INFLUENCE OF LIGHT ON DNA CONTENT OF HELIANTHUS-ANNUUS LINNAEUS

Mean nuclear 2C DNA content (C equaling haploid DNA per nucleus) of th e first leaf of the sunflower, Helianthus annuus L., is influenced by the quality and the quantity of light, Seedlings of two inbred lines, RHA 299 and RHA 271 were germinated and groan in controlled environmen tal conditions, Lighting was adjusted to provide different combination s of photon flux densities and red to far red (R:FR) ratios, At R:FR = 5.8 and photon flux densities of 170 mu mol . m(-2.) s(-1), 200 mu mo l . m(-2.) s(-1), and 230 mu mol . m(-2.) s(-1), DNA content remained high and relatively constant ((x) over bar = 6.97 pg for RHA 271 and ( x) over bar = 7.32 pg for RHA 299). When the photon flux density range (R:FR = 5.8) was elevated to 350 mu mol . m(-2.) s(-1), 410 mu mol . m(-2.) s(-1), and 470 mu mol . m(-2.) s(-1), mean DNA content was redu ced to 6.23 pg (RHA 271) and 6.46 pg (RHA 299). At R:FR = 1.5, mean DN A content was consistently high (7.2-7.9 pg) only at the lowest photon flux density of 170 mu mol . m(-2.) s(-1). Significant decreases in D NA content (less than or equal to 12%) were observed at photon flux de nsities of 200 mu mol . m(-2.) s(-1) and 230 mu mol . m(-2.)s(-1). At the higher photon flux densities (350 mu mol . m(-2.)s(-1), 410 mu mol . m(-2.) s(-1), and 470 mu mol . m(-2.) s(-1)) and R:RF = 1.5, the pl ants had extremely low DNA contents ((x) over bar = 3.36 pg for RHA 27 1 and 3.41 pg for RHA 299) and high between-plant variance, The instab ility of DNA content, particularly for plants grown under light that i s far red rich, suggests that phytochromes may be involved in regulati ng DNA content of the sunflower.