Interactions of temperature and light quality on phytohormone-mediated elongation of <Emphasis Type="Italic">Helianthus annuus</Emphasis> hypocotyls

Two important environmental signals, shade light, where the red/far-red (R/FR) light ratio is reduced, and elevated temperatures can each promote shoot growth. We examined their interactions using hypocotyl elongation of young sunflower (Helianthus annuus) seedlings, and we did this in the context of a possible hormonal mechanism for the growth increases that were induced by each environmental signal. Seedlings were subjected to combinations of six different temperatures (10, 15, 20, 25, 30 and 35°C) and four R/FR ratios (normal at 1.2 and reduced at 0.9, 0.6 and 0.3). Hypocotyl length was significantly increased by each of elevated temperature and FR enrichment. The magnitude of elongation induced by FR enrichment (low R/FR ratios) was dependent on temperature, with maximal effects of FR enrichment being seen at 20°C. Hypocotyl tissue concentrations of four endogenous gibberellins (GAs) and abscisic acid (ABA) were measured using the stable isotope dilution method. Hypocotyl ethylene evolution was also assessed. Thus, hypocotyl growth in both normal and shade light is highly dependent on temperature, with the most significant increases in FR-induced growth occurring at 20 and 25°C. A causal involvement of endogenous hormones, especially the GAs, in the growth that is induced by elevated temperatures, as well as in FR-induced growth, is strongly implied, with temperature being the stronger signal.