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Improvement in the structural root system of citrus nursery trees. Observations since 1990 showed container-grown citrus nursery
plants to develop less vigorously than expected in the field, specially when compared
to field-grown plants. In addition, container-grown trees seemed less drought
tolerant in the field, even when budded on Rangpur lime, a highly resistant rootstock.
Close examination revealed that poor structural
root system could be involved as cause of the
undesirable behavior of referred trees. Citrus plants produced in containers typically
show significant curling of the main tap root and of the pioneer lateral roots. The
worst situation is encountered in plants produced in plastic bags. Rigid containers
usually have internal ridges which prevent curling of lateral roots. Another serious
problem of container-grown citrus is the multiple
branching of the main and lateral roots, as
opposed to the typically unbranched structural-root extremities. Resulting reduction in
depth of penetration and extent of soil exploration by the defective root systems seems
to be related to the reduced drought tolerance and vigor observed. The curling of
structural roots can be somewhat reduced by the use of large containers and pruning of
defective roots before transplant. However,
special growing techniques were required to prevent multiple branching of structural
roots. Resulting plants were established in the
field in the last 3 years and have compared favorably with similar plants produced in
the traditional system, showing visually more vigor and higher drought tolerance. This may be explained by the fact that
unbranched tap roots penetrate more deeply in the soil, probably better exploring the
more humid soil layers. In addition,
the usually high number of roots resulting from branching typically grow in a tight
bundle, as seen in the pictures above. As all roots in the bundle increase in girth
simultaneously, significant mutual interference is expected, delaying growth and
reducing tree vigor. Trees with normal structural
roots as in the pictures below, on the other hand, develop freely with no limitations
in the root system, attaining maximum soil exploration. As a result, vigor is preserved
and drought tolerance maximized.
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