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Latex diagnosis (LD) is applied to optimize the natural rubber production and prevent tapping panel dryness (TPD), a physiological syndrome affecting latex production in Hevea brasiliensis. The reduced thiol content (RSH) is one of the biochemical parameters associated with the risk of TPD. However, RSH is difficult to interpret because of the influence of the environment. In order to better understand the regulation of antioxidants and to better interpret RSH, a key parameter of LD, this study analysed in latex both oxidised and reduced forms of ascorbic acid (AsA) and glutathione, and their cofactors as well as other latex diagnosis parameters in response to harvesting stress (tapping and ethephon stimulation) and TPD occurrence. The content of antioxidants in latex had a high variability among five rubber clones. The concentration in AsA was about ten times higher than GSH in laticifer, GSH accounting for about 50% of RSH. For short-term harvesting stress, RSH increased with tapping frequency and ethephon stimulation. TPD is associated with high latex viscosity and bursting of lysosomal particles called lutoids, as well as for several rubber clones with 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. RSH remains an essential physiological parameter to prevent TPD when associated with reference data under low and high harvesting stress. This study paves the way to understand the role of AsA and GSH, and carry out genetic studies of antioxidants.

It is predicted that drought will be more frequent and sustained in the future, which may affect the decline of rubber tree production. Therefore, it is critical to research some of the variables related to the drought-resistance mechanism of the rubber tree. As a result, it can be used to guide the selection of new rubber drought-resistance clones. The goal of this study was to identify drought-resistance mechanisms in rubber clones from the high drought factor index (DFI) group using ecophysiological and biochemical variables. The treatments consist of two factors, namely water deficit and contrasting clones based on the DFI variable. The first factor consisted of three levels, namely normal (fraction of transpirable soil water (FTSW) > 0.75), severe water deficit (0.1 < FTSW < 0.20), and recovery condition (FTSW > 0.75 after rewatering). The second factor consisted of seven clones, namely clones G239, GT1 (low DFI), G127, SP 217, PB 260 (moderate DFI), as well as G206 and RRIM 600 (high DFI). RRIM 600 had the highest DFI among the other clones as a drought-tolerance mechanism characteristic. Furthermore, clones RRIM 600, GT1, and G127 had lower stomatal conductance and transpiration rate than drought-sensitive clone PB 260. As a result, as drought avoidance mechanisms, clones RRIM 600, GT1, and G127 consume less water than clone PB 260. These findings indicated that clone RRIM 600 was a drought-resistant clone with drought tolerance and avoidance mechanisms.

Prolonged drought conditions pose a serious threat to plant growth and productivity due to water limitation. To survive, plants adapt by altering various biochemical pathways leading to physiological changes appropriate for growth under waterlimited condition. One chemical known to be induced during this condition is ethylene, a plant hormone that affects plant vegetative and reproductive behaviours. Ethylene is synthesized by a two-step biosynthetic pathway mediated by the enzyme 1-aminocyclopropane-1- carboxylic acid synthase (ACS) and 1-aminocyclopropane- 1-carboxylic acid oxidase (ACO). In plants, both genes are members of separate gene families. Recent data have shown that members of the ACO and ACS gene families were affected by limited water condition in Arabidopsis thaliana. The involvement of the other members of the ACC and ACO gene families is unknown. Therefore, the objectives of this study were to determine the ACS and ACO genes responding to water-limited stress condition in Arabidopsis plants and to functionally analyse the role of an inducible ACO gene in Arabidopsis plant during development and waterlimited stress. The over-expression construct was generated by using the Gateway technology and introduced into Arabidopsis by the Agrobacterium-mediated floral dip method. From nine AtACS genes, only AtACS2 and AtACS6 responded to the PEG-induced water stress. AtACS2 was induced in roots whereas AtACS6 was downregulated in leaves and roots. All six members of the AtACO gene family responded to PEG-induced water stress with four genes (AtACO3, AtACO4, AtACO5 and AtACO6) were switched-off in roots whereas three genes (AtACO1, AtACO3 and AtACO6) were induced in the leaves. Among the inducible AtACO genes, AtACO1 was unique because this gene was not expressed in roots but induced in leaves. Overexpressing AtACO1 in Arabidopsis plant changed the overall vegetative growth of the plant, particularly the root system where the root became shorter and lesser. Int