Sustainable rubber production through good latex harvesting practices: An update on mature rubber fertilization effects on latex cell biochemistry and rubber yield potential

From 2008 to 2012, under a joint funding by the Institut Français du Caoutchouc (IFC) and its affiliated societies (Michelin, SIPH and Socfinco), a network of large scale experiments has been set up and followed up in 5 agro-industrial plantations to study the influence of a gradient of fertilization of mature rubber trees on latex physiological parameters and rubber yield potential. Experiments were set up on clone PB260 (Socfindo Tanjung Maria and Aek Pamienke plantation in North Sumatra, Indonesia, SAPH-Toupah and SOGB-Grand Bereby plantations in Côte d'Ivoire) and on clone IRCA109 (RENL-Osse River plantation in Nigeria). During the first 3 years (2008/09 to 2010/11), the trees were tapped downward in S/2 d4. During the 4th year (2011/12), the trees were tapped upward in S/4 d4. Each experiment has been designed with a similar protocol: Split plot design with stimulation intensity as the main treatment and fertilizer rate as secondary treatment, with 4 replications. The 3 stimulation intensities (main plots) were INT1 (normal stimulation intensity adapted to the clone, according to Cirad stimulation recommendation grid), INT2 (intensified stimulation: INT1 + 2 stimulations/y) and INT3 (intensified stimulation: INT1 + 4 stimulations/y). The 4 fertilization rates (secondary plots) were IF0 (No fertilization control), IF1 (single dose as the standard Cirad recommendation : 50g N, 40 g P2O5, 90 g K2O per tree per year, applied in a single application 45 to 60 days after complete refoliation), IF2 (double dose : 100 g N, 80 g P2O5, 180 g K2O per tree per year) and IF3 (triple dose: 150 g N, 120 g P2O5, 270 g K2O per tree per year). The 12 main plots (4 replications x 3 stimulation intensities) were fitted to 12 tapping tasks selected for their homogeneity (girth and production) before trial implementation. After 4 years, maximum cumulative production (kg/ha) was always observed with a fertilized treatment (IF2 or IF3) combined with intensified stimulation (INT2 or INT3). Without fertilization (IF0), intensification of stimulation (INT2 or INT3) resulted in a decreased production (overexploitation) or in a reduced response to stimulation. However, these effects of combined intensification and fertilization were often below +10% when compared to the production of the unfertilized control (INT1 x IF0) and therefore hardly statistically significant at a 5% risk. Regarding the latex physiological parameters, the average latex sucrose level was significantly increased by fertilization when compared to the absolute control, even though the increased production, showing a positive physiological change in latex sugar loading or in latex metabolism. Pi and RSH latex contents were also generally increased but less significantly. This indicates that an accurate NPK fertilization might sustain a good latex physiological profile in case of intensified ethephon stimulation and would help to prevent any risk of overexploitation, likely mainly because of an improved sugar translocation towards the latex sink, due to K+ likely. This opens the way to specific recommendations of clonal stimulation recommendations depending on fertilization practices. (Résumé d'auteur