Effects of End-of-Day Red/Far-Red Ratio on Growth and Orientation of Sunflower Leaves

Sunflowers, Helianthus annuus `Contiflor' and `P75,' were grown in pots under natural daylight in glass-house and field experiments to examine the responses of extension growth and orientation of leaves to end-of-day irradiations with different red/far-red (R/FR) ratios. For individual leaves the effect on expansion was biphasic. High R/FR ratios enhanced growth in young leaves but inhibited growth as leaves became older. No differences were found for growth duration or phyllochron index. The pattern of response was similar in both cultivars. The upper leaves tracked the sun, but, in `Contiflor' at low solar elevations, solar tracking was more nearly accurate for leaves of high R/FR-irradiated plants. Conversely, the laminar angle of `P75' was unaffected by the light treatments. A similar pattern of laminar angle response occurred for plants of different ages.