## 1 Introduction  
Terpenoids (terpenes and oxygenated terpenes) are synthesized by condensation of the five carbon precursor isopentenyl diphosphate (IPP) or its isomer dimethylallyl diphosphate (DMAPP) [1]. IPP is synthesized through the mevalonate pathway occurring in the cytosol as well as through the non-mevalonate pathway in chloroplasts [2, 3]. The sequential head to tail addition of IPP and DMAPP leads to the formation of geranyl diphosphate (GPP), farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP), which are the precursors of monoterpenes, sesquiterpenenes and diterpenes, respectively [3]. Monoterpenes and sesquiterpenes are volatile compounds which are involved in plant-plant, plant-insect and plant-animal interactions [1, 4]. The precursors GPP and FPP are the substrate of hundreds of monoterpene and sesquiterpene synthases (TS) that are differentially expressed in diverse plant species and tissues [5, 6]. In general, a mono- or sesquiterpene synthase synthesizes a single mono- or sesquiterpene, but enzymes synthesizing more than one mono- or sesquiterpenes have also been described [7]. The simplest mono-, sesqui- and diterpene skeleton can be further modified by enzymes such as hydroxylases, dehydrogenases, reductases or glycosyl-, methyl- and acyl-transferases leading to the synthesis of many thousands of different terpenoids [1]. The principal families of monoterpenes are derived from limonene and pinene, and those of sesquiterpenes are derived from aromadendrene,adinene, germacrene and caryophyllene, among others (Figure 1). _Eucalyptus_ species belong to the family Myrtaceae which originated in Australia where more than 600 species have been identified [8]. _Eucalyptus globulus_ trees display rapid growth and they have been introduced in many countries with temperate weather as a source for cellulose extraction, building material and to obtain essential oils from the leaves for pharmaceutical uses. Leaves of _E. globulus_ trees contain essential oils constituted by terpenoids, mainly by the monoterpenes eucalypt (72.7%), \(\alpha\)-pinene (9.2%), \(\alpha\)-terpineol (2.5%) and the sesquiterpene aromadendrene (2.5%) having antioxidant, anti-inflammatory and antimicrobial properties [9, 10, 11]. In this sense, it has been shown that eucalypt has antiviral, antibacterial and antifungal activities _in vitro_[12, 13, 14]. In addition, eucalypt and aromadendrene have synergistic effects in regard to antioxidant and antimicrobial properties [14]. In addition, eucalypt has larvicidal effects against larvae of the mosquito _Aedes aegypti_[15], \(\alpha\)-pinene has a repellent effect against pine bark beetle [16] and \(\alpha\)-terpinene has larvicidal  
Figure 1: Precursors of mono- and sesquiterpenes. effect against _A. aegypti_ and _A. albopictus_ larvae [17]. Thus, the most abundant mono- and sesquiterpens in leaves of _Eucalyptus_ trees have antimicrobial and larvicidal effects which may protect trees against microbes and insects. Oligo-carrageenanan (OC) kappa is prepared by acid hydrolysis of pure kappa carrageenanan obtained from marine red algae. OC kappa is constituted by 20 units of galactose linked by alternate \(\beta\)-1,4- and \(\alpha\)-1,3-glycoside bonds with sulfate groups located in positions 2, 4 and 6 of the galactose ring with anhydrogalactose units (for a model see [18]). In a previous work, we determined that OC kappa increased the growth of _E. globulus_ trees cultivated for three years in the field without additional treatment, as well as the amount of total essential oils and polyphenolic compounds [19]. Recently, we showed that OC kappa induced an increase in the level of the reducing compounds NADPH, ascorbate (ASC) and glutathione (GSH) as well as in thioredoxin reductase (TRR) and thioredoxin (TRX) activities [20]. The increase in reducing compounds changed the redox status to a more reducing condition, the increase in NADPH activate TRR/TRX activities which, in turn, activate photosynthesis, C, N and S assimilation, basal metabolism and growth in _E.