Quantitative trait locus (QTL) main effects and QTL by environment (QTL × E) interactions for seven agronomic traits (grain yield, days to heading, days to maturity, plant height, lodging severity, kernel weight, and test weight) were investigated in a two-row barley (Hordeura vulgare L.) cross, Harrington/TR306. A 127-point base map was constructed from markers (mostly RFLP) scored in 146 random double-haploid (DH) lines from the Harrington/TR306 cross. Field experiments involving the two parents and 145 random DH lines were grown in 1992 and/or 1993 at 17 locations in North America. Analysis of QTL was based on simple and composite interval mapping. Primary QTL were declared at positions where both methods gave evidence for QTL. The number of primary QTL ranged from three to six per trait, collectively explaining 34 to 52% of the genetic variance. None of these primary QTL showed major effects, but many showed effects that were consistent across environments. The addition of secondary QTL gave models that explained 39 to 80% of the genetic variance. The QTL were dispersed throughout the barley genome and some were detected in regions where QTL have been found in previous studies. Eight chromosome regions contained pleiotropic loci and/or linked clusters of loci that affected multiple traits. One region on chromosome 7 affected all traits except days to heading. This study was an intensive effort to evaluate QTL in a narrow-base population grown in a large set of environments. The results reveal the types and distributions of QTL effects manipulated by plant breeders and provide opportunities for future testing of marker-assisted selection. M OLECULAR MAPS of plant genomes, used in conjunction with phenotypic measurements, can provide information about chromosome regions that affect quantitative traits. Although knowing whether such regions represent individual quantitative trait loci (QTL)

Vernalization and photoperiod requirements regulate the timing of the vegetative/reproductive transition in plants. Cereals adapted to cold winter climates regulate this developmental transition mainly through vernalization requirements, which delay transition from the vegetative to the reproductive growth stage. Recent research indicates that vernalization requirements also influence the expression of low-temperature (LT) tolerance genes in cereals exposed to acclimating temperatures. The objective of the present study was to determine if LT tolerance expression was also developmentally regulated by photoperiod response. The nonhardy, short day (SD) sensitive, wheat (Triticum aestivum L. em Thell) cultivar AC Minto, the LT tolerant, highly SD sensitive barley (Hordeum vulgare L.) cultivar Dicktoo, and a barley selection with very low sensitivity to SD were subjected to 8-h (SD) and 20-h (LD) days at cold acclimating temperatures over a period of 98 d. Final leaf number (FLN) was used to measure photoperiod sensitivity and determine the vegetative/reproductive transition point. The LT tolerance of the less SD sensitive barley genotype was similar for LD and SD treatments. In contrast, a delay in the transition from the vegetative to the reproductive stage in AC Minto and Dicktoo grown under SD resulted in an increased level and/or longer retention of LT tolerance. These results support the hypothesis that not only the level, but also the duration of gene expression determines the degree of LT tolerance in cereals. Consequently, any factor that lengthens the vegetative stage, such as vernalization or photoperiod sensitivity, also increases the duration of expression of LT tolerance genes. Key words: Triticum aestivum L., Hordeum vulgare L., low-temperature tolerance, photoperiod, developmental regulation

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Gibberellic acid (GA3), indole-3-acetic acid (IAA) or kinetin ((6)N-furfurylaminopurine) applied to the apical regions of decapitated stems of derooted Phaseolus vulgaris plants, promoted (14)C-photosynthate transport to the site of hormone application. Hormonal promotion of acropetal photosynthate transport was associated with significant increases in the pool size of free-space sugars at the hormone-treated region of the stem. The hormone-induced increases in the free-space pool size depended on continued phloem transport in the stem stumps while photosynthate leakage from the sink tissues of the stems was unaffected by the hormone treatments. On the basis of these observations, it is concluded that the increases in the pool size of sugars in the stem free-space results from hormonal action on processes that determine rates of sugar unloading from the sieve element-companion cell (se-cc) complexes. Furthermore, since loading of the se-cc complexes in the stem stumps was stimulated by GA3 and IAA and unaffected by kinetin applied at the loading site, hormonal effects on net unloading from the se-cc complexes must be caused by alterations in the efflux component. For winter-grown plants, it was found that predicted increases in sugar transfer through the stem free-space from the se-cc complexes to the sink tissues could account for the observed hormonal stimulation of photosynthate transport. In contrast, for summer-grown plants the higher sugar concentrations in the stem free-space of control plants approached saturation for the sugar-accumulation process. This caused an attenuation of the responsiveness of sugar accumulation by the stem sink tissues to hormone-induced increases in the pool size of sugars in the stem free-space. On this basis it is proposed that the bulk of photosynthates may move radially from the se-cc complexes through the stem symplast of summer-grown plants.