Abstract
The reduced thiols content (RSH) is one of the biochemical parameters of latex Diagnosis (LD) associated with the risk of Tapping Panel Dryness (TPD), a physiological syndrome induced by over-accumulation of Reactive Oxygen Species (ROS). However, RSH is difficult to interpret due to the influence of the environment. The Ph.D. thesis aims to study the regulation of antioxidants in response to harvesting stress and TPD occurrence. This work is also the first attempt at genetic analysis to identify QTL related to antioxidant traits. The research was carried out from January to July 2021 at Sembawa Research Centre, Indonesian Rubber Research Institute, Palembang, Indonesia. Three field trials were established to study the clonal variation and the evolution of yield, latex flow, physiological parameters, and antioxidants content. Clonal variation was performed on ten-year-old trees of five recommended rubber clones i.e. IRR 118, BPM 24, IRR 112, RRIC 100, and PB 260 tapped on the B0-1 (S/2 d3 ET2.5% 12/y). The effect of harvesting systems was studied on clones IRR 118 and BPM 24 using four harvesting systems (S/2 d6, S/2 d3, S/2 d1, and S/2 d3 ET2.5% 12/y). Data were collected four months after the application of treatments. The effect of TPD was examined on five recommended clones i.e. IRR 118, BPM 24, IRR 112, RRIC 100, and PB 260 tapped with S/2 d3 ET 2.5% 12/y on the B0-1 panel. Observed parameters associated with latex production included latex yield, initial flow (IF), plugging index (PI), and bursting index (BI). For LD, total solid content (TSC), sucrose content (Suc), inorganic phosphorus content (Pi), and RSH were determined. The antioxidants content observed included reduced and oxidized forms of ascorbate (AsA and DHA) and glutathione (GSH and GSSG). Cofactors associated with antioxidants regeneration were assessed including oxidised and reduced nicotinamide adenine dinucleotide (NAD, NADH), and nicotinamide adenine dinucleotide phosphate (NADP, NADPH). QTL analysis on RSH, AsA, DHA, total AsA, and the ratio of AsA/DHA was quantified from 103 of PB 260 (TPD susceptible clone) x SP 217 (TPD tolerant clone) progenies. The result showed that the concentration in AsA is about ten times higher than GSH in laticifer, while GSH accounts for about 50% of RSH. For short-term harvesting stress, RSH increases with tapping frequency and ethephon stimulation. TPD is associated with high latex viscosity and lutoid bursting as well as lower RSH and GSH contents. These results suggest that a high level of RSH shows the capacity of laticifer metabolism to cope with harvesting stress, while a drop in RSH is the sign of long stress related to lower metabolic activity and TPD occurrence. The highest heritability was RSH 0.69, while other traits were lower than 0.2. The QTL could not be detected for RSH, AsA, DHA, total AsA, and the ratio of AsA/DHA. To conclude, these results showed that RSH is affected by genotype, type and duration of stress, and environmental factors leading to difficulty for interpretation. RSH remains an essential physiological parameter to prevent TPD, yet its interpretation requires reference data under low and high harvesting stress. Further investigation under higher and longer stresses, as well as larger samples, is recommended for a clearer effect of tapping, stimulation, and TPD on the antioxidants and cofactors evolution and QTL mapping.