Physiological, biochemical and molecular responses of tea (Camellia sinensis L. O. kuntze) cultivars grown in Kenya to water deficit

Abstract

Tea is one of the most popular non-alcoholic beverages worlwide, and a leading foreign exchange earner and source of livelihood to over three million people in Kenya. Tea growing areas in Kenya often experience drought periods which cause accumulated soil water deficits. Tea plants respond to water deficit through poorly understood physiological, cellular/biochemical and molecular processes. Development of tea cultivars adapted to water-deficit stress greatly relies on an understanding of mechanisms of plant responses. The present study was therefore designed with the objective of determining the responses of tea plants to water deficit. In the study, 18-months old seedling tea plants from eight drought tolerant and susceptible cultivars were subjected to three levels of treatment consisting of high (34%), moderate (26%) and low (18%) soil moisture content in a rain-out shelter. The experiment was designed in a complete randomized design with three replications. After three months of exposure to treatment, physiological parameters (leaf water status, shoot growth and gas exchange parameters), biochemical parameters (leaf proline and glycinebetaine levels) were determined. The data generated were subjected to two-way analysis of variance using GENSTAT. Molecular responses were analysed on total leaf RNA extracted from tolerant and susceptible tea cultivars under water stressed and unstressed conditions. mRNA was extracted from the total RNA and reverse transcribed to complementary DNA. The sequences/reads generated from the cDNA libraries using a 454 GLX sequencer were analysed in silico using bioinformatic tools. The results showed that there was significant reduction (P<0.05) in shoot growth, leaf relative water content, shoot water potential and gas exchange parameters with decrease in soil water. Proline was accumulated by droughted tea plants. A total of 232,853 Raw reads generated from the sequencer, assembled into 460 long transcripts (contigs). The contigs showed similarity to proteins in the Arabidopsis proteome following annotation by BLAST. Based on Gene ontology analysis, drought response related transcripts including Heat shock proteins, Hsp70, antioxidant molecules such as Superoxide dismutase, catalase and peroxidase, signal tranducers, Calmoduline like protein, and Galactinol synthase (Gols4) were induced in the water stressed plants. Results from this study therefore confirmed that water stress response in tea is controlled at the genetic level and there is potential to manipulate the genetics of tea to develop drought tolerant cultivars.