Maize genotypes are known to differ in their capacity for morphogenesis in vitro. The degree of competence for morphogenesis can depend on the concentration of endogenous phytohormones in the cells of explants. The contents of free auxin (IAA), cytokinins (Z and ZR) and abscisic acid (ABA) were compared by immunoenzyme assay in (1) immature embryos of regenerable (A188) and recalcitrant (Gk26) lines isolated at the developmental stage, which was optimal for callus induction, and (2) mature and immature A188 embryos, which differed in their morphogenetic capacities. The analysis showed that the frequency of embryogenic callus formation was related to the concentration of hormones in the explant cells (Table1). Lower level of auxin and cytokinins and higher concentration of abscisic acid characterized the tissues with a competence for somatic embryogenesis. To temporarily decrease the auxin content in the isolated immature Gk26 embryos, one batch of the embryos was incubated with IAA-oxidase co-factors p-coumaric acid and 2,4-dichlorophenol. The second batch of Gk26 embryos was treated with exogenous ABA to elevate the concentration of the endogenous hormone. The effect of the treatment depended on the concentration of reagents. The artificial change in the balance of endogenous hormones resulted in a 3-10-times higher frequency of embryogenic callus formation by the explants (Table 2).

Table 1. The content of endogenous hormones in maize embryos (ng/g fresh weight)
 

Line	Frequency of embryonic callus induction, %	IAA	ABA	Z +ZR
A188, immature embryos	60-80	129.5±25.7	9092±2.3	15.7±1.0
A188, mature embryos	25-35	8.3±3.5	419.3±35.3	20.7±1.7
Gk26, immature embryos	2-4	3000.0 ±451.8	283.2±17.3	65.8±12.4

Table 2. The frequency of explants forming embryogenic callus.
 

Untreated	p-Coumaric acid, 5 x 10-4 M	2,4-dichlorophenol, 10-5 M	ABA, 2 x 10-5 M
2.4+1.4	12.2±3.4	13.9±3.2	20.9±4.4

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