Flow cytometry and stomata characteristics were used for screening ploidy levels in a large population of in vitro induced autopolyploids of the Musa acuminata breeding clone SH-3362 . Culturing shoot tips in liquid medium supplemented both with 5 .0 mM colchicine for 48 hours or 30 pM oryzalin (3,5-dinitro-N4,N-dipropylsulphate) for seven days, both in combination with 2% (v/v) DMSO, resulted in a high (23 .1% and 29 .1%) frequency of non-chimeric tetraploids in the fourth vegetative generation . Although mixoploidy persisted in subsequent cycles of vegetative propagation, tetraploids as identified by flow cytometry remained solid non-chimeric during two more cycles . These autotetraploids were propagated for field testing. A rough pre-selection of regenerated V4 plants based on their stomata characteristics resulted in a population in which only 56.2% of the plants were solid tetraploids . The somatic polyploidization system reported here can be utilised for banana breeding programmes .
Salinity is one of the most serious and widespread agricultural problems resulting in losses of yield and arable land. One strategy available to cope with saline soil is to choose salt-tolerant crops or to select salt-tolerant cultivars within a crop. Difficulties in breedmg for salt tolerance arise from its polygenic control and from the variable salt composition and distribution in the soil. An in vitro approach for screening large amounts of parental material is suggested and the present results confirm previous work conducted on in vitro response of the potato to a high level of sodium chloride. Eurthermore, a highly significant correlation was found between in vitro growth parameters and field performance of ten potato clones.
Shoot tip cultures from banana clones susceptible and resistant to Fusarium oxysporum f. sp . cubense (FOC) race 1 and race 4 were grown in vitro in the presence of different concentrations of fusaric acid and fungal crude filtrates or inoculated with a conidial suspension of FOC to assess correlation between in vivo and in vitro behaviour. Explants were susceptible to both filtrate and fusaric acid irrespective to their known field resistance/susceptibility response . No clear linkage between in vivo and in vitro behaviour was observed and our results suggest that the use of crude filtrate or non-host specific toxin (fusaric acid) in a screening programme for selecting a novel resistant genotype of Musa to FOC is not feasible. When peroxidase activity was used as a parameter to discriminate between susceptibility and tolerance, results were in good agreement with field response of host plant to pathogens . Early enzymatic activity increased in the incompatible host-pathogen interaction but not in the compatible interaction .
With the aim of ascertaining the existance of a correlation between in vivo resistance to Fusarium oxysporum f. sp. dianthi and in vitro response to fungal elicitors and toxic substances, phenylalanine ammonialyase and phytoalexin accumulation, on one hand, and resistance to culture filtrate, on the other, were assayed in "in vitro" cultures of three susceptible and four resistant Dianthus caryophyllus cultivars. Cultivars showing varying degrees of resistance in vivo either tolerated higher culture filtrate concentrations ('Niki') or showed high PAL activity and phytoalexin production when treated with Fusarium elicitor ('Duca'), or responded positively to both treatments ('Mei-Ling', 'Pulcino'). No such responses were shown in tissue cultures of susceptible cultivars. The differential response to the fungal elicitor seemed to be highly specific as genetic differences between cultivars were not observed in tissue cultures treated with other biotic (Phytophthora infestans) and abiotic (HgCl2) elicitors.
The growth‐promoting effects of nurse cells of carrot, tomato, patato, maize, bean, carnation and two species of tobacco were studied on carrot, tomato, tobacco and potato cells plated at low densities. In an area immediately below the nurse cells the plating efficiency was very high and found to be independent of cell density. In an area outside the nurse cells, in some cases, the plating efficiency tended to be much higher in combinations with cells from a heterologous source as compared with those from a homologous source. Moreover, in the same area with some combinations the plating efficiency decreased when cell density was lowered, while with other combinations this phenomenon did not occur. This decrease was independent of the absolute value of plating efficiency. In experiments in which the concentration of conditioning factors was presumably changed, no significant difference in the plating efficiency was noticed. We therefore suggest that different plating efficiencies observed with heterologous nurse cells were not due to a higher level of conditioning factors, but rather to the production of different types of conditioning factors that are presumably degraded with different efficiency.
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