Retardation of leaf senescence by urea cytokinins in Raphanus sativus

Approximately 500 urea derivatives and related compounds were tested for ability to retard leaf senescence as measured by chlorophyll retention in radish (Raphanus sativus) leaf discs. Of the 90 compounds found to be active, some had activity at 10^?6 M of the same order as kinetin. There was a high correlation between ability to promote chlorophyll retention and initiation of cell division. Highly active compounds had a planar ring and a HNCONH bridge; substitution with a HNCSNH bridge reduced activity and all other tested arrangements of the bridge gave inactive compounds. Substitution of both amino hydrogen atoms on one or both sides of the bridge reduced or removed activity. Some N′-substituted phenyl ureas were highly active. Introduction of a N-phenyl ring to a N-phenyl urea increased activity except where one ring was substituted in the para position with chloro, bromo or iodo. The activities of symmetrical disubstituted ureas were generally less than the corresponding N-monosubstituted derivative. The results suggest that the receptor site for cytokinin activity is the same for senescence retadation and cell division initiation.     

Changes in yield ofin-vivo fluorescence of chlorophyll a as a tool for selective herbicide monitoring

Triazines and derivatives of phenylurea, which are often found in outdoor water samples, induce specific changes in the yield of thein-vivo chlorophyll -fluorescence of PSII. These changes are correlated quantitatively with the concentration of the herbicides and can therefore be used to set-up a low-price monitor system. In order to detect selectively the herbicide-sensitive part of the fluorescence emission a pulse amplitude modulated fluorimeter was used. The bioassay system was optimised with respect to test organism, growing and measuring conditions. The relationship between fluorescence yield and herbicide concentrations were experimentally determined for the triazines atrazine and simazine and the phenylurea herbicide DCMU and mathematically fitted (r=0.99). The I₅₀-values were 0.9 µM for DCMU, 2.2 µM for simazine and 3.3 µM for atrazine. The detection limit of about 0.5 µM clearly shows that the sensitivity of this bioassay system is too low to reach the requirements of the drinking water regulation. However, due to its insensitivity against complex water matrices, there is good hope to combine this fluorometric bioassay with a potent herbicide preconcentration method like a solid-phase extraction procedure.Author for correspondence     

Quantitative structure-activity relationship of cytokinin-active adenine and urea derivatives

The substituent effect of N⁶-alkyl and -aralkyl adenines on the promotion of the growth of tobacco callus was analysed quantitatively using physico-chemical substituent parameters and regression analysis. The results indicated an optimum steric condition for activity in terms of the maximum width of the N⁶-substituents from the bond-axis connecting the N⁶-atom with its α carbon atom. The electron withdrawing effect of the N⁶-substituent enhances the activity. The substituent effect on the cytokinin activity of phenyl- and diphenyl- urea derivatives determined by Bruce and Zwar using the tobacco pith-block assay was also analysed. The results suggest that position-specific steric and hydrophobic effects of aromatic substituents participate in the variation in activity rationalizing the general trend of the activity; meta >para >ortho derivatives, for both series of compounds. The electronic effect is significant for the activity of diphenylureas but not for that of phenylureas which show somewhat different modes of interaction between the two series at the site of action. Based on inferences made from the correlations, hypothetical maps for the mode of interaction of these three sets of compounds at the site of action have been proposed.