Growth regulators (biologically active substances) in sugar beet are used to improve biological values of seed and growth regulation and development of the course of vegetation with an aim to increase the yield of roots and their sugar content. The possibilities of effect of intensive formation of new leaves in spring period or deposition of reserve substances into root in autumn, increase of sugar content and sugar production per 1 ha were investigated. Regulation of the process of formation of white sugar yield is very complicated. In view of hormonal regulation Kutina (1988) and Prochazka et al. (1997) tried to characterize it briefly. Zahradnicek et al. (1996) reports that in despite of majority of other plant materials in which cytokinins of zeatin type prevail, cytokinin N-6-(delta(2)-isopentenyl)adenin was dominant. The degree of reaction of selected sugar beet variety thigh sugar content Edda and normal Hilma) on application of growth regulators was studied. In exact small-plot field trials in the experimental station at Uhrineves the effect of growth regulators on amount and quality of harvested sugar beet roots was evaluated. Applications of growth regulators Rastim 30 DKV (0.3 l.ha(-1), active substance benzolinon), Atonik (0.6 l.ha(-1), aromatic nitrocompounds) and highly effective Cytokinin R [3.10(-6) M, (meta-hydroxy benzyl)adenosine] tin special literature denoted as mOH.BAR) were investigated. The first spraying was applied in growth stage 31 to 33 (6 to 14 leaves). The second one about six weeks before harvest, in the growth stage 42 to 46. Fertilization and agrotechnics of the trial were usual. Soils of experimental station at Uhrineves are deep, they belong to great soil group Luvisol. Average daily air temperature is 8.3 degrees C, during the growing season 14.6 degrees C. Average yearly precipitation amounts to 575 mm, out of this period April to September 380 mm. The results of three-year trials (Tabs I and II) confirm a positive effect of selected growth regulators on the amount of harvested sugar beet roots. Significant increase of root yield on average of studied years was found in autumn treatment with cytokinin, Atonik as well as Rastim 30 DKV in both the studied varieties but it was more marked in more yielding variety Hilma. The highest increase against untreated control on average of studied years was recorded after two treatments of the stand with Rastim 30 DKV (13.7% increase). Rastim 30 DKV acted most significantly in the variety Hilma in different years. The effect of studied substances on sugar content was very low in all three experimental years. Our previous opinion that application of growth regulators may rather affect amount of production than its quality given by sugar content above all. This applies to varieties with genetically conditioned higher sugar content (C and NC types). Increased root yield after stand treatment with investigated growth regulators reflected in the increase of sugar production, more significantly in the variety Hilma than Edda. On average of investigated years the yield of white sugar against untreated control increased in the variety Edda as much as by 7.1% in the variant treated with cytokinin four to six weeks before harvest. In the other studied variants an increase varied from 1.6 to 3% on average. In the variety Hilma increase was more marked particularly in variants with pre-harvest-autumn application (by 11 to 15%). Conclusions (Pulkrabek, 1995) that double treatment with Atonik or Rastim 30 DKV increase root yields per 1 ha by 3 to 5% and that it does not affect significantly the sugar content of sugar beet yield should be added by the fact that in yielding and normal varieties (e.g. Hilma) increase of the root yield is higher than in varieties with high sugar content (e.g. Edda). It follows from the previous trials (Zahradnicek et al., 1996) evaluating the effect of leaf application of Atonik that root losses during storage before their technological processing are lower than in untreated control. These conclusions were also confirmed in the laboratory tests (Kotyk ct al., 1996). They manifest decreased rate of respiration after treatment of sugar beet with Atonik. In summary the effect of leaf treatment of sugar beet with Atonik can be evaluated very positively. It increases sugar production in the field and it decreases storage losses of sugar before its technological processing by lower respiration during storage.
