RESPONSES OF YOUNG TEA (CAMELLIA-SINENSIS) CLONES TO DROUGHT AND TEMPERATURE .2. DRY-MATTER PRODUCTION AND PARTITIONING

The physiological basis for differences in yields from well-watered an d droughted plants of four contrasting clones of tea was studied in te rms of light interception, dry matter production and partitioning at a high altitude site in Southern Tanzania where there are marked season al variations in rainfall and temperature. The plant dry weights, incl uding roots, were measured eight months after held planting and subseq uently at intervals of three to four months, corresponding to the diff erent seasons, during the following two-years. Fully irrigated plants of one clone (S15/10) were also harvested after four years in the fiel d. Clones differed in the rates of canopy spread and hence in their ca pacity to intercept solar radiation. The 'radiation use efficiency' (t he net total dry matter production per unit of intercepted short-wave radiation) was similar for the four well-watered clones and ranged fro m 0.40 to 0.66 g MJ(-1), which corresponds closely to values reported for other woody tropical plants. A 16-week drought treatment imposed t wo years after planting reduced the mean light interception of the fou r clones by about 25% and the mean radiation use efficiency by 78% to 0.09 g MJ(-1). Clone S15/10, a cultivar from Kenya which produces larg e yields, partitioned a greater proportion of dry matter to leaves and harvested shoots than the other clones, and correspondingly less to l arge structural roots. This resulted in a maximum harvest index of 24% , substantially greater than other values reported in the literature. There were seasonal differences in partitioning, with more dry matter being diverted to roots and less to shoots during the cool season. Alt hough the drought treatments had no significant effect on root growth, the amount of dry matter partitioned to leaves, stems and harvested s hoots declined by 80-95%. The roots of all four clones extended in dep th at similar rates (about 2 mm d(-1)), those of Clone S15/10 reaching 2.8m after four years. The results are discussed in terms of appropri ate field cultural practices and possible selection criteria for high yielding clones.