The location of sbm-1 on the Pisum sativum genetic map was determined by linkage analysis with eight syntenic molecular markers. Analysis of the progeny of two crosses confirmed that sbm-1 is on chromosome 6 and permitted a more detailed map of this chromosome to be constructed. The inclusion of Fed-1 and Prx-3 among the markers facilitated the comparison of our map with the classical genetic map of pea. The sbm-1 gene is most closely linked to RFLP marker GS185, being separated by a distance of about 8 cM. To determine the practical value of GS185 as a marker for sbm-1 in plant breeding programs, the GS185 hybridization pattern and virus-resistance phenotype were compared in of a collection of breeding lines and cultivars. Three GS185 hybridization patterns were discerned among the lines. A strong association was found between one of these patterns and resistance to PSbMV.

The physiological basis of yield in asparagus (Asparagus offincinails L.) is poorly understood, but some reports have inferred a link between lateral fern photosynthesis (A) and annual spear yield. Gas exchange measurements were made on single lateral ferns of mature asparagus plants in the field and younger, potted plants. Among three cuitivars with different levels of yield, the light-saturated rate ofphotusynthesis (A~t) was positively associated with crop yield in both sets of plants. There was a highly significant correlation between Ant and stumatal conductance (gs). A~l also increased with fern nitrogen, chlorophyll, and soluble protein concentrations across the cultivars. These data imply that differences in both gs and photosynthetic capacity contributed to the variation in A~at, and this was supported by AJCI (internal CO2 concentration) analysis. Variation in photosynthetic capacity was related to genotypic differences in cladophyli diameter and thus the content (per unit sudace area) of photosynthetic biochemical constituents and, possibly, mesophyli resistance to COz diffusion. The possibility of genotypic variation in other photosynthetic parameters was also noted. It is concluded that fern A~l might be a predictor of yield in asparagus, and of potential use in early selection for breeding, but confirmation of such a relationship requires investigation in a wider range of genotypes.

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To assess the physiological aspects underpinning cultivar difference in asparagus (Asparagus officinalis L.) yield, diel carbon exchange, carbon partitioning and export and sucrose phosphate synthase (SPS) activity were examined in mature cladophyll tissue of two asparagus cultivars (ASP-69 and ASP-03) under field conditions. Both cultivars exhibited similar diel patterns in carbon exchange rate (CER) and carbohydrate partitioning. Rates of carbon export estimated from CER and dry mass changes were the highest at midday and coincided with maximum assimilation rate. Carbon export accounted for about 74% of carbon fixation during the photoperiod. No diel fluctuations were observed in SPS activity in either cultivar. A positive correlation between day CER and carbon export rate (r2 = 0.87) was found and this relationship did not differ between the two cultivars studied. The greater carbon export rate measured in the high-yielding cultivar (ASP-69) was associated with significantly higher CER in comparison to the low-yielding cultivar (ASP-03). However, a correlation between sucrose concentration and carbon export rate did not exist. Biochemical evidence indicated that the greater CER in ASP-69 was associated with a significantly greater SPS activity (P 0.05). Phloem 14C exudate analysis revealed that 14C flux out of cladophyll tissue in ASP-69 was significantly greater than in ASP-03. These results indicate a feed-forward effect of rate of photosynthesis on assimilate export in the two cultivars studied.