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Study of physiological parameters and antioxidants in response to harvest stress and tapping panel dryness in latex of Hevea brasiliensis (Willd. Ex. A. Juss.) Muell. Arg.
Dissertation Abstract:
The reduced thiols content (RSH) is one of the biochemical parameters of latex Diagnosis (LD) in rubber trees (Hevea brasiliensis (Will. ex A. Juss.) Muell. Arg.) 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 dissertation research aims to observe the variation of antioxidant content in latex of rubber clones, to study the evolution of RSH content in response to different stress of tapping, ethephon stimulation, and TPD occurrence, to understand the RSH regulation in different stress types and duration, to identify the better way to interpret RSH value in relation with harvesting stress and TPD occurrence, and to examine the ratio of reduced to oxidized form of antioxidants as the estimator of stress level in H. brasiliensis latex. This work is also the first attempt at genetic analysis to identify Quantitative Trait Locus (QTL) related to antioxidant traits. The project was started in February 2019. Following preparation and a series of laboratory training in 2019-2020, field experiments were carried out from January to July 2021 at Sembawa Research Centre, Indonesian Rubber Research Institute, Palembang, Indonesia. Four field trials were established to study: 1. the clonal variation of antioxidants in five recommended rubber clones, 2. the effect of tapping and ethephon stimulation on the yield, latex flow, and antioxidants parameters, 3. the effect of TPD occurrence, and 4. genetic analysis of main antioxidant parameters. 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 which were selected based on latex metabolism characteristics. The trees were tapped on the B0-1 with S/2 d3 ET2.5% 12/y tapping system. 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. Heritability and QTL analysis was performed on 103 PB 260 (TPD susceptible clone) x SP 217 (TPD tolerant clone) progenies. 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 observed antioxidants included reduced and oxidized forms of ascorbate (AsA and DHA) and glutathione (GSH and GSSG). Cofactors associated with antioxidants regeneration were assessed, including oxidized and reduced nicotinamide adenine dinucleotide (NAD, NADH), and nicotinamide adenine dinucleotide phosphate (NADP, NADPH). Heritability and QTL analysis was performed on RSH, AsA, DHA, total AsA, and the ratio of AsA/DHA. The results showed that the concentration in AsA is about ten times higher than GSH in laticifer, while GSH accounts for about 50% of RSH. The RSH and its related parameters (GSH and GSSG) varied among rubber clones, while AsA, DHA, and antioxidant cofactors (NADH, NAD, NADPH, and NADP) were not significantly different among observed clones. For short-term harvesting stress, RSH increases with tapping frequency and ethephon stimulation and then drops in long-term stress, usually accompanied by TPD occurrence which is associated with high latex viscosity and lutoid bursting. 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 a sign of prolonged stress related to lower metabolic activity and TPD occurrence. For better interpretation, reference of maximum RSH value under short-term high-stress and minimum value in long-term high-stress conditions are required as well as information related to harvesting system history, plant age, TPD percentage, and other LD parameters data (TSC, Suc, and Pi). The analysis of the reduced and oxidized forms of antioxidants could not give a clear direction of the oxidation, regeneration, and biosynthesis level of antioxidants. The high proportion of reduced antioxidant forms suggests that laticifers have a strong capacity to maintain reduction capacity. 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. Further investigation under higher and longer stresses, as well as larger samples, is recommended for a clearer effect of tapping, ethephon stimulation, and TPD on the antioxidants and cofactors evolution and QTL mapping.