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Direct spectrophotometric determination of quercetin content in onions (Allium cepa L.) was investigated as a possible alternative to high-performance liquid chromatography (HPLC) analysis. Quercetin content in five onion varieties was monitored at 362 nm and quantified using simple spectrophotometric and HPLC methods. HPLC revealed that 3,4'-Qdg and 4'-Qmg comprised up to 93% of total flavonol content detected in the studied varieties. These major quercetin conjugates combined (3,4'-Qdg + 4'-Qmg) and total flavonol conjugates quantified by HPLC correlated closely with spectrophotometer values. Correlation coefficients were 0.96 (P < 0.0001) for 3,4'-Qdg + 4'-Qmg and 0.97 (P < 0.0001) for total flavonol conjugates in onion. Simple spectrophotometric procedure proved to be a valid, efficient, and cost-effective method for the quantification of total quercetin in onion. Chemical names used: quercetin-3,4'-O-diglucoside (3,4'-Qdg); quercetin-4'-O-glucoside (4'-Qmg).

A biotin- and fluorescein-labelled probe of Helianthus argophyllus has been used to map specific repeated rDNA sequences by in situ hybridization on mitotic chromosomes of Alliwn cepa, Allium fistulosum, a diploid interspecific (Allium fistulosum x Allium cepa) F1 hybrid, and a triploid interspecific (2 x = A. cepa, 1 x = A. fistulosum) shallot. Hybridization sites were restricted to satellited and smallest pairs of chromosomes in both A. cepa and A. fistulosum. The number, size, and position of the hybridization sites distinguish homologous chromosomes and identify the individual chromosomes carrying the nucleolus organizing region (NOR) at the secondary constriction, as well as the individual chromosomes carrying an additional NOR. This in situ hybridization technique is the first reported in a plant species and offers new cytogenetic markers in Allium.

Onions were grown in environmentally controlled growth chambers for 85 days to investigate the effect of relatively low light intensity (350 mmol m À2 s À1 ) at two different total irradiance periods (12-h and 24-h photoperiods) on growth and photosynthetic performance. To test whether photosynthetic downregulation occurred due to carbohydrate feedback, we used onions that differed in bulbforming capacity. Allium fistulosum (L. cv. 'Kinka') is a non-bulbing onion, with potentially limited carbohydrate storage capacity, while Allium cepa (L. cv. 'Cal 296') is a bulb-forming onion with possibly greater carbohydrate storage capacity. In A. fistulosum, photosynthetic downregulation was observed in 24-h plants as indicated by reductions in the light-and CO 2 -saturated photosynthetic capacity (A sat and A max , respectively) by 26%, reduced maximum rate of carboxylation (V cmax ) by ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) by 33%, reduced maximum rate of electron transport (J max ) by 27% and 3-fold higher foliar sugar concentration. In contrast, the photosynthetic and biochemical capacity of A. cepa was not affected by exposure to 24-h photoperiod, presumably because substantial amounts of foliar carbohydrates were re-allocated to bulbs. In 24-h A. cepa, up to 84% of total plant mass was allocated to bulbs, while in 12-h plants, more mass was allocated to leaves. Production of greater leaf area in 12-h plants compared with 24-h plants compensated for lower total daily irradiance such that 12-h and 24-h plants of both species exhibited similar daily total leaf net CO 2 exchange and plant mass at the end of the experiment.Abbreviations -A max , photosynthetic capacity at saturating light level (1500 mmol m À2 s À1 PPFD) and saturating atmospheric CO 2 (C a of 80 Pa CO 2 ) (mmol CO 2 m À2 s À1 ); A sat , net photosynthesis at saturating irradiance (1500 mmol m À2 s À1 PPFD) and C a of 37 Pa CO 2 (mmol CO 2 m À2 s À1 ); C a , atmospheric CO 2 partial pressure (Pa); C i , intercellular CO 2 partial pressure (Pa); J max , maximum rate of electron transport (mmol m À2 s À1 ); L s , relative stomatal limitation to photosynthesis (%); Q comp , photosynthetic light compensation point (mmol m À2 s À1 ); Q sat , photosynthetic light saturation point (mmol m À2 s À1 ); V cmax , maximum rate of Rubisco carboxylation (mmol m À2 s À1 ).

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