It is known that inbreds of maize are discriminated by the rate of callus growth, frequency of totipotent callus formation, plant regeneration and duration of callus cultivation in undifferentiated state. These differences were explained by genotypic features of inbreds (Duncan D.R. et al., Planta, 165:322-332, 1985). Without any doubt in the basic rules of genetic factors in the control of the morphogenetical potential of maize, we attempted to consider this problem from another point of view and compared the peculiarities of callusogenesis and regeneration in maize inbreds in connection with the length of their vegetative period. The preliminary study (Piralov et al., Bull.Inst.Grain Farming, 1997, N4) carried out with 9 early-late (length of vegetative period 90-105 days) and middle-late (115-130 days) maize inbreds revealed higher morphological potential in the middle-late inbreds. In the present experiment we compared morphogenetic potential of early-late inbred DK2/66 and middle - late inbred DK675.

The calli were induced from immature embryos (1.0-2.0 mm) on medium containing the inorganic nutrients of N6 and MS media, vitamins of D medium (Duncan et al.,1985), L-proline (690 mg/l), myo-inositol (100 mg/l), casein hydrolysate (100 mg/l), silver nitrate (10 mg/l), sucrose (20 mg/l), 2,4-D (1 mg/l), agar (7 g/l). The explants were grown in the dark at 25-27 C and transferred on fresh medium every 15 days. For differentiation and regeneration, calli were transferred on media either with 0.1 mg/l 2,4-D or without hormone.

Summary data for both media are given in the table. As shown in this table, the frequency of totipotent callus formation and regeneration was higher in DK675. In particular, the frequency of regeneration on media with different levels of hormone exceeded inbred DK2/66 by 14-20 times. In addition, if the differentiation and regeneration process in inbred DK2/66 is restricted by primordia and small seedlings (up to 1 sm) formation in DK675 in the same period of time plantlets are formed about 5-10 sm and higher. Some of these plantlets were grown to maturity, selfed or sibbed and their seeds were collected for field evaluation. Regenerants of the early-late inbred were not obtained. We can support calli of DK2/66 in culture only 3-4 months, while DK675 calli kept about 5 years without any loss of regenerative potential.

Table. The frequency of totipotent callus formation and regeneration of inbreds DK2/66 and DK675.
 

		Frequency of Regeneration
Inbred	Frequency of totipotent callus formation (%)	Number of calli	Aging of calli (days)	2,4-D content in medium (mg/l)	Number of plantlets/callus
D_2/66	54.8 a	338	29-36	1.25	0.1 a
-"-		59	46-52	0.1	1.4 a
-"-		54	94	0	0.4 a
D_675	61.6 b	355	29-38	1.25	1.5 b
-"-		20	53	0.1	14.3 b
-"-		84	87	0	8.5 b

Thus, if the morphogenetic potential of inbreds is evaluated with the combination of such properties as frequency of totipotent callus formation, quantitative and qualitative indexes of regeneration, rate of callus growth and duration of callus growth without loss of regenerative ability, we can draw the conclusion that the middle-late inbred possesses much higher morphogenetical potential than the early-late ones. The explanation of the differences in morphogenetic potentials among inbreds with different length of vegetative period should be searched for in genetic and physiological peculiarities of inbreds, in the distinctions among the inbreds in the pool of endogenous hormones and in the activity of enzymes, that are metabolizing the hormones.

The question about relations between the length of the vegetative period of maize inbreds and their morphogenetical potentials may be interesting for the creation of a selection system in vitro for the length of the vegetative period.
 

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