Cold storage of potato (Solanum tuberosum L.) tubers is known to cause accumulation of reducing sugars. Hexose accumulation has been shown to be cultivar-dependent and proposed to be the result of sucrose hydrolysis via invertase. To study whether hexose accumulation is indeed related to the amount of invertase activities, two different approaches were used: (i) neutral and acidic invertase activities as well as soluble sugars were measured in cold-stored tubers of 24 potato cultivars differing in the cold-induced accumulation of reducing sugars and (ii) antisense potato plants with reduced soluble acid invertase activities were created and the soluble sugar accumulation in cold-stored tubers was studied. The cold-induced hexose accumulation in tubers from the different potato cultivars varied strongly (up to eightfold). Large differences were also detected with respect to soluble acid (50-fold) and neutral (5-fold) invertase activities among the different cultivars. Although there was almost no correlation between the total amount of invertase activity and the accumulation of reducing sugars there was a striking correlation between the hexose/sucrose ratio and the extractable soluble invertase activity. To exclude the possibility that other cultivar-specific features could account for the obtained results, the antisense approach was used to decrease the amount of soluble acid invertase activity in a uniform genetic background. To this end the cDNA of a cold-inducible soluble acid invertase (EMBL nucleic-acid database accession no. X70368) was cloned from the cultivar Desirée, and transgenic potato plants were created expressing this cDNA in the antisense orientation under control of the constitutive 35S cauliflower mosaic virus promotor. Analysis of the harvested and cold-stored tubers showed that inhibition of the soluble acid invertase activity leads to a decreased hexose and an increased sucrose content compared with controls. As was already found for the different potato cultivars the hexose/sucrose ratio decreased with decreasing invertase activities but the total amount of soluble sugars did not significantly change. From these data we conclude that invertases do not control the total amount of soluble sugars in cold-stored potato tubers but are involved in the regulation of the ratio of hexose to sucrose.

Genetic diversity of crop plants resulting from breeding and selection is preserved in gene banks. Utilization of such materials for further crop improvement depends on knowledge of agronomic performance and useful traits, which is usually obtained by phenotypic evaluation. Associations between DNA markers and agronomic characters in collections of crop plants would ͑i͒ allow assessment of the genetic potential of specific genotypes prior to phenotypic evaluation, ͑ii͒ identify superior trait alleles in germplasm collections, ͑iii͒ facilitate high resolution QTL mapping and ͑iv͒ validate candidate genes responsible for quantitative agronomic characters. The feasibility of association mapping was tested in a gene bank collection of 600 potato cultivars bred between 1850 and 1990 in different countries. The cultivars were genotyped with five DNA markers linked to previously mapped QTL for resistance to late blight and plant maturity. Specific DNA fragments were tested for association with these quantitative characters based on passport evaluation data. Highly significant association with QTL for resistance to late blight and plant maturity was detected with PCR markers specific for R1, a major gene for resistance to late blight, and anonymous PCR markers flanking the R1 locus at 0.2 Centimorgan genetic distance. The marker alleles associated with increased resistance and later plant maturity were traced to an introgression from the wild species S. demissum. These DNA markers are the first marker that are diagnostic for quantitative agronomic characters in a large collection of cultivars.

No abstract

No abstract

Experiments were conducted from 1985 to 1989 to evaluate the response of radishes (Raphanus sativus L.) to N fertilization on Histosols. Three of these experiments used 15N-labeled fertilizer to evaluate the recovery of N by radishes. There was no response to N fertilization in seven of the eight experiments, even though some of them were conducted under conditions of high rainfall. The one experiment in which radish yields increased with N was conducted in a poorly drained, waterlogged field that was atypical of normal radish production fields. Recoveries of fertilizer N in the marketable radish roots averaged 19%. The results of N and 15N analysis showed that although fertilizer N was available for uptake, so was an ample amount of soil mineralized N. These results indicate that under typical growing conditions, radishes produced on Florida Histosols do not respond to N fertilization.