- In the mild climate of the Pacific Northwest, the harvesting of
deciduous shade trees in wholesale nurseries is prolonged from
October to January because of autumn growth and prolonged leaf
retention. Economically, it is advantageous to defoliate nursery trees
at the earliest possible date. Premature defoliation, however, can
cause die back or predispose trees to over-wintering damage or poor
performance the subsequent year.
The purpose of this study was to identify and characterize the
physiological stage of development at which leaves can be removed
from deciduous trees without adversely affecting survival and
regrowth. This physiological stage was termed "interfronoce,"
Interfronoce could not be reliably predicted from visual growth
phenomena such as leaf coloration, leaf fall, and terminal bud
Preliminary data showed that stems and roots of plants unharmed
by defoliation were less hydrated and contained higher concentrations
of soluble carbohydrates than less mature plants which were damaged.
It was hypothesized that osmotic potential of cells and the ionic status
of cytoplasm were considerably different in interfronoce vs. pre-interfronoce
plants, and that these differences could be used to predict
The hypothesis was tested by studying the influence of defoliation
on uniform Cornus stolonifera Michx. plants grown under controlled
SD or LD conditions for 10 wks, and 2 years of study on field-grown
nursery stock. In the controlled study, xylem water potential
and turgor pressure of SD-10 hr plants in interfronoce were generally
lower than in pre-interfronoce plants grown at LD-16 hr days.
Sequential hand defoliation showed that 60 days of SD treatment
induced interfronoce in C. stolonifera. The xylem water potential
also reached its lowest point after 60 days of SD exposure. Defoliation
prior to interfronoce caused bud and stem die back and stimulated
In 1974, phenological observations were made on 59 cultivars of
25 species. Observations were made of leaf expansion, autumn leaf
coloration, and terminal bud formation. On 12 cultivars of 5 species,
measurements were made of xylem water potential and osmotic
potential on terminal shoots. Turgor pressure was calculated. The
timing of these phenological and physiological changes was correlated
with weather data. Data on each species were grouped for a multiple
discriminant function analysis to determine the most reliable interfronoce
indicators. Response to hand defoliation was used as the
standard for establishing when plants entered interfronoce. A combined
index of interfronoce based on xylem water potential, osmotic
potential, and turgor pressure predicted the onset of interfronoce
with a high degree of accuracy in Acer, Malus, Quercus, and Sorbus.
In terms of subsequent performance (growth), the time of interfronoce
ranged from Sept 16 to Nov 11 in the species studied.
Seasonal changes in water status of 38 cultivars of 22 shade tree
species were determined under field conditions in 1975 since certain
changes in water status appeared to coincide with onset of interfronoce
in 1974. Predawn xylem water potential, osmotic potential, and
turgor pressure values (water status) were determined at 2 wk intervals
from July 26 to Nov 21. A multiple discriminant function analysis
indicated that stem water status was reliable for measuring interfronoce
in cultivars of Acer, Betula, Carpinus, Crataegus, Fraxinus,
Gleditsia, Malus, Platanus, Quercus, Sorbus, and Tilia studied.
Chemical defoliants were safely used to hasten leaf abscission
by 4-8 wk when water status was used to predict interfronoce. Comparatively
low xylem water potential measurements effectively timed
the application of chemical defoliants.
Relationships between interfronoce, dormancy, and the rest
period and quiescence phases of dormancy are discussed and