Floral Humidity in Flowering Plants: A Preliminary Survey

Abstract

The area of space immediately around the floral display is likely to have an increased level of humidity relative to the environment around it, due to both nectar evaporation and floral transpiration. This increased level of floral humidity could act as a closedistance cue for pollinators or influence thermoregulation, pollen viability and infection of flowers by fungal pathogens. However, with a few exceptions, not much is known about the patterns of floral humidity in flowering plants or the physiological traits that result in its generation. We conducted a survey of 42 radially symmetrical flower species (representing 21 widely spread families) under controlled conditions. Humidity was measured using a novel robot arm technique that allowed us to take measurements along transects across and above the floral surface. The intensity of floral humidity was found to vary between different flower species. Thirty of the species we surveyed presented levels of humidity exceeding a control comparable to background humidity levels, while twelve species did not. Patterns of floral humidity also differed across species. Nevertheless, floral humidity tended to be highest near the center of the flower, and decreased logarithmically with increasing distance above the flower, normally declining to background levels within 30 mm. It remains unclear how physiological traits influence the diversity of floral humidity discovered in this survey, but floral shape seems to also influence floral humidity. These results demonstrate that floral humidity may occur in a wide range of species and that there might be greater level of diversity and complexity in this floral trait than previously known.

Bumblebees can detect floral humidity

“…Investigation of the capacity of pollinators other than H. lineatala to respond to floral humidity is limited (von Arx, 2013), with non-experimental observations that flies may use floral humidity in addition to other floral display traits produced within Indian alpine environments (Nordström et al, 2017). Given that floral humidity is present in many flower species, as recently measured by Harrap et al (Harrap et al, 2020a), it is most likely that floral humidity is regularly encountered as part of flowers’ multimodal displays by a wide range of generalist pollinators and influences their foraging behaviours.…”

“…Insects can therefore detect both the humidity at a given time, and also the rate and direction of humidity changes, getting drier or moister. The sensitivity to humidity reported in Apis mellifera (Tichy and Kallina, 2014) echoes the humidity changes produced by flowers (Harrap et al, 2020a), suggesting that floral humidity differences could feasibly be detected by these pollinators, allowing them to use floral humidity while foraging.…”

“…Floral humidity, an area of elevated humidity within the headspace of the flower, has been demonstrated to occur in a number of flower species (Corbet et al, 1979; Nordström et al, 2017; von Arx et al, 2012). Floral humidity is created by a combination of nectar evaporation and floral transpiration (Azad et al, 2007; Corbet et al, 1979; Harrap et al, 2020a; von Arx et al, 2012) although the contribution of these two influences may vary between species. Transects of the flower headspace of 42 species found 30 (71%) produce floral humidity of an intensity greater than would be expected from any conflating environmental humidity sources (Harrap et al, 2020a) (such as the minimal humidity differences due to uneven air mixing in the sampling room, or humidity produced by the capped horticultural tubes that flowers were mounted in during sampling).…”

“…Floral humidity is created by a combination of nectar evaporation and floral transpiration (Azad et al, 2007; Corbet et al, 1979; Harrap et al, 2020a; von Arx et al, 2012) although the contribution of these two influences may vary between species. Transects of the flower headspace of 42 species found 30 (71%) produce floral humidity of an intensity greater than would be expected from any conflating environmental humidity sources (Harrap et al, 2020a) (such as the minimal humidity differences due to uneven air mixing in the sampling room, or humidity produced by the capped horticultural tubes that flowers were mounted in during sampling). The intensity of floral humidity produced by flowers, represented by (the average peak difference in relative humidity in the flower species’ headspace, compared to the background), reached up to 3.71% (in Calystegia sylvatica ).…”

“…The intensity of floral humidity produced by flowers, represented by (the average peak difference in relative humidity in the flower species’ headspace, compared to the background), reached up to 3.71% (in Calystegia sylvatica ). Floral humidity occurs widely and varies between species (Harrap et al, 2020a) and does not appear to be limited to species visited by a particular group of pollinators (Harrap et al, 2020a), elevated floral humidity intensity has been observed in flowers pollinated primarily by moths (von Arx et al, 2012), flies (Nordström et al, 2017) and bees (Corbet et al, 1979).…”