Responses of composite tea plants to drought and irrigation in the Southern Highlands of Tanzania.

The responses of composite tea plants to drought and irrigation were studied in a line-source experiment in the Southern Highlands of Tanzania. Two scion clones (SI5/10 and K35) were grafted onto each of three rootstock clones (6/8, SFS150 and PC81). The scions were also grafted onto each other (i.e. SI5/10 on K35 and vice versa), and cuttings of ungrafted clones SI5/10 and K35 were prepared at the same times as the controls. In February 1996, the plants were field planted at a spacing of 1.2 x 0.8 m. Six differential irrigation treatments ranging from rainfed conditions (no irrigation) to full irrigation were imposed during the dry seasons, though from planting up to the end of August 1998, the experiment was uniformly irrigated to ensure successful crop establishment. The experiment was irrigated whenever the potential soil water deficit in the fully irrigated plots reached 40 - 50 mm. Between December 1996 and May 1997 shoots above 0.5 m in length were removed or “tipped” to produce a level plucking surface. Routine harvesting started on 16 June 1997, and continued at intervals of 13 to 17 days during the warm season (September to May) and 22 to 29 days in the cool season (June to August). The choice of the scion influenced both the response to water stress and the yield response to drought of the composite plants. Similarly, the soil water availability appeared to modify the effects of rootstock on the yield of the scion. Although not reaching statistical significance, the use of composite plants led to an overall mean annual yield increase of 9% (range between combinations 4 to 21%) in the unirrigated plots, but less than 1% (range -6 to 6%) in the well watered plots when clone S I5/10 was used as a scion. By contrast, using clone K35 as a scion, the overall mean annual yield increase whether irrigated or unirrigated was less than 3% (range -12 to 11%). In the driest plots, rootstocks 6/8 and SFS150 consistently increased the yields of scion clones S I5/10 (by 21%) and K35 (by 11%) respectively. During the dry season, some composite plants survived the drought better than the ungrafted plants. Assessing the effect of rootstocks on the composition of harvested shoots (i.e. shoots of different sizes/stages and the proportion of the total weight that were composed of coarse and broken leaf) revealed that none of the rootstocks had a significant effect on the composition of harvested shoots. Likewise, the rootstocks did not affect the bush morphology as assessed by ground cover development, shoot population density, stem diameter and number of branches. The use of composite plants in tea production by both smallholders and commercial companies is technically feasible, but the benefits to be realised depend on the rootstock/scion combinations used. The extra cost of producing plants by grafting when large-scale tea growers opt to use composites rather than the conventional plants was estimated to be only US $ 100 ha⁻¹. Due to low labour costs, the corresponding cost for the smallholders was US $ 47 ha⁻¹. The payback period when composite plants were used in tea production was the same as that of the conventional plants, but this period was shorter for smallholders than the commercial companies. The payback period was in year 1 (i.e. the first season of harvesting) for the small-scale growers compared with year 2 or 3 (depending on the prices of made tea) for the commercial companies. Assessing the probability of getting a yield increase necessary to make the use of composites worthwhile revealed that for some combinations, there were higher chances of getting such yield increase. For example, the probability of getting a yield increase of 200 kg of made tea ha⁻¹ in one year was 81% when scion clone SI5/10 was grafted onto rootstock 6/8. The corresponding probability was 74% when clone K35 was grafted onto rootstock SFS150. These results and their implications for the tea industry are discussed. The way forward commercially as well as the future research to increase our knowledge for efficient use of composite plants in tea production is proposed.