Physiological Responses of Deciduous Tree Root Collar Drenched with Flurprimidol

Materials and Methods

Results and Discussion

The effect of 1985 flurprimidol root collar drenches was not evident until mid-June 1986. Terminal growth was inhibited (43 to 70%) on all species-treatment-time combinations by the end of 1986 except for August and October-treated white ash (Figure 1). In 1986 lateral shoot growth of pin oak and red maple was inhibited by June and August treatments (50 to 92%) but the June treatment was most effective for white ash in 1986 (Figure 2). Lateral growth of black walnut in 1986 was unaffected by flurprimidol. Increasing the level of flurprimidol from 2 to 4 g per tree did not increase inhibition at the same rate as 0 to 2 g per tree. This is indicated by a significant linear and quadratic component to the regression equations. Terminal shoot growth in 1986 of October treatments on ash and walnut was significantly more from 4 g versus 2 g flurprimidol (Figure 1). A possible explanation for this anomaly is that root absorption was slowed in the presence of the high concentration of flurprimidol (16,000 ppm versus 8,000 ppm in water) thus slowing transport to the terminal shoot apices. However, by the end of the 1987 growing season inhibition from both rates was not different.

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Figure 1.

Terminal shoot growth at the end of the second and third growing seasons following root collar treatment of 11-year-old trees with flurprimidol in June, August, and October 1985. Within species, means are separated with Waller-Duncan K-ratio t test, 5% level. Contrasts above histograms display probability values of linear (L) or quadratic (Q) effects due to flurprimidol rate and to month of application effect (Mo).

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Figure 2.

Lateral growth per shoot at the end of the second and third growing seasons following root collar treatment of 11-year-old trees with flurprimidol in June, August, and October 1985. Within species, means are separated with Waller-Duncan K-ratio t test, 5% level. Contrasts above histograms display probability values of linear (L) or quadratic (Q) effects due to flurprimidol rate and to month of application effect (Mo).

By the end of the 1987 growing season terminal and lateral growth of all species was significantly inhibited by all six treatments (33 to 88%) except for terminal shoot growth of red maple from October treatments and the 2-g rate in June and August treatments (Figures 1 and 2). Red maple was the first to show obvious signs of inhibition in 1986 but was the only species to regain normal terminal growth in 1987; although, the 4-g rate applied in June still effectively inhibited terminal shoot growth of red maple in 1987. Excavations around the root collars of the four species treated revealed that red maple had a much higher number of nonwoody (fibrous) roots growing from older woody roots in the vicinity of the root collar zone than the other three species. This may explain in part, red maples’ faster inhibition response to flurprimidol. Although not significant at the 5% level due to variability among replications, there appeared to be some inhibition of diameter growth (data not shown).

Leaf area in 1986 was reduced for pin oak, red maple, and white ash (22 to 65%), but not for black walnut. In 1987 leaf area was inhibited (17 to 65%) for all species treatment combinations (Table 1). Leaf rachis length in 1986 was reduced by June and August treatments on white ash (20 to 45%) resulting in a more compact leaf. Rachis length of black walnut was not affected in 1986, but in 1987 rachis length was signficantly reduced (P ► 0.01) for both white ash and black walnut 21 to 50% (data not shown). In 1986, leaf dry weight of red maple treated in June and August differed from controls, but leaf dry weight in 1987 was reduced for all species (28 to 64%) with the exception of white ash (Table 2). Increasing flurprimidol level from 2 to 4 g per tree did not decrease leaf area or dry weight at the same rate as 0 to 2 g per tree since there were signficant linear and quadratic components in the regression equations (Tables 1 and 2).

With the exception of pin oak terminal growth, less inhibition of growth and leaf expansion occurred from the October root collar drenches than June or August as indicated by the significant month effect in Figures 1 and 2, and Table 1. The less effective inhibition in October was probably because the trees were not actively translocating acropetally at treatment time and had one less growing season to absorb and translocate flurprimidol than trees treated in June or August. Also, no significant effect on growth was observed on any species-treatment-time combination during the growing season (1985) the root collar drenches were performed. Since trunk-injected trees usually respond to flurprimidol within a month or two after treatment (9), time of absorption through the older root bark must be responsible for the slow inhibition response. However, by the end of the 1987 growing season, shoot growth inhibition of both terminal and laterals was both effective and very uniform without apparent shoot “escapes” except for red maple treated with the lower rate. Gilliam et al. (4) observed that red maple “escape” shoots were even longer than control tree shoots three years after treatment of seedlings. The “escape” shoots of treated trees in this study after three years were not significantly longer than control trees.

In addition to the reduction in leaf area and dry weight, flurprimidol caused a slight decrease in net photosynthesis and an increase in chlorophyll concentration (Table 3). The transpiration rate was not affected. Steffens and Jacobs (7) also found a lower relative photosynthetic rate of apple trees with the gibberllin biosynthesis inhibitor paclobutrazol. One would expect leaf photosynthetic rates to be higher than control trees since chlorophyll concentration was higher. However, Anderson (1) found that the photosynthetic rate under natural conditions does not vary much over a considerable range of chlorophyll concentrations and that the rate of photosynthesis is usually well below the maximum possible rate (6). In this study, the reduction in the photosynthetic rate, leaf dry weight and expansion that coincided with the increased chlorophyll concentration did not appear to adversely affect plant vigor or aesthetic quality. The only visible symptoms of flurprimidol in 1986 and 1987 were small, dark-green leaves and shorter shoots. In an earlier study, white ash and black walnut trees treated in April 1984 with a dose as high as 12 g/tree of flurprimidol were still visibly healthy in June 1988 with obviously high concentrations of leaf chlorophyll and shorter shoots (10).

Conclusions

The results of this study indicate that root collar treatments of flurprimidol conducted during the growing or early dormant season inhibited growth uniformly without visible phytotoxicity for two years and, with the exception of growth inhibition, had a relatively minor effect on physiological processes. In addition to consistent uniform inhibition of terminal and lateral shoot growth, which is difficult to obtain with trunk injection techniques, the root collar drench method has several advantages especially for controlling the growth of young trees. Root collar zones of city trees are usually accessible whereas the fibrous roots (targets for the soil injection technique) that extend beyond the foliage canopy may be unavailable; e.g., covered with concrete or macadam. Confining flurprimidol to the root collar zone reduces the possibility of contacting nontarget plants because flurprimidol remains close to the root collar zone. In another study, using a bioassay technique, flurprimidol activity was still confined laterally within 10 cm of the treated root collar 1 ½ years after drenching the base of the trunk (10). Before flurprimidol can be registered for root collar drench use, more information is needed regarding effective doses for large trees and a variety of other species. Also, information is needed regarding longevity and movement of furprimidol residues in various soil types. When drenching the root collar zone, some of the wettable powder formulation, although directed at the trunk base, makes contact with the soil.