Among citrus rootstocks, higher specific root length (SRL, root
length/d. wt) has been linked to several specific
morphological and physiological traits, including smaller average root
diameter, higher root hydraulic conductivity
and higher rates of root proliferation. In this study, thickness of the
outer tangential exodermal (hypodermal) wall
and its suberin layer, number of passage cells, presence of epidermis,
and stelar anatomy were examined and
related to variation in root diameter of field roots of known maximum age.
We also compared root morphology
and anatomy of young roots in the field with those of potted rootstock
seedlings in the glasshouse. Fibrous roots
were measured separately from pioneer (framework) roots. Among the fibrous
roots, only the first-order (root links
having a root tip) and second-order (root links bearing first-order roots)
laterals were examined. Among first-order
field roots, larger root diameter was caused by larger rather than more
numerous cells in the cortex. Root diameter
of first-order roots was positively correlated with both number of passage
cells in the exodermis and thickness of
the secondary walls of the exodermis in both field and potted plants.
Exodermal walls were about 80% thicker in field- than pot-grown roots.
In the field, in more than 50% of the
first-order roots examined less than 30% of the root surface was still
covered by epidermis, with few differences
among rootstocks. In contrast, in roots of 19-wk-old glasshouse plants
generally 70–100% of the epidermis was
still intact. There was no evidence of secondary xylem development in second-order
fibrous roots in the field; in
seedling, pot-grown rootsystems, 75–97% of second-order roots had
formed secondary xylem despite their small
diameter (<0.8 mm).
It is argued that there can be suites of physiological, morphological
and anatomical traits in roots that co-vary
with specific root length. Investigations of how root morphology and anatomy
are linked to root function,
moreover, need to recognize trait variability and the potentially important
differences between field- and pot-
grown (seedling) roots.