Seasonal changes in sugar beet photosynthesis as affected by exogenous cytokinin N6-(m-hydroxybenzyl)adenosine

Abstract

Foliar sprays with N6-(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 (Chl) a and b and carotenoids, nor on the rates of net photosynthesis (P(N)) or photorespiration (R(L)), or on CO2 compensation concentration (Γ) measured during the whole vegetation period on detached leaves under optimum environmental conditions. Increased values in P(N), R(L) and Chl 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.