Abstract:
Techniques to measure vegetative maturity and dormancy
of deciduous nursery stock were investigated in red-osier
dogwood. Previous methods which were based on: survival
or tip dieback, change in level of ethylene production,
xylem water potential, and alteration of square wave traces
traces, provided no better indication of dormancy than did
a knowledge of calendar date. Tissue response to and modification
of applied electrical currents however, indicated
that electrical impedance ratios could be used successfully
not only in plant breeding, presymptomatic disease
diagnosis, water relations, and frost hardiness studies,
but as a predictor for vegetative maturity and dormancy
development as well.
Impedance ratios and water contents were determined
concomitantly during the development of vegetative maturity
in a clone of red-osier dogwood which enters dormancy at
a photoperiod of 14.2 hours or less. Plants were exposed
to 16 hr. and 12 hr. photoperiods in growth chambers to
selectively prevent (16 hr.) or induce (12 hr.) dormancy. The onset of vegetative maturity was measured by defoliating
plants at weekly intervals and subsequently observing
whether or not previously dormant buds were stimulated to
grow. There was a consistent and significant increase in
the impedance ratios of dogwood stems at time plants became
vegetatively mature. During the first 5 weeks, impedance
values obtained from frequencies 10⁴/10⁵ Hz remained relatively
stable at 2.3 in plants grown at the 12 hr. photoperiod.
After week 5, a significant increase in the ratio
occurred, rising to a peak of 4.0 by the 7th week, and
remained at that level. Stem moisture content declined
gradually as the plants matured, and was approximately
inversely proportional to impedance value. Changes in
impedance ratios were consistent and less subject to variation
than stem water contents. Impedance ratio measurements
have potential for predicting the timing of vegetative
maturity and dormancy in woody deciduous species.
