Changes in the water status and rheological characteristics of pea seedling axillary buds during their transitions growth-dormancy-growth in apical dominance

Abstract

The technique of isopiestic thermocouple psychrometry was used for the analysis of bud transition from dormancy to growth and back in 8-18-day-old pea (Pisum sativum L.) seedlings. We monitored changes in the water (ψw) and osmotic (ψs + m) potentials and also turgor pressure (ψp) in dormant buds and threshold turgor (Y) in growing buds, the latter being one of the cell-wall rheological characteristics. Seedling decapitation resulted in a decrease of Y in the bud, which coincided with the start of its outgrowth. The replacement of terminal shoot with exogenous auxin (IAA or NAA) retarded bud outgrowth and maintained the high level of Y, which argues for the auxin control of this parameter. When growth of the first axillary bud was inhibited by the second one, positioned higher and remained on the plant, the beginning of Y increase preceded visible correlative growth suppression; this makes this rheological index an early marker of bud transition from growth to dormancy. The effects of the terminal shoot part and auxin application on the bud osmotic status differed substantially. In fact, bud transition to dormancy in the presence of the terminal shoot, the main or developing from the second axillary bud, was accompanied by the rise in ψs + m, whereas, in the case of the replacement of the second bud with exogenous auxin, the first bud growth suppression occurred with the decrease in ψs + m. The low value of the bud ψs + m is a factor for creating a considerable gradient of the water potential between the stem and bud supporting water transport to the bud, which was much more active than in plants with a terminal shoot. It seems likely that this is the reason for the absence of complete growth suppression observed by us and other researchers even after application of high auxin concentrations. Immediately after seedling decapitation, ψs + m in the buds reduced; however, this was not the result of trophic metabolite redistribution due to the loss of their active sink because ψs + m reduced also in experiments with complete isolation of the bud releasing from dormancy in the chamber at 100% humidity. Auxin application to the cut surface of decapitated seedlings did not affect the ψs + m decrease. Like decapitation, cotyledon removal resulted in the increase in the bud turgor pressure. However, in this case, water stress did not change the bud osmotic status. Thus, the induction of osmotica accumulation in the bud after the removal of the terminal shoot is evidently related to neither trophic, nor auxin, nor hydraulic signal. The data obtained allowed us to conclude that both components of the bud water potential—ψs + m and Y—play an important role in the control of bud growth at apical dominance. Auxin produced in the shoot apex is involved in the control of Y, whereas the nature of the signal controlling the ψs + m level is unclear.