Gas Exchange and Cytokinins

Abstract

Plant hormones including cytokinins (CK) are the main signals in root-to-shoot communication and vice versa. Among others, CK antagonize some processes induced by water stress, mainly those mediated by abscisic acid. However, it is still an open question whether the stimulation of stomatal conductance (gs) and in consequence of transpiration rate (E) and net photosynthetic rate (PN) by CK is of general or exceptional character. Therefore, a set of experiments was done aiming to determine how effects of CK on gas exchange parameters depend on CK type and concentration, way of application, and plant species used. The effect of N6-benzyladenine (BA) on bean plants was dependent on concentration and time: in the first hours after application the positive effect on PN was observed at all concentrations used (1, 5, 10, and 20 m M), but after 24 and 72 h only at lower concentrations. The way of application was not decisive: similar effects on gs, E, and PN were found when BA solutions were sprayed on leaves, added to the substrate, or when roots were directly immersed in BA solutions. PN was usually more stimulated than E by BA and therefore WUE was increased. Positive effect of 1 m M BA was also observed during rehydration of water-stressed bean plants. In sugar beet plants, N6-(m-hydroxybenzyl)adenosine (HBA) was tested in addition to BA. However, the effects of both CK in different concentrations (1 - 20 m M) and ways of application were inconsistent, and only rarely the increase in gs, E or PN was observed. Even if these experiments did not prove general stimulation of gas exchange parameters by CK, further preliminary experiments suggested possibilities of alleviation of their inhibition induced by abscisic acid.