A comparison was made of the type and frequency of mutational events found in the progeny of tomato plants regenerated after one passage in vitro with those induced by chemical mutagenesis with ethyl methane sulphonate. Several mutants were recovered in the progeny of regenerated and mutagenized plants of two cultivars of tomato. They can be grouped into the following categories: seedling lethality, male sterility, resistance to Verticillium, short stature, change in number of lateral shoots or in leaf shape. The results indicate that the two sources of variability differ in their effect, changing the spectrum and frequency of the mutants as well as, at least in some cases, their pattern of segregation.

Dap (Defective aleurone pigmentation) is the designation for mutations in maize that give rise to a characteristic dappled endosperm phenotype, consisting of patches of purple tissue, of variable size and shape, on a yellow background. Features shared by all Dap mutants are: dominant expression when they are maternally derived, lack of expression or transmission when they originate from pollen, failure to recover homozygous Dap genotypes, reduced frequency of Dap seeds in the progeny of outcrosses of Dap/+ females, association of the dappled phenotype with reduction in seed size. The mutants so far tested, six in all, can be grouped into two classes, one including male-transmissible (MT) isolates not expressed in the endosperm if their contribution is paternal, and a second class of isolates (NMT) that are permanently lost following paternal transmission. We suggest that the NMT mutations are on a chromosome that carries an intercalary deletion. Assuming linkage between the mutant and the deletion, selection against the deficient chromosome during male gametogenesis would account for the failure to recover Dap seeds in the progeny of Dap/+ male parents. We have obtained genetic evidence supporting this hypothesis. This interpretation, however, does not apply to MT alleles. For these, other mechanisms, such as imprinting and/or dosage effects may be proposed. The mutable pattern in the endosperm to which all Dap mutants give rise is an intriguing phenotype which remains to be clarified. An unexpected finding is that aleuronic and subaleuronic cells corresponding to the colourless areas are abnormal in shape and anthocyanin biosynthesis is blocked in these cells. This finding calls for further investigation in light of a possible connection between flavonoid precursors and cell shape.

The progeny of 104 regenerated maize plants were screened for tolerance to the safe broad-spectrum herbicide glyphosate during seed germination and early growth . Seven somaclones showed varying degrees of resistance to the application of the herbicide at 1 .2 mM (0.1 kg ad . in 400 1 ha ' of water) . Plants capable of a normal growth following treatment with 2 .4 mM (0 .2 kg ha ') glyphosate at the three leaf stage were selfed, and their progeny analyzed . A family able to tolerate the exposure to glyphosate at 2 .4 mM was isolated and shown to maintain a photosynthetic rate comparable with control after the application of the herbicide .The selfed progeny of the tolerant somaclone was characterized as to the properties of two targets of glyphosate, the shikimate pathway enzymes 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase and 3-deoxy-D-arabinoheptulosonate-7-phosphate (DAHP) synthase . In vitro tests ruled out the possibility that the tolerance was due to altered forms of these enzymes . Families showed significant variability with regard to EPSP and DAHP synthase levels, measured at different stages during seedling growth ; however, not even these traits were correlated with in vivo response to glyphosate. The possible role of other physiological processes in determining the increased tolerance to the herbicide is discussed .Abbreviations: DAHP -3-deoxy-D-arabino-heptulosonate-7-phosphate, EPSP -5-enol-pyruvyl-shikimate-3-phosphate, CER -carbon-exchange rate, ID50 -concentration causing 50% inhibition

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