Touch induces rapid floral closure in gentians

This study is led by Dr. Can Dai (Hubei University), Dr. Yanbing Gong (Wuhan University), and Dr. Qingfeng Wang (Wuhan Botanical Garden, the Chinese Academy of Sciences). During the second Tibetan plateau scientific expedition in 2020, the research team accidentally found that the corollas of Gentiana clarkei and G. pseudoaquatica (white morph) (Gentianaceae) closed within seconds after they touched the flowers at Xiongmuco, Nagqu. “It was startling to witness with naked eyes. The flowers disappeared momentarily in front of you,” Dai says. After visiting more than twenty sites, they found four species of Gentiana that responded to mechanical stimulation with rapid corolla contraction and full closure (see the video below). It only took 7–210 s for flowers of G. pseudoaquatica (both blue and white morphs), G. prostrata var. karelinii, G. clarkei, and an unidentified gentian species to exhibit such movements.

What selective pressures might have given rise to the evolution of rapid thigmonastic petal movements in gentians? The researchers firstly looked into abiotic factors, since rain (hail) or wind is very common at plateaus and can interact with flowers in the form of mechanical perturbation. In the field observation, however, the flowers of Gentiana were typically well closed before a thunderstorm, rain, or hail could physically hit the corolla. Previous studies have shown that environmental cues, particularly temperature, are the key factors controlling flower opening and closure in gentians. It is evident that when the sunlight disappears and precipitation approaches plateaus, the temperature drops dramatically. Thus, it seems that flowers are well protected by temperature induced closure other than mechano-sensitivity. As to the possibility of wind, although the researchers have not caught any evidence of induced floral closure by wind in nature, they conducted experiments using artificial wind and found that only very strong wind (24-27 m/s) could elicit responsive petal behaviors in gentians. However, wind of such power is devastating and very unlikely to take place during summer (anthesis) in Tibet.

A type of large floral visitors, bumblebees, perhaps lies in the kernel of floral thigmonasty in gentians. The researchers noticed that three species of bumblebees visited flowers of G. clarkei and made slits in floral tubes in order to obtain nectar illegitimately. Due to bumblebees’ large body sizes and vigorous manipulations, 98.8 % of the flowers they visited displayed induced floral closure (see the video below). The injuries caused by nectar robbing were substantial. Nearly 80% of flowers experienced exterior damage, of which 6% showed injuries in ovary. Hence, the touch-response machinery in gentian flowers might be useful in escaping from further visits when a robber still hovered nearby. The relatively low incidence of ovary damages likely implied effective protection against lethal injuries caused by repeated floral larceny.

To add another level of complexity, some bumblebees also displayed regular legitimate visiting behavior to flowers of G. clarkei, resulting in rapid floral closure as well. If the bumblebees can transfer pollen among flowers, it is then reasonable to extrapolate that induced floral closure may also play roles in promoting pollen deposition, stimulating pollen germination, or encouraging outcrossing. “Undoubtedly, further examinations on fecundity consequences are needed,” Wang says, “without which, we won't be able to disentangle the adaptive significance of thigmonastic corolla behaviors.” An induced defense? A pollination stimulator? Both explanations sound very exciting and call for in-depth studies.

See the article:

Touch induces rapid floral closure in gentians

https://doi.org/10.1016/j.scib.2021.12.026

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