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• Rooibos plants had 50% less biomass when grown at low moisture. • Plants avoided drought stress through the production of long roots. • Rooibos exhibited non-stomatal limitations to photosynthesis during drought. • Longer roots and high root/shoot ratios allowed for higher RWC well into drought. • Production of polyphenols allowed the plants a degree of protection. In recent decades climate change has impacted ecosystems negatively globally with Mediterranean systems being particularly vulnerable. Changes in rainfall pattern and amounts have led to increases in flooding and droughts in certain areas of the world. Many important agricultural crops such as Aspalathus linearis (Burm.f.) R. Dahlgren, rooibos tea, rely on the Mediterranean-type seasonality in the Western Cape, South Africa. Aspalathus linearis plants, which occur naturally in the Cedarberg, grow in hot and dry summer conditions and have a root that penetrates the soil down to 2 m, yet no work has looked at the root response to drought under low rainfall environments. The objective of this study was to determine the physiological and morphological response of A. linearis plants grown at two different water regimes to drought. A potted glasshouse experiment was carried out where drought stress (withdrawing watering) was imposed on well-watered and low moisture supplied plants. The low moisture droughted plants exhibited drought tolerant mechanisms which included higher root/shoot ratios as well as thinner roots, both of which are effective for water and nutrient uptake. Overall, plants in the 30% FC treatment recorded lower P max , g s and E after three days in the drought conditions while 70% FC plants were only affected after five days. Furthermore, plants grown under low moisture conditions produced 50% lower biomass compared to plants grown under adequate moisture conditions. This implies that low rainfall and the occurrences of dry spells and drought, associated with climate change are likely to reduce the production of A. linearis in the Cederberg area. [ABSTRACT FROM AUTHOR]

• A. linearis plants lower their leaf temperatures through transpirational cooling • Low chlorophyll and high phenol could be acclimatized adaptive changes to heat • Soil quality and increasing temperatures negatively affect biomass of A. linearis • Younger plants invested more resources to ensure their protection and survival Mediterranean systems, such as in South Africa, are particularly vulnerable to the predicted increases in mean surface temperatures which will likely affect the growth and physiology of many plants and subsequently affect agricultural productivity. Aspalathus linearis (Burm.f.) R. Dahlgren (rooibos) is an important commercial, endemic crop that is produced in the Cederberg region, South Africa. This region experiences a Mediterranean-type ecosystem that is characterised by hot, dry summers (November – February) and cool, wet winters (May – August). This study assessed the effects of temperature on the growth and stress tolerant traits of A. linearis over two years. Crop biomass and selected physiological traits were determined in the field, along a temperature gradient, at four farm sites in the Cederberg during the summer and winter 2017 and summer 2018. Aspalathus linearis plants showed evidence of transpirational leaf cooling during summer and this, combined with lower chlorophyll and high phenolic content, could be considered acclimatized adaptive changes allowing the plants to mitigate the heating effects of elevated temperatures. Although changes in soil nutrients and increasing temperatures had a negative impact on crop biomass, the phenolic content, a measure of tea quality, did not vary with sites. This suggests that a shift in rooibos farming to cooler and wetter areas further south, for better growth and higher yields, would not compromise the quality of rooibos tea. [ABSTRACT FROM AUTHOR]