Analysis of Variability in the Amaranthus Bioassay for Cytokinins: Effects of Water Stress on Benzyladenine- and Fusicoccin-dependent Responses.

The use of only the upper part of the hypocotyl and the cotyledons in the Amaranthus tricolor bioassay for cytokinins, instead of the whole seedling, was found to reduce the endogenous response and give a higher benzyladenine-dependent response. There is no marked difference in the uptake or metabolism of benzyladenine in whole seedlings compared with that in excised cotyledons.Analysis of variability in the bioassay showed that water availability to the cut seedlings and to whole seedlings is a major factor in the amounts of betacyanin accumulated during the subsequent induction period. The increase in the amount of betacyanin accumulated in response to benzyladenine, following conditions of water stress, is not correlated with differences in benzyladenine uptake. Endogenous production and fusicoccin stimulation is also increased following water loss by cut seedlings. Possible explanations for this stress induction may be found in responses of active transport to changes in turgor pressure.Although pretreatment of the roots of seedlings with mannitol stimulated subsequent induction by excised cotyledons, the presence of mannitol during the induction period inhibited the accumulation of betacyanin. This inhibition is not due to any effect on benzyladenine uptake. The susceptibility of amino acid uptake and polysome profiles to water stress suggests that the inhibition of betacyanin synthesis, a process dependent on protein synthesis, may be due to inhibition either of precursor (tyrosine) uptake or of the synthesis or activity of some enzyme in the pathway.