Environmentally correlated variation in 2C nuclear DNA content measurements in Helianthus annuus L.

From observations of inbred lines of Helianthus annuus grown under controll ed environmental conditions, we hypothesized that sunflower plants regulate DNA content in response to light quality and quantity. This hypothesis was tested under field conditions by measuring DNA content (propidium iodide f luorescence) of embryo and leaf nuclei from plants of a natural population of Helianthus annuus L. The population was divided into three sites. Site A was barren, consisting of widely dispersed plants, which were exposed to d irect sunlight. The mean DNA content of leaves sampled from plants at this site was 6.56 +/- 0.06 pg. Plants at site B received various proportions of direct sunlight and irradiance reflected from neighbouring vegetation. The mean DNA content of leaves of young plants at this site was 6.21 +/- 0.07 pg. Leaves of plants from this site sampled at the time of flower bud forma tion had even lower mean nuclear DNA content (5.29 +/- 0.11 pg). Embryos ex cised from achenes of these plants possessed a high mean DNA content (7.51 +/- 0.06 pg). Plants growing in shade under a canopy of trees (Site C) had a relatively high mean DNA content (6.99 + 0.12 pg). The current results ar e compatible with the hypothesis that far-red-induced reduction of DNA cont ent in sunflower nuclei represents an adaptation for shade-avoidance in com petition with neighbouring plants. A model is presented in which the sunflo wer plant self-regulates its DNA content in response to environmental stimu li, thereby obtaining a more optimal DNA content for the environment in whi ch it resides. An alternative hypothesis is that the variation in DNA conte nt may be, at least in part, due to the differential accumulation in leaves of one or more secondary products which interfer with the intercalation an d/or fluorescence of propidium iodide. (C) 1998 Annals of Botany Company.