral colour dynamics, and nectar chemistry

Abstract

Floral characteristics such as shape, symmetry, coloration, and nectar rewards collectively influence pollinator preferences and drive plant–pollinator coevolution. In this study, we examined pollinator preferences in the sympatric congeneric species, Ixora chinensis and Ixora coccinea by integrating geometric morphometrics (GMM) analysis of flower, complemented by pollinator observations, temporal colour analysis, and biochemical assessment of nectar traits. GMM analysis showed greater phenotypic plasticity and fluctuating asymmetry in I. chinensis than in I. coccinea, with the two species occupying distinct regions of morphospace. In the present study, we found that Butterflies (Lepidoptera) emerged as the dominant pollinators, accounting for 69% of visits, and their proboscis length showed a strong positive correlation (r = 0.988) with corolla tube length. I. coccinea possessed significantly longer corolla tubes (40.7 ± 0.98 mm) than I. chinensis (35.0 ± 0.75 mm), thereby restricting access primarily to butterflies with longer proboscides. Freshly opened flowers of I. chinensis changed colour from orange to red as they aged, signalling the presence of high-quality nectar enriched with essential elements and crystalline structures. In contrast, I. coccinea did not exhibit this colour transition. Although I. coccinea produced a higher volume of amorphous nectar and reached peak coloration during maximum pollinator activity around 11:00 h under moderate light (~ 3000 lx), pollinators consistently preferred I. chinensis. This preference demonstrates that multimodal integration of shorter corolla tubes, greater floral plasticity, dynamic colour signalling, and superior nectar quality exert a stronger influence on pollinator attraction than nectar volume alone. This study demonstrates how multiple floral traits collaboratively drive differential pollinator visitation among sympatric congeneric species.