Exploring sexual dimorphism in Viscum album: Fewer, smaller, and more yellow males

Mistletoe (Viscum album) shrubs exhibit a yellowish hue during host dormancy, a trait that is more pronounced in male individuals. Moreover, in populations of this hemiparasite, a female-biased sex ratio is commonly observed. In the present study, we raised the question of the physiological basis and consequences of these between-sex differences. We hypothesized that males might be more impacted by host dormancy and restricted resource acquisition. Furthermore, we considered the possibility that females could be more efficient at acquiring resources from host tissues compared to males. The population of V. album subsp. album growing on Malus sp. within the collection of the Institute of Den­drology, Polish Academy of Sciences in Kórnik, was studied during the host’s (Malus sp.) dormant period. Measurements included number, location and size of shrubs, chlorophyll fluorescence, chlorophyll and carotenoid contents, water contents, leaf morphology, and the content of selected elements. We observed a female-biased sex ratio in the mistletoe population, and females were larger than males. The females had a lower chlorophyll a / b ratio due to the greater concentration of chlorophyll b, higher chlo­rophyll to carotenoid (chl / car) ratio and potassium concentration in leaves. Moreover, a slightly stronger correlation was observed between the nitrogen concentration and the chl / car ratio in females than in males. No significant differences between sexes were observed in other parameters, including specific leaf area, relative water content, magnesium, phosphorus, calcium concentrations, or chlorophyll fluorescence parameters. Our results showed that V. album males were more yellowish during host dormancy than fe­males because they had a lower chl / car ratio. The disparities in pigment concentrations, coupled with the smaller size and fewer male individuals in populations, suggest that male individuals are in an overall weaker condition than females.