Journal of Experimental Botany, 33(137): 1286-1292, (Dec 1982)
Environmental Factors involved in the Regulation of Sprouting of Basal Buds in Rose Plants
E. Khayat and N. Zieslin

ABSTRACT

The plant top is the main factor inhibiting the formation of juvenile-like shoots from the basal part of the rose plant. In plants with the whole top removed a cooling period of one week at 4°C had a promotional effect on the outgrowth of basal buds. Six days of darkness partially inhibited sprouting of these buds and 9 d suppressed sprouting completely. On the other hand. 3 d in light following plant top removal were sufficient to cause sprouting.

Buds on the upper parts of lateral shoots were not affected by the dark treatment.

INTRODUCTION

Vigorous, erect shoots, arising from various parts of the stem are common in many perennial woody plants. They most commonly appear on the basal part of the stem, especially around the trunks of cut-down trees. Other sites are the connection nodes of the lateral branches with the stem and the curved parts of bent-down branches. This type of growth is also known as 'water sprouts', 'bottom breaks', renewal shoots or epicormic shoots (Busgen and Munch, 1929; Kohl and Smith, 1970; Kramer and Kozlowsky, 1979: Zieslin, Mor, Bachrach, Haaze, and Kofranek, 1976), and due to their vigorous growth, the absence of flowering (excluding roses) and relative ease of root regeneration from cuttings, these shoots are often categorized as 'juvenile-like' (Tubbs, 1974; Wareing, 1959; Zimmermann and Brown, 1971).

These proleptic shoots originate from dormant, inhibited or latent buds formed in the axils of scales which protect lateral buds (Church and Godman, 1966; Oshri, 1981). In some species, epicormic shoots originate from cellular formations known as sphaeroblasts (Baldini and Mosse, 1956; Dermen, 1948; Hatcher and Garner, 1954: Wellensiek, 1952).

Light is important in promoting basal bud growth in forest trees (Kramer and Kozlowsky, 1979) and in roses (Fischer and Kofranek, 1949). Artificial lighting of roses enhances the growth of basal shoots (Carpenter and Anderson, 1972: Carpenter and Rodriguez, 1971a. Khosh-Khui and George, 1977). while a decrease in the irradiance or darkening is inhibitory (Zieslin and Mor, 1981).

Air and root temperature have also been found to affect sprouting of the basal buds (Brown and Ormrod, 1980; Shrock and Hanan, 1981; Zieslin and Mor, 1981).

Treatments resulting in the cessation of plant growth such as water stress (Post, 1952). pruning (Kohl and Smith, 1969), shoot bending (Faber and White, 1977: Moe, 1973: Zieslin and Halevy, 1978) and removal of lateral growths (Kohl and Smith, 1970: Zieslin and Mor, 1981; Zieslin et al., 1976) also tended to promote 'bottom break' formation.

Stimulation of basal bud growth by various growth regulators, particularly by cytokinins and ethylene, is well documented (Carpenter and Rodriguez, 19716: Faber and White, 1977: Ohkava, 1979; Parups, 1971; Zieslin, Halevy, Mor, Bachrach, and Sapir, 1972).

In spite of all these treatments, the appearance of renewal shoots in rose plants is rather rare and sporadic, although the complete removal of the plant top always results in a burst of epicormic growth.

The purpose of the present study was to investigate the involvement of temperature and light in the control of growth of buds from the basal part ('crown') of the plant.

MATERIALS AND METHODS

Three-year-old plants (cv. Baccara) grown in 10 l containers were used in all the experiments. The plants were grown in a greenhouse under natural day lengths in Israel. Minimum night temperatures of 18°C and maximum day temperatures of 30°C were maintained throughout the experimental period. Cooling treatments were applied by growing plants at 4°C in growth chambers with a daily lighting period of 12 h, using mixed light from incandescent and fluorescent lamps of 62 W m-2. The plants were fertilized with a complete nutrient solution of N:P:K (20:20:20) with supplementary micro-elements. Darkening of the rose 'crowns' after canopy removal was achieved by covering with black polyethylene under aluminium foil. The details of the individual experiments will be described with the results.

