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

From observations of inbred lines of Helianthus annuus grown under controlled 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 fluorescence) 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 direct 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 formation had even lower mean nuclear DNA content (5.29 +/- 0.11 pg). Embryos excised 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 are compatible with the hypothesis that far-red-induced reduction of DNA content in sunflower nuclei represents an adaptation for shade-avoidance in competition with neighbouring plants. A model is presented in which the sunflower plant self-regulates its DNA content in response to environmental stimuli, thereby obtaining a more optimal DNA content for the environment in which it resides. An alternative hypothesis is that the variation in DNA content may be, at least in part, due to the differential accumulation in leaves of one or more secondary products which interfer with the intercalation and/or fluorescence of propidium iodide.