Maize (Zea mays ssp. mays) and eastern gamagrass (Tripsacum dactyloides) are known for their susceptibility to chilling injuries. Their hybrid (Z. mays T. dactyloides) showed higher tolerance to low temperatures (-2 °C) in the field than its parents. Exposure to 5 °C for 2 or 3 d reduced the variable to maximal chlorophyll fluorescence ratio (F V /F M ), an indicator of the maximum photochemical efficiency of the photosystem 2, and the variable to minimal fluorescence ratio (F V /F 0 ) more in maize and eastern gamagrass than in hybrid plants. Chlorophyll contents for rewarming plants (25 °C for 3 d) were lower than before chilling in both parents while values for hybrid plants were similar. Electrolyte leakage was higher in chilled than control plants but it did not show significant differences among genotypes. Our data suggest that hybrid plants have higher capacity to recover from chilling injury in controlled conditions than their parents.

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High salt concentrations in soils negatively affect maize growth. Techniques such as remote hybridization and in vitro selection have been extensively used to accelerate breeding processes. In order to determine the usefulness of Tripsacum to improve salt tolerance in maize, the effects of NaCl, in vitro and in vivo, were evaluated in an intergeneric hybrid (MT) obtained from crossing Zea mays with Tripsacum dactyloides. Organogenic calli, induced from immature MT hybrid embryos, were exposed to different NaCl concentrations and the survival and regeneration percentages were calculated. Plants of the MT hybrid, obtained from the organogenic calli, were exposed to NaCl concentrations considered harmful for maize. The shoot dry weights of plants exposed to 250 mM NaCl did not show significant differences respect to the control ones. Although sodium content in shoots was incremented 2,5 fold, it was not toxic for this material. The MT hybrid showed better behavior, in vitro and in vivo, that maize genotypes exposed to similar conditions.High salt concentrations in soils negatively affect maize growth and, consequently, produce a large drop in yield (Pasternak et al. 1995). In many countries of the world soil salinity is a serious problem for agriculture and,

Since corn has been classified as one of the most sensitive crops to soil salinity, the evaluation of NaCl toleranceof a hybrid of Zea mays ssp. mays (2n=40) and Tripsacum dactyloides (2n=72), was considered of interest to determinethe possibility of using this germplasm in corn tolerance improvement. Long-term organogenic calli, obtainedfrom immature hybrid embryos, have shown high rates of multiplication and plant regeneration. After in vitro treatmentof these calli with different levels of salinity, the survival and regeneration percentages and the daily relativeweight increments were calculated. Regenerated plants were transplanted to pots and watered with NaCl solution (6.1dS m-1) in order to assess tolerance at a whole plant level. Measurements of height, number of leaves per plant, andfresh and dry weights were taken. Exposure to 170 mM NaCl in vitro during 35 days and in vivo during 20 days produceda fresh weight decrease of 51% in calli and 31% in plants, respectively. These results indicate an improved toleranceof the maize/Tripsacum hybrid to salinity stress, in vitro and in vivo, compared with results from previous reportsusing other corn genotypes under similar conditions.

The aim of this study was to analyse phenotype and cytogenetic behaviour of Zea mays ssp. mays and Zea mays ssp. parviglumis (Zpa) hybrids with different ploidy levels. Maize 2n =20 (Zm20) and Zpa hybrid plants (MPa20) were obtained naturally, they showed regular meiosis and fertile progeny (+80%). Otherwise, maize 2n=40 (Zm40) and Zpa hybrids (MPa30) were obtained by embryo rescue, they showed irregular meiosis and sterile progeny (0-8%). Meiotic configurations more frequently observed in the species and the hybrids were: Zm20 (1011); Zm40 (3.2411+8.341V); Zpa (0.461+9.7611); MPa20 (0.401+9.5411+0.05111+0.65IV); MPa30 (5.971+5.9311+4.05111). After colchicine treatment the number of IV increased in Zm20, Zpa and MPa20 and 10III were observed MPa30, because of homoeologous chromosomes pairing. In conclusion, Zm20 (AmAm BmBm) and Zpa (ApaApa BpaBpa) are allotetraploids with 2 homoeologous genomes. Whilst in the hybrids, the homoeologous genomes A pair in all cases, genomes B only do if their homologous competing during pairing, does not exist except in colchicine-treated plants which also show homoeologous genomes B pairing.