Soil Physical Environment and Root Growth
in Northern Climates
GURDARSHAN S. BRAR AND CHARLES M. REYNOLDS
INTRODUCTION
cal impedance, and spatial variability. The soil
Plant roots grow because new cells form in the
physical environment required for the develop-
meristematic tissues near the root tip and the
ment of an efficient root system must be free from
newly formed tissues expand in volume, pushing
stresses caused by mechanical, water, oxygen,
the root tip forward. The amount of water and the
and cold-temperature conditions. Soils, because
nature of nutrient uptake from the root zone are
they vary texturally, structurally, in porosity, and
established by the interactions of various physi-
pore size distribution, differ greatly 1) in their re-
cal, chemical, and biological processes. After
sistance to root penetration, 2) how they collect,
many years of scientific discoveries, we under-
store, and release water for root uptake, and 3)
stand how water and chemicals enter the soil and
how they maintain aeration by gas exchange with
are transported through the root zone. However,
the atmosphere.
our understanding of the fundamental aspects of
The objective of this report is to present an
growth and proliferation of plant roots within the
overview of plant root growth and development
cold soil environment is still incomplete, because
under soil physical environments with special
of the vast complexity of the various interacting
emphasis on models that are available for root
processes involved.
growth simulation. The physical edaphic factors,
During the last decade, root research has re-
as they occur naturally or are created by human
ceived its strongest emphasis from plant physiol-
activities, and their effects on root growth are pre-
ogists, ecologists, and soil scientists. Limited in-
sented, beginning with brief background infor-
formation is available on water and nutrient
mation on root origin and distribution, growth,
uptake by roots in cold regions. Billings (1992) re-
development, and functions.
ported that in northern climates, plant metabo-
lism, growth, and reproduction are controlled by
ROOT ORIGIN AND DISTRIBUTION
the interactions of the soil physical environment
and the genetic structure of cold-adapted flora. In
Seeds germinate and primary roots originate
these cold regions, the major constraints to root
from the radicle. Roots also originate from stem
growth are the presence of continuous perma-
nodes on stolons, bulbs, and rhizomes of grasses,
frost in the subsurface and the short growing sea-
or from the hypocotyl of certain dicotyledonous
son (Fig. 1). The literature provides information
plants (dicots, Fig. 2). As dicots grow, the laterals
on the genetic structure of cold-tolerant plants;
of the first order emerge from the main root fol-
however, limited information is available on the
lowed by laterals of second, third, fourth, and
root growth and development in northern cli-
even fifth order (Barley 1970). In monocotyledon-
mates. Therefore, this review focuses on the soil
ous plants (monocots, Fig. 2), the seminal (pri-
physical environment and its effect on root
mary) roots originate from primordia in the em-
growth in cold climates.
bryo, and crown (nodal, adventitious, or brace)
The soil physical environment will be charac-
roots arise from foliar nodes of the growing plant
terized by temperature, water, aeration, mechani-
(Russell 1977, Klepper 1991). The growth rate of
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