Abstract
Success in reproduction is subject to the successful initiation as well as successful completion of a chain of consecutive events starting from flower formation and ending with viable seed production. A pivotal role in this chain is played by the stigma which is the seat of pollen recognition and initiation of pollen-pistil interaction. An interesting feature of the family Bignoniaceae is the presence of thigmosensitive stigmas which open, close and re-open in response to touch. Kigelia pinnata bears a touch sensitive stigma and is a low fruit setter in Jammu and Kashmir (India). One possible reason might be pollen limitation coupled with reported self-incompatibility. However, not much is known about the mechanism of self-incompatibility in K. pinnata or of the role of its thigmotropic stigma. Carefully designed manual pollination experiments along with critical field observations revealed naturally deposited pollen load to be too low to cause permanent closure of stigma lobes. A strong relationship exists between the threshold pollen load on stigma, its permanent closure and fertilization. Of the various pollination treatments undertaken, fruits were formed only in open and manual cross pollinations when ∼9200 pollen of legitimate type is deposited on the stigma. Thigmosensitivity further limited the opportunity for the deposition of optimum pollen loads. Although, frequented by as many as seven different visitors, an average of 8 bats per night are available for as many as 30 trees which reflects a baseline deficiency of effective pollinators. This limitation in pollen and pollinator availability affects pollination success of this species and ends up with low fruit set. Fluorescence microscopy reveals successful germination and tube growth of pollen grains of both self and cross type but fruit-set is 100 % in cross-pollinated pistils only. Despite slower rates of ovule penetration and evidence of delayed fertilization, absolutely no fruit initiation occurs in self-pollinated pistils. This strongly points towards self incompatibility being late acting.
Highlights
The Bignoniaceae with $900 species includes trees, shrubs and woody vines in the neotropics (Lohmann andUlloa 2006; Rodriguez et al 2016)
Kigelia pinnata is a moderate-sized tree with grey-brown bark and a green, dense round canopy
It can be inferred that presence of thigmosensitive stigma and self-incompatible nature of K. pinnata coupled with the pollen limitation are likely to tell upon its distribution as well as perpetuation over a long time range
Summary
The Bignoniaceae with $900 species includes trees, shrubs and woody vines in the neotropics (Lohmann andUlloa 2006; Rodriguez et al 2016). Low fruit set in these taxa is generally attributed to high cost of fruit production (Ehrlen 1991) or to extrinsic causes like pollen limitation (Knight et al 2005) and flower predation and/or internal factors like genotype and stored resources (Stephenson 1980, 1981; Weins et al 1987) Such details on trees in India are limited (Tandon et al 2003). Only 2 of the 28 reportedly chiropterophilous plants (Subramanya and Radhamani 1993; Vikas et al 2009) namely Ceiba protandra of Bombaceae (Nathan et al 2005) and Oroxylum indicum of Bignoniaceae (Vikas et al 2009) have been investigated in detail In the latter, low fruit set is attributed to the poor pollination efficiency imposed by the thigmotropic stigma and a single visit of its legitimate pollinator Cynopterus sphinx Vahl
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