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ment of pigweed under the maize canopy was attributed to the reduced level of PPFD, a potential light-quality Light quantity (photosynthetic photon flux density, PPFD) and effect on pigweed development in the latter study should quality (red:far-red ratio, R:FR) may affect phenological development of weed species growing under a crop canopy. An indoor study was not be excluded. conducted to quantify the effects of incident PPFD and R:FR on Light transmitted through a crop canopy is enriched development and dry matter accumulation of redroot pigweed (Amain far-red radiation (730-740 nm) and depleted in red ranthus retroflexus L.). Pigweed was grown in growth cabinets from radiation (660-670 nm) due to selective absorptance of the one-leaf stage to the initiation of seed set under three different red light and transmittance and reflectance of far-red PPFD/R:FR treatments: (i) high PPFD (550 mol m Ϫ2 s Ϫ1 ) and high light by green leaves. Consequently, the R:FR ratio R:FR (1.4) (HH), (ii) low PPFD (180 mol m Ϫ2 s Ϫ1 ) and high R:FR (light quality) is reduced from ≈1.2 above the canopy (1.4) (LH), and (iii) low PPFD (180 mol m Ϫ2 s Ϫ1 ) and low R:FR to anywhere between 0.1 and 1.0 under the crop canopy (0.8) (LL). The experiment was undertaken at 12-and 16-h daylengths (Rousseaux et al., 1999). The R:FR ratio is known as with three replications. Rate of leaf appearance (RLA) was accelerphotomorphogenic light, and affects many plant morated with an increase in PPFD (HH vs. LH) at both daylengths. The FR enrichment (LL) negated the effect of low PPFD on RLA under phological characteristics (e.g., stem elongation, branchthe 12-h but not under the 16-h daylength. Low PPFD delayed the ing, apical dominance, and leaf area distribution) (Salisoccurrence of floral primordia, flowering and initiation of seed set. bury and Ross, 1991; Ballaré and Casal, 2000). Most Plant height was a result of the complementary effects of PPFD research into effects of light quality on plants has foand R:FR. Total dry matter accumulation and partitioning, with the cused on morphological changes and dry matter distriexception of dry matter accumulation to the stem, were influenced bution (see Ballaré and Casal, 2000, and references by PPFD only. Results of this study show that both light quality and therein). The few papers that discussed development quantity influence the phenology of pigweed.

Climate variability adversely impacts crop production and imposes a major constraint on farming planning, mostly under rainfed conditions, across the world. Considering the recent advances in climate science, many studies are trying to provide a reliable basis for climate, and subsequently agricultural production, forecasts. The El Niño-Southern Oscillation phenomenon (ENSO) is one of the principle sources of interannual climatic variability. In Iran, primarily in the northeast, rainfed cereal yield shows a high annual variability. This study investigated the role played by precipitation, temperature and three climate indices [Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and NINO 3.4] in historically observed rainfed crop yields (1983-2005) of both barley and wheat in the northeast of Iran. The results revealed differences in the association between crop yield and climatic factors at different locations. The south of the study area is a very hot location, and the maximum temperature proved to be the limiting and determining factor for crop yields; temperature variability resulted in crop yield variability. For the north of the study area, NINO 3.4 exhibited a clear association trend with crop yields. In central locations, NAO provided a solid basis for the relationship between crop yields and climate factors.

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