Responses of young tea (Camellia sinensis) clones to drought and temperature .3. Shoot extension and development

The effects of temperature (14-19 degrees C) and mean soil water deficit (up to 90 mm) on the shoot extension and development rates of six contrasting tea clones in southern Tanzania were studied 10-28 months after field-planting. An exponential function with two constants, an initial shoot length and a relative extension rate (r), provided a reliable description of the length of axillary shoots for the period from the release of apical dominance to the unfurling of three true leaves. Apparent base temperatures (T-be) and thermal extension rates (rho(c)), derived from linear relations between r and the mean air temperature, were able to explain 85-92% of the seasonal variation in r for each fully irrigated clone. The mean soil water deficit was used to explain 80% of the remaining variation in r across three drought treatments. Clonal differences in the values of T-be (8.9-11.3 degrees C) and rho(c) (7.8-11.8 x 10(-3) mm mm(-1) (degrees Cd)(-1)) can be used to explain differences in the seasonal distributions of yield. The apparent base temperatures for development (T-bd) Were consistently 1.7-3.4 degrees C below those for extension; an observation supported by the effects of temperature on the length of shoot internodes. The results are compared with those obtained in other studies and are discussed in terms of clonal selection, the choice of harvest intervals and the prediction of green-leaf quality.