Cytological and genetical analyses were made of the breeding system of embryo-cultured interspecific tomato hybrids between L. esculentum and L. peruvianum. It was found that fluorescence techniques and electron microscopy allowed a distinction to be made between pollen tubes inhibited by a unilateral incompatibility reaction and pollen tubes inhibited by a self-incompatibility reaction, after self-pollination of the hybrids or after reciprocal crossing between the hybrid and the parental species. The observed differences, if real and reliable, demonstrate that unilateral incompatibility in esculentum pollen tubes is governed by a single gametophytic factor which is either linked or allelic to the S-locus. This finding is discussed with reference to recent reports that unilateral incompatibility is controlled, in peruvianum styles, by a number of different dominant genes and it is concluded that these dominant genes, the S-locus of self-incompatibility and the gametophytic factor regulating the unilateral reaction in esculentum pollen belong to the same linkage group. The strong sterility barriers which prevent practically all backcrosses between the hybrid and the parental species were shown to be independent of the factors regulating stylar incompatibility. L. peruvianum is heterozygous for the sterility genes which prevent fertilization or embryo formation when the interspecific hybrid is crossed, as pistillate parent, to different accessions of L. peruvianum. One peruvianum stock was found which, as a pollinator, was highly cross-fertile with the hybrids.The presence of a concentric endoplasmic reticulum in inhibited pollen tubes was observed to be a constant feature of both the self- and the unilateral incompatibility reactions and was interpreted as an indication that incompatibility might lead to a general cessation of protein synthesis. Although incompatible tubes very much resemble, in this respect, the pollen tubes cultured in vitro, it seems probable, on theoretical grounds, that the inhibition of pollen tubes in incompatible styles does not result from an absence of growth promoting substances but from the presence of a metabolic inhibitor.

After diethylsulphate treatment of seeds of the pea variety 'Parvus', a new floral mutation was isolated in the second generation. This mutation, named stamina pistilloida, is characterized by a partial fusion of the androecium with the gynoecium; the two marginal stamens of the staminal column are transformed in rudimentary carpels more or less differentiated according to ecoclimatic conditions. The genetic analysis has shown the monogenic and recessive behaviour of the mutation (gene proposed stp) and its linkage with the gene oh in the chromosome II.

By inhibiting the a-like DNA pofymerase, and therefore nuclear DNA synthesis, aphidicoiin induces accumulation of suspension cultured carrot ceils at the Gr/S boundary of the cell cycle. After a 24-h treatment with the drug the accumulation is complete, affecting all the cycling cells (95% of the population). Upon removal of the inhibitor, all cycling cells immediately resume nuclear DNA synthesis and move synchronously throughout the S-phase. Aphidicolin Synchronization Culfured plant cellDaucus carota Nuclear DNA synthesis

This research, partly supported by a grant of the Italian National Research Council (C.N.R.) and by a subcontract between the Association EURATOM-ITAL and the CNEN, is contribution n. 857 from the Biology Division of the European Communities and contribution n. 363 from the Laboratorio Applicazioni in Agricoltura del CNEN. ** Biology Division of the European Communities.

Gamma ray treatments at various doses were applied to flowers after cross-compatible pollinations (S1S4 × S12S13) and self-incompatible pollinations (S1S4 × S1S4). After 200 kR treatment (highest dose) a high percentage of pollen became non-viable, and tube growth of all the germinated pollen was inhibited at the first third, or just before half the length of the style. Observations on the ultrastructural aspects revealed that the pollen tubes were destroyed by a precise degradation process which led to the disappearance of the inner wall, lysis of the tube, accumulation of several bipartite particles and alteration of endoplasmic reticulum into a whorl of concentric circles. These results indicated that the high dose of gamma rays probably interferes with protein synthesis. The ultrastructural aspects of compatible pollen tubes after gamma irradiation were similar to those of self-incompatible tubes and therefore it is suggested that cessation of protein synthesis might also be a result of incompatibility.Electron microscopy observations after treatment with 200 kR gamma rays on flowers after self-incompatible pollination (S1S4 × S1S4) showed that gamma irradiation affects the self-incompatibility reaction; but the results obtained so far are difficult to explain.