Presence of Offsets Reduces Hosta's Response to Benzyladenine

Gary J. Keever and Thomas J. Brass
Vegetative buds of hosta develop on rhizomes and in leaf axils, but the shoot apex appears to suppress outgrowth of these buds by apical dominance. A primary factor in the mechanism of apical dominance is a hormonal interaction between auxins and cytokinins. Auxin, produced at the shoot tip and transported basipetally, inhibits the release of buds from apical dominance. This action of auxin is antagonized by cytokinin.

A previous study demonstrated that application of the synthetic cytokinin, benzyladenine (BA), to single-shoot plants of blue hosta induced the outgrowth of lateral and rhizomic buds. Offsets formed from BA-induced buds were removed from treated plants within 30 days of BA application and rooted under intermittent mist. Application of this technique to hosta production would likely employ stock plants with various numbers of initial offsets. However, because apical dominance is at least partially controlled by auxins produced in the shoot tip, the presence of multiple shoot apices may alter the response of hosta to BA application. The objective of this study was to determined effects of number of initial offsets on response of blue hosta to a range of BA rates.

Methods
Uniform plants of blue hosta were selected for uniformity and divided into three groups based on the number of offsets present: those with no offsets, those with one emerged offset, and those with two or three emerged offsets. Plants in each group received a single foliar spray of 0, 1,250, 2,500, or 3,750 ppm BA. At 30, 60, and 90 days after treatment (DAT), offset counts were recorded for each plant.

Results
Generally, offset number increased with increasing BA rate, regardless of initial offset number. These offsets developed from both axillary and rhizomic buds. For plants with no initial offsets, offset counts increased from 0.1 to 3.1 at 30 DAT, with no further increase at 60 DAT, even with 3,750 ppm BA. Change in offset number 90 DAT, averaged across initial offset number, again increased with increasing BA rate.

At 30 DAT, change in offset number among controls was greatest for plants with no initial offsets, followed by those with one initial offset, and least for plants with two or three initial offsets. When treated with 1,250 ppm or 3,750 ppm BA, plants with no or one initial offset formed similar numbers of offsets 30 DAT but more offsets than those with two or three initial offsets. A similar trend was detected 60 DAT with controls and plants treated with 1,250 ppm BA. However, plants with different initial offset numbers formed similar numbers of offsets 60 DAT when treated with 2,500 ppm or 3,750 ppm BA. At 90 DAT, plants with no initial offsets had a similar increase in offset number as those with one initial offset and a 143% greater increase compared to plants with two or three initial offsets.

These results indicate that the presence of one offset on stock plants had little or no effect on hosta's response to BA compared to stock plants with no initial offsets. However, the presence of two or three offsets reduced subsequent formation of offsets in response to BA application. One possible explanation for the observed response in hosta is that plants with two or three initial offsets produced more auxin than those with zero or one offset, thus apical dominance was stronger and BA was less effective in promoting outgrowth of additional axillary or rhizomic buds.

The change in offset number decreased in controls with one (30 DAT) or two or three (30 and 60 DAT) initial offsets and at 30 DAT in plants with two or three initial offsets treated with 1,250 ppm BA. This decrease in offsets may reflect a random abortion of offsets.

These data show that BA stimulates outgrowth of lateral buds in hosta. However, fewer additional BA-stimulated buds are likely to develop into shoots when two or three initial offsets are present on the stock plants than when zero or one offset is present. These results provide a clearer understanding of stock plant response to BA application and should facilitate the development of a practical system for the accelerated multiplication of hosta.

CHANGE IN INITIAL OFFSET NUMBER OF BLUE HOSTA STOCK PLANTS
TREATED WITH 0, 1,250, 2,500 OR 3,750 ppm BA
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Change in offset number
.30 DAT60 DAT90 DAT1
Initial
offset
number
012,3. 012,3. 012,3
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BA rate (ppm)
00.10.8-1.4. 0.10.1-0.7. ---0.3--
1,2501.52.3-0.4. 2.32.80.3. --1.5--
2,5001.31.31.4. 1.91.4 1.7. --1.4--
3,7503.12.41.1. 3.02.11.9. --2.4--
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1Average across initial offset number
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