The functions of foliar nyctinasty: a review and hypothesis

Abstract

Foliar nyctinasty is a plant behaviour characterised by a pronounced daily oscillation in leaf orientation. During the day, the blades of nyctinastic plant leaves (or leaflets) assume a more or less horizontal position that optimises their ability to capture sunlight for photosynthesis. At night, the positions that the leaf blades assume, regardless of whether they arise by rising, falling or twisting, are essentially vertical. Among the ideas put forth to explain the raison d'être of foliar nyctinasty are that it: (i) improves the temperature relations of plants; (ii) helps remove surface water from foliage; (iii) prevents the disruption of photoperiodism by moonlight; and (iv) directly discourages insect herbivory. After discussing these previous hypotheses, a novel tritrophic hypothesis is introduced that proposes that foliar nyctinasty constitutes an indirect plant defence against nocturnal herbivores. It is suggested that the reduction in physical clutter that follows from nocturnal leaf closure may increase the foraging success of many types of animals that prey upon or parasitise herbivores. Predators and parasitoids generally use some combination of visual, auditory or olfactory cues to detect prey. In terrestrial environments, it is hypothesised that the vertical orientation of the blades of nyctinastic plants at night would be especially beneficial to flying nocturnal predators (e.g. bats and owls) and parasitoids whose modus operandi is death from above. The movements of prey beneath a plant with vertically oriented foliage would be visually more obvious to gleaning or swooping predators under nocturnal or crepuscular conditions. Such predators could also detect sounds made by prey better without baffling layers of foliage overhead to damp and disperse the signal. Moreover, any volatiles released by the prey would diffuse more directly to the awaiting olfactory apparatus of the predators or parasitoids. In addition to facilitating the demise of herbivores by carnivores and parasitoids, foliar nyctinasty, much like the enhanced illumination of the full moon, may mitigate feeding by nocturnal herbivores by altering their foraging behaviour. Foliar nyctinasty could also provide a competitive advantage by encouraging herbivores, seeking more cover, to forage on or around non-nyctinastic species. As an added advantage, foliar nyctinasty, by decreasing the temperature between plants through its effects on re-radiation, may slow certain types of ectothermic herbivores making them more vulnerable to predation. Foliar nyctinasty also may not solely be a behavioural adaptation against folivores; by discouraging foraging by granivores, the inclusive fitness of nyctinastic plants may be increased.

Apex structures enhance water drainage on leaves

“…S4). The leaves of subtropical zone possess a relatively large inclination angle, β, in a range of 50°-70°under sunshine for better photosynthesis and a relatively small inclination angle, β, in a range of 20°-40°for better drainage and avoiding damage (26,27). In the rainforest with frequent heavy rainfall, the leaf of A. locasia macrorrhiza (Lour.)…”

Proportional fitness loss and the timing of defensive investment: a cohesive framework across animals and plants

“…For example, foliar nyctinasty (daily movement of plant leaves) may reduce encounters with herbivores by reducing leaf availability at night (Minorsky 2019). There is also evidence that plants may preempt attack in time by adjusting their defensive investment to be greater during times of day when encounter with an attacker is more likely (Falk et al 2014).…”

A Clustering Framework for Monitoring Circadian Rhythm in Structural Dynamics in Plants From Terrestrial Laser Scanning Time Series

“…The capability of mapping plant' short-term structural dynamics would afford valuable means for plant sciences to test open hypotheses concerning circadian motion and foliar nyctinasty. In his review, Minorsky (2019) lists several hypotheses concerning foliar nyctinasty that have been presented over the last century. These include internal plant processes such as better temperature control and water shedding.…”