Optimization of fertilizer application rate for jatropha zeyheri tea quality under greenhouse, microplot and field conditions

Abstract

Jatropha zeyheri is an indigenous crop to South Africa, which contains tea brewing and medicinal properties commonly utilized by local communities in rural areas of South Africa. Domestication and commercialization of indigenous herbal teas have gained popularity in recent years however, domestication of J. zeyheri will require optimum nitrogen, phosphorus and potassium (NPK) fertilizer mixture to improve yield and quality. Tea fertilization is among the key factors influencing tea’s physiological, physical and chemical qualities. However, the influence of NPK fertilizer on J. zeyheri tea has not been documented. The objectives of the study were to assess the effect of different NPK fertilizer application rates on (1) growth and yield, (2) phytochemical composition and antioxidants activity, (3) mineral element composition and (4) microbial composition of J. zeyheri tea under greenhouse, microplot and field conditions. In achieving Objective 1, J.zeyheri seeds were collected in the wild at Khureng village, Lepelle-Nkumpi Municipality, Limpopo Province, South Africa. At two-leaf phase, J. zeyheri seedlings were transplanted into 25 cm diameter plastic pots. Each pot was filled with heated-pasteurised sandy soil and Hygromix at 3:1 (v/v) ratio and placed in a spacing of 0.30 m × 0.30 m inter-and intra-row spacing. Each experiment comprised six treatments of NPK fertilizer 2:3:2 (22) rates (0, 2, 4, 8, 16 and 32 g per plant) translating to (0, 20, 40, 80, 160 and 320 kg NPK mixture/ha, unit area) arranged in a RCBD, with 5 replications. NPK fertilizer rates had effects on chlorophyll content, leaf width and dry root mass, vine length, stem diameter and leaf length. However, treatments had no effects on NDVI, number of leaves and dry shoot mass under greenhouse conditions. Under microplot conditions, NPK fertilizer rates significantly affected on leaf width, vine length, stem diameter, leaf xxxvii length and dry root mass. In contrast, NPK fertilizer effects were not significant on chlorophyll content, NDVI, dry shoot mass, number of fruit, leaf area index. Under field conditions, NPK fertilizer rates had effects on stem diameter and leaf length and leaf width. NPK fertilizer rates did not have effect on vine length, chlorophyll content, number of leaves and dry shoot mass. NPK fertilizer requirements for J. zeyheri were optimized at 3.34, 3.30 and 4.21 g fertilizer/plant under greenhouse, microplot and field conditions, which translates to approximately 33, 33 and 42 kg NPK fertilizer mixture/ha for 10 000 plants of J. zeyheri plants. In achieving Objective 2, leaves from greenhouse, microplot and field experiments were harvested and oven-dried for 72 hours at a temperature of 60°C. After laboratory preparations, phytochemicals and antioxidant activity were analysed using the UV-visible spectrophotometer. Under greenhouse, treatments had effect on antioxidant activity (AA), total flavonoids content (TFC), total phenols content (TPC) and total tannins content (TTC). Under microplot conditions, NPK fertilizer rates had effects on TTC and TFC. Antioxidant activity and TPC were not affected by NPK fertilizer rates. Under field conditions, NPK fertilizer rates had effect on TPC, whereas AA, TTC and TFC were not affected by NPK fertilizer rates. Antioxidant activity, TTC, TFC and TPC were not affected under field conditions, Fertilizer requirements for phytochemicals and antioxidant activity were optimized at 3.97 g and 4.64 g fertilizer/plant under greenhouse and microplot conditions, which translates to approximately 40 kg and 46 kg NPK fertilizer/ha for 10 000 plants of J. zeyheri, respectively. However, under field conditions phytoconstituents and antioxidant activity were not optimized due to the insignificance effect of NPK fertilizer rate. In achieving Objective 3, samples were prepared using a microwave digestion system, both essential minerals (Fe, P, K, Mg, Ca, xxxviii Cu, Mn, and Zn) and non-essential mineral elements (Al, Co, Cr, Cd, Na, Pb, Ni and As) were determined using an ICPE-9000 Spectrometer. Under greenhouse conditions, NPK fertilizer rates had effects on essential mineral elements Ca, Fe, K, Mg, Zn and Mn. However, Cu and P were not affected. NPK fertilizer effects were observed on non-essential mineral elements, Al, Na and Cd. However, NPK fertilizer rates did not affect Co, Cr, As, Pb and Ni in J. zeyheri leaf tissues. Under microplot conditions, NPK fertilizer rates had effects on essential mineral element Mn, Ca, Cu, K and Fe. NPK fertilizer rates did not have significant effects on Mg, P and Zn. NPK fertilizer rates had significant effects on non-essential mineral elements Al, Cd, Co, Na and Ni. NPK fertilizer rates did not have significant effects on As and Cr. Under field conditions, NPK fertilizer rates had effects on essential mineral element P, Ca, Fe and Mn. NPK fertilizer rates did not have effects on K, Mg, Cu and Zn. NPK fertilizer rates had effects on non-essential mineral elements Al, Na, Cd, Co, Cr, As and Ni. NPK fertilizer rates did not have significant effects on Zn. Fertilizer requirements for J. zeyheri tea leaf tissues were optimized at 2.77, 2.51 and 4.10 g for essential mineral elements under greenhouse, microplot and field conditions, which translates to 28, 25 and 41 kg NPK fertilizer mixture/ha, respectively. Non-essential mineral elements were optimized at 2.48, 2.10 and 2.71 g, which translates to approximately 25, 21 and 27 kg NPK fertilizer mixture/ha under greenhouse, microplot and field conditions, respectively. In achieving Objective 4, microorganisms on tea leaves were determined using Tempo (BioMerieux, Rodolphe, Durham, United States) instrument. Data were subjected to ANOVA using the statistix 10.0 software. Under greenhouse, increasing NPK fertilizer rates had effects on Bacillus cereus and Enterobacteriaceae. Increasing rates of NPK fertilizer did not influence total coliforms xxxix contained in J. zeyheri tea leaf tissues. The optimum NPK fertilizer application rate under greenhouse conditions was optimized at 1.75 g, which translates to approximately 18 kg NPK fertilizer mixture/ha. Under microplot, increasing NPK fertilizer rates had effects only on Enterobacteriaceae, while Bacillus cereus and total coliforms were not affected. Under field conditions, increasing rates of NPK fertilizer did not influence Bacillus cereus, total coliforms and Enterobacteriaceae in J. zeyheri tea leaf tissues. In conclusion, in terms of growth and yield, NPK fertilizer increased growth and yield at low rates ranging from 3.3 and 42 g which translate to 33 to 42 kg NPK fertilizer mixture/ha. Phytochemical composition and antioxidant activity were stimulated when NPK fertilizer was applied from 3.97 and 4.6 g which translate to a range of 40 to 46 kg NPK fertilizer mixture/ha. Accumulation of essential and non-essential mineral elements improved when NPK fertilizer rates were applied at 2.5 and 4.1 g which translate to a range of 25 to 41 and 2.1 and 2.7 g which translate to a range from 21 to 27 kg NPK fertilizer mixture/ha. Microbial composition on J. zeyheri leaves were improved at 1.78 g NPK fertilizer which can be translated to 18 NPK fertilizer rates under greenhouse conditions. In reference to the current study, J. zeyheri appears to qualify as a low-input tea crop as observed by stimulation at lower NPK fertilizer rates (18 to 50 kg NPK fertilizer mixture/ha).