Rapid floral senescence following male function and breeding systems of some tropical orchids

No comparative study of floral senescence following male function among a range of tropical orchid genera has previously been undertaken. The timing and pattern of floral senescence and occurrence of fruit formation were studied following self-, geitonogamous and cross-pollination in 14 epiphytic and two terrestrial orchid species to determine their breeding system and assess the occurrence of floral abscission following pollinaria removal. Both pollination and pollinaria removal caused rapid floral senescence, and the pattern and timing of the floral changes were the same in all treatments. Six Dendrobium species and Pelatantheria insectifera were self-incompatible (SI) and eight other species, including one terrestrial species, were self-compatible (SC). Capsules produced from outcrossing in four SC species, Phalaenopsis cornu-cervi, Eria pubescens, Cleisostoma appendiculatum and Arundina graminifolia, were larger and heavier than those produced after selfing. Reductions in flower life span following pollinaria removal were positively correlated with flower size and longevity of unpollinated flowers but not with position in the inflorescence or nature of the breeding system. Rapid flower senescence following pollinaria removal reported here suggests that it may be widespread in tropical species. The significant association of the response with size of flowers and inflorescences among the species studied suggests that the cost of flower maintenance outweighs the benefit of remaining open for female function after pollinaria have been removed. Both SC and SI species were found among tropical orchids, but variation in capsule size following self- and cross-pollination indicates that there may be a reduction in seed production following selfing, even in SC species, and that fruit formation alone should not be taken as reliable evidence of full self-compatibility.