Species‐specific responses to combined water stress and increasing temperatures in two bee‐pollinated congeners (Echium, Boraginaceae)

Abstract

Water stress and increasing temperatures are two main constraints faced by plants in the context of climate change. These constraints affect plant physiology and morphology, including phenology, floral traits, and nectar rewards, thus altering plant–pollinator interactions. We compared the abiotic stress responses of two bee‐pollinated Boraginaceae species, Echium plantagineum, an annual, and Echium vulgare, a biennial. Plants were grown for 5 weeks during their flowering period under two watering regimes (well‐watered and water‐stressed) and three temperature regimes (21, 24, 27°C). We measured physiological traits linked to photosynthesis (chlorophyll content, stomatal conductance, and water use efficiency), and vegetative (leaf number and growth rate) and floral (e.g., flower number, phenology, floral morphology, and nectar production) traits. The physiological and morphological traits of both species were affected by the water and temperature stresses, although the effects were greater for the annual species. Both stresses negatively affected floral traits, accelerating flower phenology, decreasing flower size, and, for the annual species, decreasing nectar rewards. In both species, the number of flowers was reduced by 22%–45% under water stress, limiting the total amount of floral rewards. Under water stress and increasing temperatures, which mimic the effects of climate change, floral traits and resources of bee‐pollinated species are affected and can lead to disruptions of pollination and reproductive success.

The earlier the better? Nesting timing and reproductive success in subalpine cavity‐nesting bees

“…Indirect effects of temperature could also be resource‐mediated. Elevated temperatures can reduce flower abundance and/or reward production (Descamps et al., 2018, 2020); however, we see no indication of a negative relationship between floral density and temperature in our data (results not shown). Foraging activity of pollinators other than our focal species may also increase in warmer conditions, possibly increasing competition for floral resources.…”

Bumblebee Behavior on Flowers, but Not Initial Attraction, Is Altered by Short-Term Drought Stress

“…For example, a decrease in flower size, in the number of flowers and/or in floral height under drought conditions can reduce transpirational water loss through flowers ( Feild et al, 2009 ; Teixido and Valladares, 2014 ; Lambrecht et al, 2017 ) and can decrease water consumption for flower maintenance ( Galen et al, 1999 ). However, these changes are often species-specific ( Burkle and Runyon, 2016 ; Descamps et al, 2018 , 2020 ; Glenny et al, 2018 ).…”

“…In addition to the modulation of morphological floral traits under drought conditions ( Carroll et al, 2001 ; Caruso, 2006 ; Halpern et al, 2010 ; Burkle and Runyon, 2016 ; Glenny et al, 2018 ; Descamps et al, 2020 ; Walter, 2020 ), plasticity can also occur in scent emission. Drought stress increases floral scent emission and causes a shift in the composition of floral VOCs as some compound pathways might be up- or down-regulated.…”

Extreme Drought Affects Visitation and Seed Set in a Plant Species in the Central Chilean Andes Heavily Dependent on Hummingbird Pollination

“…Outcrossing species that require external pollinators are expected to be affected more severely than species capable of autonomous self-pollination, and species with specialised pollination, more than species with generalised pollination. Although many studies have addressed the effects of drought and temperature on specific aspects of plant-pollinator interactions, studies on how water and temperature stress integrally impact plant-pollinator interactions and fitness are still scarce [16][17][18][19][20]. Pollinator activity is well known to be directly affected both positively and negatively by changes in ambient temperature [21][22][23][24][25][26][27][28][29][30][31][32].…”

“…More viscid nectar could potentially affect extraction ability in some pollinator groups. Other expected indirect effects of drought in combination with warmer global temperatures relevant to pollinator activity include reduced corolla sizes [ 16 , 19 , 20 ] and reduced potential flower longevity as a result of more rapid flower development under warmer temperatures [ 44 , 45 ] and accelerated floral senescence as a result of soil drying and water stress [ 45 ].…”