Mechanism of paclobutrazol regulation of shoot growth in Sturt's desert pea, 'Swainsona formosa' (G.Don) J. Thompson (syn. 'Clianthus formosus')

Abstract

Sturt's Desert Pea (SDP) ('Swainsona formosa') is an Australian native plant species with considerable potential as an ornamental crop, including as a flowering pot plant, provided compact forms can be produced. Plant growth regulators (PGR's) are widely used to control plant growth. A range of PGR's were screened for use on SDP and paclobutrazol (PBZ), which has previously been used with variable results on SDP, was found to be the least toxic and most effective. Little previous research has been done to determine its mode of action in SDP. Understanding the mode of action of PBZ may enable more precise and more efficient use, and minimise concern regarding the impact of PGR's on safety standards and environmental pollution. PBZ has been shown to inhibit GA biosynthesis in a range of plant species and may also affect the levels of other endogenous hormones. It is also thought to inhibit sterol synthesis. To understand its mode of action, particularly its interaction with the plant hormones, GA₃, IAA and BAP were applied alone and in various combinations with PBZ, to intact or decapitated plants and to shoot cuttings obtained from main or lateral shoots of SDP. The anatomical responses following PBZ and PGR applications were also examined. In intact plants, PBZ reduced main shoot elongation, increased lateral shoot number but reduced lateral shoot elongation. Simultaneous or delayed application of GA₃reversed the inhibitory effect of PBZ on main shoot growth. However, the reversal of PBZ inhibition of growth in lateral shoots was only achieved if the application of GA₃ was delayed until after the release of the lateral buds from apical dominance or the commencement of lateral shoot growth. Simultaneously applied GA₃ increased apical dominance in intact plants and correlative inhibition in decapitated plants thereby inhibited the growth of further lateral shoots. That was why GA₃ still did rot reverse the PBZ inhibition of lateral shoot growth in intact or decapitated plants. ... This thesis clearly established an inherent difference between the responsiveness of the main shoots and lateral shoots to PBZ. This appears to be due to differences in the levels of endogenous hormones.