Mpn. Gent, PERSISTENCE OF TRIAZOLE GROWTH-RETARDANTS ON STEM ELONGATION OF RHODODENDRON AND KALMIA, Journal of plant growth regulation, 16(4), 1997, pp. 197-203
Triazole growth retardant chemicals may inhibit stem elongation of woo
dy ornamental species for several years after application. Potted plan
ts of large-leaf Rhododendron catawbiense and Kalmia latifolia were tr
eated with a single spray application of paclobutrazol or uniconazole
in the 2nd year from propagation. They were transplanted into the fiel
d the next spring. The elongation of stems was measured in the year of
application and in the next 2-4 years. Treatments with a wide range o
f doses were applied in 1991, 1992, or 1995, For all except the most d
ilute applications, stem elongation was retarded in the year after app
lication. At the highest doses, stem growth was inhibited for 2 years
after application. The results were fit to a model of growth regulator
action which assumed that stem elongation was inversely related to th
e amount of growth regulator applied. For paclobutrazol, the dose per
plant that inhibited stem elongation half as much as a saturating dose
was tenfold that for uniconazole, about 0.5 and 0.05 mg, respectively
. For both chemicals, the dose-response coefficient decreased exponent
ially with time after application, with an exponential time constant o
f about 2 year(-1). A dose of growth regulator which reduced stem elon
gation by half immediately after application would only inhibit 12% of
stem elongation the next year. However, a tenfold greater dose would
result in less than half the stem elongation of untreated plants in th
e next year.