EXPERIMENTAL PROCEDURE AND RESULTS

The influence of cooling on sprouting of epicormic buds after removal of the plant top was examined in the following experiment. The tops of 30 out of 60 plants previously selected for uniformity were removed, leaving only the bases of the main branches—the 'crowns'. In the second group of 30 plants, two main branches, 50 cm long were defoliated and left intact. Twenty-four plants of each group were exposed to 4 °C in the growth chamber, while 6 plants of each group, were left in the greenhouse as uncooled controls. One, 2, 3 and 5 weeks after the beginning of cooling, groups of 12 plants (6 with and 6 without branches) were returned to the greenhouse. The new sprouted buds were continuously removed to avoid possible correlative inhibition from the first sprouted buds. The total number of buds sprouted was determined 6 weeks after the end of the cooling treatments and plants returned to the greenhouse. The results show (Table 1) that 5 weeks at 4 °C was not sufficient to cause epicormic growth in plants with branches left intact. However, 1 week of cooling had a pronounced stimulatory effect on the breaking of 'crown' buds of the topped plants. The number of buds sprouted on plants receiving 1—2 weeks of cooling was more than 3 times higher than in the uncooled control plants, while a longer period of cooling resulted in partial inhibition of bud breaking.

TABLE 1. The effect of cooling at 4 °C on the sprouting of basal buds in rose plants (cv. Baccara) after complete plant top removal

Plants with 2 intact defoliated branches were left as a control. The data were taken 6 weeks after the end of the cooling treatments. Each value is the mean from 6 plants.

Cooling
(weeks)
Buds per plant
Plant top removed Two defoliated branches remaining intact
0 4.5 0.2  
1 16.2 0.0  
2 13.2 0.2  
3 7.4 0.0  
5 8.0 0.0  

The effect of elimination of light on sprouting from the 'crown' was tested in the following experiment. After the removal of the top, the 'crowns' of 12 plants were wrapped with a polyethylene sheet and aluminium foil. For a comparison, 12 uncovered plants were left as a control treatment. Six plants of each group were kept in the growth chamber at 4 °C. After 2 weeks of cooling the plants were returned to the greenhouse. Eight days later the number of sprouted buds was determined. The results are presented in Table 2. The data show that prevention of light reaching the woody 'crown' almost completely inhibits bud breaking, while in spite of the stimulation (Table 1), the cold treatment was unable to prevent this dark inhibition. In the light, a 50% increase in the number of sprouted buds was obtained in plants given the cold treatment, compared with the uncooled control plants.

TABLE 2. The effect of darkening on the sprouting of basal buds from the 'crown' of rose plants

Each value is the mean from 6 plants. Figures accompanied by different letters are significantly different (P = < 0.05). (Multiple range test, MRT.)

Treatment Buds per plant
Greenhouse, light 8 b
4°C, light 12 a
Greenhouse, dark 1 c
4°C, dark 2 c

TABLE 3. The period of darkness required for the inhibition of sprouting of basal buds

The dark treatment started immediately after plant top removal Each value is the mean from 6 plants. Figures accompanied by different letters are significantly different at P = 0.05. (MRT.)

Days in
darkness
Days after the uncovering Total per
plant
6 10 14 19
0 2.5 2.8 3.3 4.0 4.0 a
3 3.6 4.6 5.0   5.0 a
6 1.0 3.8     3.8 a
9 0.6       0.6 b

The minimal dark period required to prevent sprouting of the 'crown' buds was examined in the next experiment.

Immediately after removal of the top, 'crowns' of 18 plants were covered, while a group of 6 plants was left in the light. Groups of 6 plants were uncovered at 3 d intervals and the number of buds sprouted was determined during 19 d after the dark cover was removed. The results are summarized in Table 3. Six days after the end of the dark treatment, those plants which had received 6 d of dark treatment showed 60% inhibition of bud sprouting compared with the undarkened plants, while an additional 4 d in light completely released the inhibition. However, an additional 3 d in the dark inhibited the buds almost completely and no sprouting could be detected after 19 d in light following the dark treatment.

In another experiment the minimal period of exposure to light necessary for bud sprouting following the top removal, was examined. After topping, groups of 6 plants were covered at 3 d intervals, while one group of 6 plants remained uncovered. The covering of all the plants was removed 18 d after the topping and the number of sprouted buds was counted. The results presented in Table 4 show, that 3 d of light were sufficient to maintain the bud breaking at 62% as compared to the plants remaining in the light. Three additional days of exposure to light led to complete bud breaking, while buds of plants left in the dark remained inhibited.

The inhibitory effect of the dark treatment on the axillary lateral buds located on different positions along the rose shoot was also examined. Five flowering shoots were cut above the third from the base, fully expanded five-leaflet leaf. The 3 leaves were removed and the remaining stump was covered with black polyethylene. Five additional shoots were cut above the seventh from the base, five-leaflet leaf. The 3 uppermost leaves were removed and the defoliated stem was covered as above. For a comparison 5 covered and 5 uncovered 'crowns' were left. The covering of all the shoots and 'crowns' was removed after 12 d. The results of this experiment presented in Table 5 illustrate the difference in the response of buds on the various locations along the shoot to darkening. While all the buds close to the shoot base were still completely inhibited as were the buds on the covered 'crowns', the more distal buds were indifferent to the dark treatment. However, the elongation of these buds was suppressed.

