Morphological and physiological indicators of tolerance to atmospheric stress in two sensitive and two tolerant tea clones in South Africa

Tea (Camellia sinensis) clones (PC113 and SFS204) sensitive to very dry air and clones (PC114 and SFS150) that are, tolerant, were studied at two tea estates (Tshivhase and Grenshoek) in the Northern Province of the Republic of South Africa. Among the morphological leaf traits studied, stomatal density, pore diameter and pore depth were not linked consistently to stress tolerance. Cuticle thickness was not a good indicator of stress tolerance because genetic differences between clones were confounded by the clonal response of wax production to stress. In contrast, measured leaf conductance to water vapour transport was larger and leaf water potential was lower in sensitive clones, but only with more severe atmospheric stress (Grenshock). Also the ratio of the calculated maximum stomatal conductance in old and young leaves was higher in sensitive clones, suggesting that the loss of a larger fraction of the total stem flow by old leaves enhanced the stress experienced by the young leaves. However, this indicator was valid only under the more stressful microclimate of Grenshoek. The results indicate that even promising criteria for stress tolerance should be tested by exposure to stress during selection.