The relationship between the regeneration system and genetic variability in the cucumber (Cucumis sativus L.)

Durum wheat has been subjected to intense breeding in Italy due to its local economic importance. Four groups of Italian cultivars representative of different breeding eras were compared in northern Syria for yield potential and morphophysiological features at a moderately favourable site, and drought tolerance at a stressful site. Group 1 included indigenous landraces; Group 2 comprised genotypes selected from exotic landraces (released in 1920's–1930's); Group 3 included genotypes selected from crosses or mutagenesis involving Group 2 materials (1950's–1960's); Group 4 comprised genotypes selected from crosses between CIMMYT and Group 2 materials (from 1970's). Under moderately favourable conditions, a yield increase of 1.03 t ha–¹ was observed from Group 1 through Group 4, corresponding to a genetic gain of about 0.017 t ha–¹ per year. Such increase was only partly accounted for by a parallel increase in spike fertility and seed weight. Plant stature decreased dramatically from Group 1 to Group 4; a remarkable reduction of height was already attained in Group 3, before the introduction of dwarfing genes from bread wheat. Earliness of heading and grain filling duration increased consistently across breeding phases, the length of crop cycle remaining almost unaltered. No significant improvement of drought tolerance resulted from the breeding activity, suggesting the need to put more emphasis on selection for real stress tolerance rather than escape. Overall variation for morphophysiological traits, assessed by a principal components analysis, highlighted the great diversity among the Group 1 cultivars, while variation within Groups 3 and 4 was lower. Indigenous landraces, little used in the breeding history, as well as novel, unexploited exotic germplasm sources could contribute to broaden the crop genetic base in the region.