DISCUSSION

From the results it is clear that the major factors controlling the out-growth of basal, epicormic buds are located in the top part of the plant (Table 1). Removal of the top leads to the release of many of those buds from inhibition, while in plants with tops, sprouting was negligible during the experimental period. This phenomenon is frequently observed following cutting down of trees. Stimulation of epicormic growth in plants with intact canopies is possible by means of horticultural treatments, cooling or by treatments with growth regulators (Zieslin and Mor, 1981). However, sprouting is consistent and the number of buds sprouting  relatively small. The inhibition of buds by the plant top may be a result of inhibitory factors produced in the canopy and accumulating in the basal parts of rose shoots (Zieslin, Spiegelstein, and Halevy, 1978) or from auxin synthesized in lateral buds along the stem. However, the level of auxin found in the upper buds of rose plants was much higher than that in the lower buds (Moe, 1971).

TABLE 4. The minimal period of light, prior to the dark treatment, required for stimulation of sprouting of basal buds

The dark covering was removed simultaneously 18 d after the plant top removal. Figures accompanied by different letters are significantly different at P = 0.05 (MRT). In brackets is the number of days plants were exposed to the dark treatment.

Days in light before
exposure to dark
Number of buds
per plant
0(18)   0.0 c
3(15)   2.5 b
6(12)   4.1 ab
9(19)   5.0 a
12(6)   5.8 a
18(0)   4.0 ab

TABLE 5. The response of various buds on rose plants to the dark treatment

Each treatment was replicated five times. The length of the most advanced shoot was measured.

Plant part Number of 'crowns' or
shoots with sprouted buds
Shoot length (cm)
Light Dark Light Dark
'Crown' 5 0 3.2 0.0
Short shoots 5 0 1.8 0.0
Long shoots 5 5 0.3 3.5

The bud inhibition caused by the prevention of light reaching the woody tissue has been reported in rose plants with intact canopies (Zieslin and Mor, 1981) and also in other plants (Hartmann and Opitz, 1966). Surprisingly, the covered buds were unable to sprout although the main inhibitory factor of the whole top part of the plant was removed (Table 2). Thus, it is possible that light is required for the elimination of part of the inhibitory complex remaining in the 'crown' tissue. This assumption is supported by the results of the experiment where various buds along the stem were darkened (Table 5). The buds of the upper part of the shoot with a lower level of inhibitors (Zieslin et al., 1978) were not inhibited by the dark treatment. On the other hand, buds on more basal parts, with a higher content of inhibitors, remained completely inhibited (Table 5). Furthermore, 9 d of darkening were sufficient to prevent sprouting of basal buds (Table 3), but 12 d exposure of the upper buds to darkness had no effect on bud breaking (Table 5).

Three days in light prior to darkening resulted in a low rate of inhibition, and after 6 d of exposure to light no inhibition could be detected (Table 4).

Inhibition of the basal buds in roses was also affected by low irradiance (Zieslin and Mor, 1981) and a higher rate of inhibition was found in plants illuminated with FR light (Healy and Wilkins, 1980).

Cooling of plant 'crowns' caused a 3-fold increase in the number of buds sprouted (Table 1). An increase in the sprouting of basal buds in intact rose plants was demonstrated following a prolonged period of cooling of 23 weeks (Hanan, 1979; Schrock and Hanan, 1981). A promotive effect of cooling on sprouting of basal buds was also reported in chrysanthemum (Schwabe, 1950), and statice (Shillo, 1976) and resembles the effect of cooling on tillering in grasses. This response of the lower buds to cold treatments may indicate residual dormancy in the basal buds of rose plants. A more dormant state of lower buds was mentioned in several tree species (Nooden and Weber, 1978). This may also be attributed to activation of cytokinins accumulating in the basal part of the rose plant (Zieslin and Khayat, 1982) by lower temperature as demonstrated in rose flowers (Zieslin, Madori, and Halevy, 1979) and in rootless almond shoots stored at low temperature (van Staden and Dimalla, 1981).

The data reported in this study show that epicormic growth in roses, which may be related to the bushy pattern of growth in other woody species is a complex phenomenon. In its control, environmental factors as well as correlative factors play an important role. These two groups of factors may affect the growth of the buds by controlling the balance of plant growth regulators involved in this phenomenon.

LITERATURE CITED