Uptake, transport and bioactivity of exogenously applied ABA and ABA analogues in white spruce and wheat seedlings

There are significant differences between conifers and herbaceous species in their stomatal sensitivity to exogenously applied ABA. Experiments on white spruce (Picea glauca (Moench) Voss) and wheat (Triticum aestivum L. cv Katepwa) seedlings, whose roots were sealed in an aeroponic misting chamber, confirmed that 200-fold higher concentrations (2 x 10-3 M) of exogenously applied (+/-)ABA were required to close stomata in spruce than in wheat (10-5 M). I tested the hypothesis that this difference in response between species was because: (i) stomata are inherently more sensitive to ABA in wheat than in spruce; (ii) in wheat, ABA is taken up more efficiently by roots and more ABA is subsequently delivered to the shoots and (iii) a combination of (i) and (ii). Tritiated ABA was applied to plants over approximately 10 hours and their water uptake measured continuously.;ABA uptake efficiency (UE) was almost twice as high in spruce as in wheat. However, in spruce, virtually all of the ABA taken up remained in the roots. In contrast, in wheat, a much higher proportion of ABA taken up by the plant was delivered to the shoots. Thus transport efficiency (TE) was almost 9 times higher in wheat than spruce. When the roots were excised from spruce seedlings, there was a 55-fold increase in ABA delivery to shoots and a concomitant 20-fold increase in stomatal sensitivity to the application of ABA. Immunofluorescence labeling showed that cortical cells around the endodermis were the main site of exogenous ABA accumulation in spruce roots. In wheat, the major portion of the exogenous ABA was found inside the vascular tissue in the roots. I conclude that in spruce, the roots provide a major barrier to the transport of ABA to the shoots.;Experiments were also conducted, to determine the uptake and transport from roots to shoots of (+)- and (--)-ABA enantiomers and their respective methyl ester derivatives. I tested the hypothesis that the higher biological activity, of ABA enantiomers or analogues would be related to their being more efficiently incorporated into roots and subsequently transported to shoots.;In both species, (+)-ABA was more biologically active than (--)-ABA. Differences in TE between the enantiomers; were significant only in wheat. In spruce, UE of the methyl ester enantiomers was almost twice as high as that of the respective ABA enantiomers. However, virtually all of the MeABA taken up remained in the roots. Thus, despite its higher transport across root membranes, MeABA had a lower biological activity than ABA. Adding an isopropyl ester to the C-1 carbon of ABA brought about an increased bioactivity only in spruce where (+/-)-iPrABA induced stomatal closure at a 10-fold lower concentration than (+/-)-ABA. Thus, in spruce, any increased biological activity of ABA analogues depends on how effectively they are transported from roots to shoots. (Abstract shortened by UMI.).