865 Latex Yield of Hevea brasiliensis Poorly Indicates Soil Drought S. Isarangkool Na Ayutthaya 1,a and F.C. Do 2 1 Horticultural Section, Department of Plant Science and Agricultural Resources, Faculty of Agriculture, Khon Kaen University, Thailand 2 IRD, UMR Eco&Sols, SupAgro-INRA, Montpellier F-34060, France Keywords: rubber tree, Hevea brasiliensis, yield, soil drought, indicator Abstract Predawn leaf water potential, sap flux density or tree transpiration and whole-tree hydraulic conductance are known indicators of soil drought on rubber tree (Hevea brasiliensis). However, these parameters are difficult to access for farmers, in contrast to latex yield which is recorded every day. Hence, the aim of this study was to evaluate the potential use of latex yield as indicator of soil drought in the marginal rainfall areas. The experiment was done during intermittent drought in the rainy season in a representative rubber plantation located in Buriram province, northeast Thailand. The results showed that while the relative transpiration declined by 66% for severe soil drought, the number of rubber sheets per hectare and per tapping decreased slightly by 18% and did not differ for mild drought. Other yield parameters per tapping such as fresh latex per tree or dry rubber content did not differ significantly despite tendencies between severe drought and well-watered period. Finally, the latex yield per tapping does not appear to be a sensitive indicator of soil drought. INTRODUCTION To benefit from the increasing demand for natural rubber, the cultivation of Hevea brasiliensis has been extended into drought-prone areas like those in the southern part of northeast Thailand. However, severe droughts have induced important branch diebacks and tree mortality (Isarangkool Na Ayutthaya et al., 2011) and in combination with latex tapping it can induce tapping panel dryness or trunk phloem necrosis syndromes (Pellegrin et al., 2007; Do et al., 2011). Hence, an indicator of water stress can be useful to suspend latex tapping when trees suffer a substantial drought. Leaf water potential, transpiration or sap flux density, tree hydraulic conductance, stomata or leaf conductance, photosynthesis, or trunk fluctuation are known indicators of soil drought (Orwig and Abrams, 1997; Chandrashekar et al., 1998; Goldhamer et al., 1999; Moriana et al., 2003; Intrigliolo and Castel, 2005; Isarangkool Na Ayutthaya et al., 2011). However, these parameters are difficult to access for farmers. In contrast to this, latex yield is recorded almost every day and previous results support its response to soil moisture status (Gururaja Rao et al., 1990; Raj et al., 2005). Hence, the idea of this study was to evaluate the potential use of latex yield as indicator of soil drought. The benefit of this work could help the farmers for decision of stop tapping when the soil drought occurs for tree protection. In northeast Thailand the tapping period lasts between 8 and 9 months including the rainy season and the first part of the dry season before defoliation. The system of tapping includes two days of tapping and one day of rest (2d/3). Hence there are 150-160 days of tapping during the stage of fully mature leaves. Isarangkool Na Ayutthaya et al. (2011) have assessed for the main clone used in the area (RRIM600) thresholds of relative extractable water (REW; dimensionless) corresponding to drought intensity: mild-drought between 0.5 and 0.2 REW and severe drought below 0.2 REW. Thus, the aim of this work was to quantify the effect of soil drought intensity in a constraining rainy season on latex yield as measured by the number of rubber sheets per hectare and tapping, and on latex yield parameters as fresh latex volume and dry rubber content. a isupat@kku.ac.th Proc. II International Symposium on Horticulture in Europe Eds.: J.-C. Mauget and S. Godet Acta Hortic. 1099, ISHS 2015