Irradiance-induced alterations of growth and cytokinins in Phaseolus vulgaris seedlings

Light is a major environmental factor affecting plant growth and development. The cytokinins have many similar effects on these processes and may be involved in photomorphogenesis. In order to study the correlation between light and endogenous cytokinins, we have examined growth parameters and endogenous cytokinins in stems, leaves and other organs of Phaseolus vulgaris, cultivated for 10 days under a range of irradiances (25, 110, 350 and 500 mu mol m(-2) s(-1)). The nucleotides isopentenyladenosine-5'-monophosphate and zeatin riboside-5'-monophosphate were the dominant cytokinins, whereas both free bases and ribosides were below the detection level (0.5 pmol g(-1)). Plants grown at the highest irradiance had in their stems, leaves, petioles and roots significantly higher levels of cytokinins than had plants grown at the lowest irradiance. As expected, increased light influx increased the dry weight of the root, petiole and leaf, and increased the leaf area, with concomitant increases in the cytokinins in these plant parts. However, the stem showed a different and more complex relationship with irradiance. Stem cytokinin levels increased drastically between 350 and 500 mu mol m(-2) s(-1), but this was not correlated with any change in stem length; the light inhibition of stem elongation was mainly seen when irradiance was increased to 110 mu mol m(-2) s(-1). Taken as a whole, the results are consistent with an effect of irradiance and cytokinins on the processes favouring biomass production.