SEASONAL-CHANGES IN SUGAR-BEET PHOTOSYNTHESIS AS AFFECTED BY EXOGENOUS CYTOKININ N-6-(M-HYDROXYBENZYL)ADENOSINE

Foliar sprays with N-6-(m-hydroxybenzyl)adenosine, (mOH)-[9R]BAP, one of the synthetic cytokinins, were applied to field-grown sugar beet tw ice during the vegetation period: before full canopy closing (I), 6 we eks before harvest (II) or both, Application of (mOH)[9R]BAP retained high cytokinin content that usually declines prior to harvest. The tot al content of isoprenoid cytokinins at harvest was 2.6-fold higher in (mOH)[9R]BAP-treated plants as compared to the controls. Treatment I h ad no significant effect on contents of chlorophylls (Chi) a and b and carotenoids, nor on the rates of net photosynthesis (P-N) Or photores piration (R(L)), Or on CO2 compensation concentration (Gamma) measured during the whole vegetation period on detached leaves under optimum e nvironmental conditions, Increased values in P-N, R(L) and Chi a and b contents were found in variants II and I+II linked with a delay in le af senescence before harvest. Transpiration rate and stomatal conducta nces of adaxial and abaxial epidermes were not significantly affected by any treatment.