Domestication of the shea (vitellaria paradoxa c.f. gaertn) tree: developing improved propagation techniques for accelerated plant growth.

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 vii 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.

Source: KNUST