Studies of vegetative propagation of the lychee (Litchi chinensis ...

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Fig. 10. Transverse section through a graft union showing the bridging callus tissue (ct) advancing from the scion (s) and the rootstock (R) together with the inactive pith cells (p) at this stage (x 219). Fig. II. Transverse section through a 115-day-old graft union showing the developing cambial zone (cz) from the Kwai Mi scion (5) and the Hak Ip rootstock (R) (x 9). Fig. 12. Transverse section through the parenchymatous cells (n) of the cambial zone showing their detailed structure (x 139). Fig. 13. Transverse section through a 115-day-old sidegraft union showing the callus tissue (ct) developing from Kwai Mi scion (5) on Hak Ip rootstock (R) (x II). Fig. 14. Transverse section through a side-graft union of Brewster scion (5) on Kwai Mi rootstock (R) showing the extent of the developing callus tissue (ct) 115 days after grafting (x 12). Fig. 15. Transverse section through a 115-day-old sidegraft union of Brewster scion (5) on its own rootstock (R) showing the extent of the bridging callus tissue (ct) (x 217).
With the other cultivars, it was noticed that there was a new flush of leaves 4 weeks after the cuttings were set. Since a source of energy is required for this process, it is likely that the small carbohydrate reserves in the cuttings were used up for this initial growth, thus depleting the reserves which could have been drawn upon for root development. This carbohydrate depletion theory may explain the rooting failures in Kwai Mi and Brewster. Even though the percentage of successful rooting was low with either NAA-Na or IBA in both Kwai Mi and Brewster, nevertheless, IBA consistently induced a higher number and greater length of roots per cutting than NAA-Na. This suggests that IBA may be more suitable for rooting lychee cuttings than NAA-Na. Traub (1937) also found IBA particularly useful with L. chinensis and others like Psidium guajava. Perhaps at the same concentration, NAA-Na may be more toxic to lychee cuttings than IBA. Doran (1941) found in his experiments that naphthalene-acetic acid (NAA), at a given concentration, was more toxic to cuttings of most species than IBA or indoleacetic acid (lAA). Since callus formation was generally slow in the three cultivars, desiccation of the cuttings, especially within the polyethylene enclosure, may have contributed to rooting failures. The results of carbohydrate analyses on stems taken for rooting showed some correlation with rooting percentages obtained in Kwai Mi and Brewster. Better rooting was obtained with higher rather than lower starch content in the stems. Cultivar differences undoubtedly played a part in the success of rooting obtained; for even though Hak Ip showed a slightly higher starch content in the stems than Kwai Mi, there was no rooting under any environment. Here rooting response seems to be correlated with factors other than starch level only. The important role played by carbohydrates in the rooting of cuttings has been shown by Murneek (1941), Reid (1926) and Shippy (1930). These investigators generally found high correlation between carbohydrate levels in stem cuttings and rooting facility. Brandon (1939), however, failed to find the relationship between starch content and ease of rooting in species of the genus Rosa. The results of this study did not, on the whole, sustain 4' the finding that high carbohydrate level in stem cuttings is correlated with facility of rooting. Unpublished reports in the Department of Horticulture, University of Hawaii, have indicated that when treated with IAA and planted in an unheated propagation bed, only 10 or 20 % of lychee cuttings rooted, but if the propagation bed was maintained at 3Q-32°C, root formation was rapid and large numbers of roots were formed. The reports concluded that for rooting lychee cuttings, treatment with IAA and heated propagation beds were essential. Cooper & Knowlton (1939) obtained 100% rooting oflychee cuttings under 100 % humidity in propagating boxes. In the study reported here, the average daily relative humidity was 78 % and temperature 25°C within the polyethylene enclosure. The conditions in the enclosure were perhaps not ideal for rooting oflychee cuttings and the low percentage rooting obtained was perhaps due in part to the unsuitable conditions for the cuttings. From the results of the side-wedge grafting experiments, it seemed that Brewster may be more readily grafted on Kwai Mi than on Hak Ip. Since Brewster and K wai Mi are faster growers than Hak Ip, graft success was perhaps the result of more rapid development of the unifying callus from these cultivars compared with Hak Ip. The percentage graft take under normal day recorded using Brewster girdled scions, coincided at 9 weeks with the highest level of starch (Fig. 2) in the stems at this time. If graft take was solely dependent on the level of starch in the stems, then in Kwai Mi the graft failures may be explained by assuming that scionwood collection was ill-timed. The starch level in prepared scionwood was highest at 6 weeks and lowest at 9 weeks when the scionwood was taken for grafting. The starch level alone cannot explain the observed results in the Hak Ip. Failure may be more aptly explained on the basis of relatively low rate of growth of this cultivar, coupled with factors other than starch level. Various investigators have described the origin of callus tissue in the healing process of various tree species. Mendel (1936) and Juliano (1941) have all described callus tissue as originating from the parenchymatous cells from the medullary and phloem rays and the cortex. Similar observations were made by Artschwager (1941), Bradford &
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