ABSTRACT
The aims of this study were: i) To assess the effect of embryo size, genotype and 2,4-dichlorophenoxyacetic acid (2,4-D) on plant regeneration from hybrid embryos of maize x Tripsacum dactyloides; ii) To assess the effect of salinity on diverse physiological parameters of plants of maize (line B73),  T. dactyloides and the fourth offspring (MT41 and MT51) of an F1 hybrid  of maize x T. dactyloides; iii) To assess the effect of salinity and alkalinity on plants of maize and T. dactyloides. To fulfill the aim i), four genotypes of maize (2n = 20) were pollinated with T. dactyloides (2n = 72). Embryos were isolated from the caryopsis 13, 14, 15, 16 ó 19 days after pollination and placed on the basic medium free of plant growth regulators or supplemented with 4.6 ?mol L-1 2,4-D. To fulfill the aim ii), plants of maize and hybrids MT41 and MT51 were grown in complete nutrient solutions at pH 5.8, under two salinity levels (in mM): 1 and 101. To fulfill the aim iii), plants of maize and T. dactyloides were grown in complete nutrient solutions supplemented with 5 mM of buffer tris(hydroxymethyl)aminomethane, at two levels of pH and NaCl (in mM): 1-pH=5.8 (control); 1-pH=8.5; 101-pH=5.8; 101-pH=8.5. Treatments were applied 7 (maize and hybrids) and 14 (T. dactyloides) days after sowing (DAS) and the determinations were carried out 22 and 38 DAS, respectively. Final concentration of NaCl was reached in two days (51 and 101 mM). Embryos of 1.25 mm length or less originated 99.4 % of regenerant calli. Three of the hybrids originated a similar number of plants up to 6 months after embryo plating. The fourth hybrid, maize line B73 x T. dactyloides, originated caulogenic calli, but neither shoot elongation nor complete plants were obtained. Embryos plated on 2,4-D free medium did not originate viable plants. Regenerated plants showed a chromosomal somatic number of 2n=46, characteristics similar to that of T. dactyloides plants and did not produce viable seeds. Decrease of shoot dry weight (DW) caused by salinity was lower in maize (12 %) that in hybrid MT41 (32 %) and hybrid MT51 (22 %) showed intermediate values. T. dactyloides was more sensitive to salinity (101 mM NaCl-pH=5.8) than maize (51 % vs. 28 % of shoot DW decrease related to control plants, respectively). Alkalinity (combined with 1 or 101 mm NaCl) induced similar reductions of shoot DW of maize (45 % and 51 %, respectively) and T. dactyloides (51 % and 58 %, respectively). Root DW was neither affected by salinity nor by alkalinity. Among control plants, T. dactyloides showed values of specific leaf mass (SLM) and percentage of electrolyte leakage from leaves (PELl) higher than maize (20%, 202 %, respectively) and a total chlorophyll content 42 % lower that that of maize. Only PELl (%) differed between hybrids MT41 (16) and MT51 (14) and maize (12). Relative water content (RWC) and total chlorophyll content of maize, T. dactyloides and MT41 and MT51 hybrids were reduced by salinity but neither PELl nor SLM were affected by salinity, related to the controls. Under salt stress, maize and hybrids MT41 and MT51 showed similar leaf Na+ contents (media ? SE in mmol g-1 DW): 0.74; 0.78 and 0.82 respectively. On the other hand, leaf Na+ content of T. dactyloides was 3.7 fold lower that that of maize. This difference could be explained by a lower uptake (total plant Na+ accumulation/root DW) and translocation of Na+ to the shoot (total shoot Na+ accumulation/total root Na+ accumulation), which were 50 % lower in T. dactyloides than in maize. Although T. dactyloides showed higher salt sensitivity than maize (expressed as shoot dry weight relative to the control), most of the evaluated parameters (RWC, SLM, PELl, total chlorophyll content) showed similar responses to salinity in both species. Moreover, T. dactyloides showed greater efficiency than maize in some mechanisms associated with salinity tolerance, such as the ability to maintain a low leaf Na+ content and a reduced root Na+ uptake and translocation to the shoot. These features were not express in the hybrids, which were similar to maize in most of the evaluated parameters, both under salt and non-salt stress conditions. In this context, the higher sensibility of T. dactyloides to salinity, compared to maize, may be related, at least partially, to a poor resistance of leaf tissue to Na+, i.e. low ability to compartmentalize Na+ into the vacuole. 

      Key words: maize, T. dactyloides, interspecific hybrids, post-cygotic incompatibility, caulogénesis, salinity, alkalinity, leaf Na+ content,  relative water content, specific leaf mass, clorophyll content.