FAR-RED ENRICHMENT AND PHOTOSYNTHETICALLY ACTIVE RADIATION LEVEL INFLUENCE LEAF SENESCENCE IN FIELD-GROWN SUNFLOWER

Basal leaves frequently senesce before anthesis in high population den sity crops. This paper evaluates the hypothesis that quantitative and qualitative changes in the light environment associated with a high le af area index (LAI) trigger leaf senescence in sunflower (Helianthus a nnuus L.) canopies. Mean leaf duration (LD, time from achievement of m aximum leaf area) of leaf 8 was significantly (P<0.05) reduced from 51 to 19 days as crop population density was increased from 0.47 to 4.76 plants m(-2). High compared to low plant population density was assoc iated with earlier reduction in the photosynthetically active radiatio n (PAR) and red/far-red ratio (RFR) reaching the target leaf. However the changes in RFR preceded those in PAR. When the light environment o f individual leaves of isolated plants growing under field conditions was manipulated using filters and PR-reflecting mirrors, LD was positi vely and linearly related with the mean daily PAR (MDR) received in th e FR- (no FR enrichment) treatments. FR enrichment of light reaching t he abaxial surface of the leaf significantly (P<0.05) reduced LD by 9 days at intermediate PAR levels with respect to FR-controls, but did n ot affect LD at the maximum PAR used in these experiments. However, wh en light reaching both leaf surfaces was enriched with FR, LD (for lea ves receiving maximum PAR) was 13 days shorter than that of the FR- co ntrol. These results show that basal leaf senescence in sunflower is e nhanced both by a decrease in PAR and by a decrease in R/FR.