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 twice during the vegetation period: before full canopy closing (I), 6 weeks before harvest (II) or both, Application of (mOH)[9R]BAP retained high cytokinin content that usually declines prior to harvest. The total content of isoprenoid cytokinins at harvest was 2.6-fold higher in (mOH)[9R]BAP-treated plants as compared to the controls. Treatment I had no significant effect on contents of chlorophylls (Chi) a and b and carotenoids, nor on the rates of net photosynthesis (P-N) Or photorespiration (R(L)), Or on CO2 compensation concentration (Gamma) measured during the whole vegetation period on detached leaves under optimum environmental 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 leaf senescence before harvest. Transpiration rate and stomatal conductances of adaxial and abaxial epidermes were not significantly affected by any treatment.