Domestication of the shea (vitellaria paradoxa c.f. gaertn) tree: developing improved propagation techniques for accelerated plant growth.
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Date
2015-11-08
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Abstract
The shea (Vitellaria paradoxa Gaertn) tree which grows wild is a multi – purpose species
highly valued for oil obtained from its seed which is similar to that of cocoa butter. The wild
nature of the shea tree, slow growth and long gestation period does not make its development
attractive thus hampering its domestication. However, worldwide, vegetative propagation
methods have been used as a means for domesticating endangered and wild species which
will give a promising future to the shea industry. A series of field and laboratory experiments
were carried out at the Cocoa Research Institute Sub-station, Bole and KNUST, Kumasi from
2012 to 2014. The objectives of the study were to (i) assess the rooting performance of air-layered stems under dry and wet conditions (ii) determine the effect of propagating structures
and seedling types on the weaning and field survival of rooted propagules (iii) assess the role
of some endogenous growth promoting substances and application of some exogenous
growth regulators on rooting of air-layered stems (iv) investigate the physiological and
environmental effects of scions and rootstocks on grafting success (v) study the biochemical
constituents and anatomical features of shoots and rootstocks used for grafting, layering and
rooted cuttings . A series of factorial experiments in randomized complete block design were
used for the field studies. The data were analyzed using Analysis of Variance (ANOVA),
regression and correlation analysis. Rooting of the air-layered shoots was better in both
number and length of the roots with the application of 10,000 ppm indolebutyric acid and
using Sphagmum moss as rooting medium than the other treatments. Ambient low
temperatures (22-24
o
C) had significant positive effect on rooting of air-layered shoots.
Weaning of propagules showed high survival for the rooted cuttings in the mist propagator
(93.3 %) comparable to that of the seedlings (100 %). Plant height and stem girth of the
rooted cuttings were however similar for all the propagating structures during the weaning
period of three months. In comparison with the normal (seedling) plantlet, the rooted cuttings
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in the mist propagator recorded better growth in terms of leaf production, plant height and
stem girth. There was a significant relationship between the canopy architecture of the
selected tree and the root production of the layered shoots expressed as Y
(rooting)
= 113.87 -23.697 X
(canopy spread); R
2
= 0.89; P < 0.002; n = 9. There was also a significant relationship
between field survival of propagules and the month of establishment expressed as Y (percent
survival) = -2844 + 0.070 X (month); p < 0.001; R
2
= 0.68; n = 90. For field establishment,
planting at a soil depth of 52 cm was suitable for transplanting weaned propagules as it
produced the highest survival, biggest girth and highest number of leaves. Biochemical
studies on stem portions showed the presence of nitrogen, protein, simple sugars, total free
phenols and auxins which played very significant role in vegetative propagation (air-layering,
cuttings and grafting) in producing high rooting and graft success. Anatomical studies
revealed that some growth hormones and medium promoted cell differentiation (vascular
tissues, xylem, cambium and phloem) in the shoots to enhance rooting. The best wounding
method on scions to yield significant graft success and enhanced growth was pre-curing
whilst rejuvenated shoot was preferred to young plants as root stocks. Grafting using the top
cleft technique with partially sprouted scions significantly gave the best graft success.
Coppicing trees in May/June produced the highest number of shoots within sixty days after
coppicing as well as the highest graft success. Stages of wound healing and graft union
formation were also revealed through the anatomy (differentiation of the xylem phloem and
cambial tissues) of the grafts which were indicative of compatibility or incompatibility.
Conclusions of the study indicate that the top cleft grafting technique results in the highest
graft success.
Description
A thesis submitted to the School of Research and Graduate Studies, Kwame Nkrumah University of Science and Technology in partial fulfilment of the of the requirement for the award of the degree of, Doctor Of Philosophy In Fruit Science, 2015