Physiological parameters linked to stress for optimisation of hevea latex production

Bark tapping of Hevea brasiliensis enables to collect latex containing natural rubber. It is common practice to increase latex production by applying an ethylene generator (Ethrel) on a bark cut. Ehtylene stimulates both flow duration and latex regeneration between two tappings. However, excessive stimulation results in stress phenomena caused by active oxygen species (AOS). To avoid damage to rubber plantations and production losses, it is important to study the physiological parameters linked to the stress caused by stimulation frequency (0 to 78 per year). This study was conducted in Ivory Coast, based on 8 stimulation frequencies and 10 replications one. The concentrations of two types of antioxydants able to destroy AOS, namely, thiols and ascorbic acid, were measured in fresh latex and analyzed in relation with concentrations of malondialdehyde, reflecting lipid peroxidations. Rates of latex production were also analyzed in relations to stimulation frequency. Latex production increased until 13 annual stimulations then decreased, probably due to cellular dysfunctioning. Antioxidant concentrations increased until the level of 8 annual stimulations and then decreased for higher stimulation frequencies, probably indicating metabolic disorders related to a breakdown of biochemical defenses. Malondialdehyde concentration by contrast showed a minimum at 8 annual stimulations and increased at higher stimulation frequencies. It was therefore negatively correlated with the antioxidants, confirming that this substance is an indicator of effective oxidative dammage. In conclusion, without stimulation, the metabolism was relaxed with a low synthesis of protectors and thus, some lipidic peroxidation. Moderate activation of the metabolism with Ethrel increased production and protector synthesis and consequently reduced peroxidation. Excessive stimulation, however, decreased production, caused functional disorder and weakened system defenses against lipidic peroxidation. These finding might allow optimizing annual stimulation intensity while taking into account the intrinsic metabolic activity of the trees and their tolerance to stimulation, in order to avoid over-exploitation leading to a decrease in latex production. (Texte intégral