Development of redroot pigweed is influenced by light spectral quality and quantity

Light quantity (photosynthetic photon flux density, PPFD) and quality (red:far-red ratio, R:FR) may affect phenological development of weed species growing under a crop canopy. An indoor study was conducted to quantify the effects of incident PPFD and R:FR on development and dry matter accumulation of redroot pigweed (Amaranthas retroflexus L.). Pigweed was grown in growth cabinets from the one-leaf stage to the initiation of seed set under three different PPFD/R:FR treatments: (i) high PPFD (550 mumol m(-2) s(-1)) and high R:FR (1.4) (HH), (ii) low PPFD (180 mumol m(-2) s(-1)) and high RXR (1.4) (LH), and (iii) low PPFD (180 mumol m(-2) s(-1)) and low R:FR (0.8) (LL). The experiment was undertaken at 12- and 16-h daylengths with three replications. Rate of leaf appearance (RLA) was accelerated with an increase in PPFD (HH vs. LH) at both daylengths. The FR enrichment (LL) negated the effect of low PPFD on RLA under the 12-h but not under the 16-h daylength. Low PPFD delayed the occurrence of floral primordia, flowering and initiation of seed set. Plant height was a result of the complementary effects of PPFD and RXR. Total dry matter accumulation and partitioning, with the exception of dry matter accumulation to the stem, were influenced by PPFD only. Results of this study show that both light quality and quantity influence the phenology of pigweed.