Effect of irrigation rate on growth parameters, yield and tea quality of Cyclopia subternata

Abstract

Cyclopia, generally known as honeybush, and belonging to the Fabaceae family, originates from the Cape Floristic Region of the Eastern and Western Cape provinces of South Africa. Honeybush (Cyclopia spp.) plants are a rich source of antioxidant properties and phenolic compounds, and the extracts are utilized as beverage and other aesthetic products. Currently, six honeybush species are commercially cultivated, but to date, there are limited trials attempting to study their agronomic water demand. Two pot trials and a field study were conducted at three different sites where Cyclopia subternata plants were cultivated on different soil types (Stellenbosch granite, Stellenbosch shale and Stellenbosch clovelly) and subjected to three different water deficit stress levels (well-watered, semi-stressed and stressed). Remarkably, irrigation treatments and soil types did not significantly affect the growth of the plants. However, the well-watered treatment consistently had higher yields compared to the other two treatments. Proline which generally accumulates in plants that undergo different stresses (biotic and abiotic) was studied in all three experiments using the colorimetric method. The water-stressed (semi-stressed and stressed) treatments had higher proline concentrations with lower relative water contents (RWC), which signify water stress. Stomatal conductance was also investigated only in one pot study and field experiment and generally, lower in stressed plants and highest in well-watered plants. The drop in stomatal conductance in the stressed plants was due to the induction of stomatal closure which is a coping mechanism to aid survival by reducing transpiration rate. The development, growth and yields of the plant can be limited by water availability. Thus, this study also investigated the changes in molecular functions, cellular components, and biological processes of C. subternata exposed to different water stress conditions (T1, T2, T3, T13, T17, and T19). The proteins found in C. subternata leaves were differentially identified and quantified with quantitative mass spectrometry. A total of 11 proteins were differentially expressed proteins (DEPs) were identified using Fisher's Exact Test (p < 0.00100). Only α-glucan phosphorylase was found to be statistically common between T17 and T19 (p < 0.00100). In T19, 5 DEPs were upregulated and 6 were downregulated. Based on gene ontology, the DEPs in the stressed plant were associated with cellular and metabolic processes, response to stimulus, binding, catalytic activity, and cellular anatomical entity. Majority of these proteins were involved in photosynthesis, phenylpropanoid biosynthesis, thiamine, and purine metabolism. This study revealed the presence of trans-cinnamate 4-monooxygenase, an intermediate for the biosynthesis of large number of substances, such as phenylpropanoids, coumarins, and flavonoids. As much as consumers prefer healthy food, they are mostly not eager to compromise on taste and other sensory properties. Studies have proven that water stress has various effect on the sensory quality of different plant food products. In terms of tea quality, a descriptive sensory analysis was used for field grown C. subternata to compare the infusions prepared from three water deficit treatments. Severe water-deficit stress seems to boost the sensory profile of the infusions, in particular the ‘woody’, ‘fynbos-floral’, ‘rose perfume’, ‘fynbos-sweet’ and ‘sweet spice’ aromas as well as a sweet taste.