The influence of soil water content and nitrogen supply on growth, yield and polyphenol content of selected tea [Camellia sinensis (l.) O. Kuntze] clones in Kenya

Abstract

Tea is a major foreign exchange earner in Kenya and it accounts for about 24% of the total value of the domestic exports for the last 10 years. The sector employs 10% of Kenya’s population directly and indirectly, and contributes to infrastructural development which includes construction of schools and health facilities, rural access roads and rural industries. However, tea suffers frequent droughts with significant yield decline, often accompanied by plant deaths, a condition which seems to be worsened by fertilizer input. The objectives of this work were to determine: i) the critical minimum soil water requirements in selected tea clones and show how it varies with nitrogen (N) supply ii) the effect of N supply and progressive decline of soil water content on tea shoot growth, leaf yield and black tea quality iii) the levels of tea polyphenols and define their association with water stress in tea and their suitability as indicators for drought tolerance. Two experiments were set up in a rain-out shelter at Tea Research Foundation of Kenya (TRFK) and a field experiment conducted at three different field sites in tea growing areas. The first experiment consisted of six different tea clones which were subjected to 4 levels of soil water content (SWC) (38, 30, 22 and 14% v/v) for 12 weeks. The second experiment consisted of clone BBK 35 treated with 5 different rates of N (0, 75, 150, 225 and 300 kg/ha) using NPKS (25-5-5-5) fertilizer material, and 5 levels of SWC (38, 34, 30, 26, 22, 18% v/v). Shoot and leaf growth, dry matter partitioning, photosynthesis, stomatal conductance, transpiration, leaf anatomy and shoot polyphenol content were determined. The field experiment consisted of clone BBK 35 with five rates of N, similar to the rain-out shelter and where shoot population density and yield were determined during dry months. SWC limit for tea was shown to be about 20% v/v and that fertilizer supply increased the susceptibility of tea to drought. Contrary to earlier speculations that fertilizer raises the optimal SWC for tea, it emerged that fertilizer rates above 200 kg N ha-1 influenced assimilate partitioning to shoot and foliage, making tea more vulnerable to drought effect. Clones with high total polyphenol content and whose levels varied less with changes in SWC were more tolerant to drought. Two catechin variants, epicatechin and epigallocatechin correlated with SWC, water stress index and shoot growth in tea, which suggests they are potential indicators for drought stress in tea.