Abstract

Earlier studies in India have shown that Eucalyptus can consume excessive amounts of water. In this study, the water relations of 6 species (E. tereticornis, E. camaldulensis, E. urophylla, E. brassiana, E. pellita and E. deglupta) were compared in an experimental plot in Kerala, and the rooting depth of the species commonly planted in Kerala (E. tereticornis) investigated. Diurnal variations in leaf water potential, stomatal conductance and net photosynthesis were measured at monthly intervals forall 6 species over 3 yr, starting when they were 11 yr old. Leaf water potential measurements gave a predawn value greater than -0.5 MPa, indicating the absence of any serious water stress for any of the species, throughout the year. The lowest midday water potential did not fall below -2.0 MPa despite the continuous dry period of 4-5 months, indicating that the root systems of all the species go very deep in the soil. Stomata were found on the abaxial and adaxial sides of the foliage in all species except E. pellita and E. deglupta. The stomatal frequency varied between 150 and 485 mm-2 in all species. There were significant variations between species in the stomatal conductance, which were lowest in E. urophylla and highest in E. camaldulensis. Therewas no midday closure of stomata in any species except E. deglupta. There were significant variations between premonsoon and postmonsoon stomatal conductance. Stomatal closure in response to increasing atmospheric vapour pressure deficit was most developed in E. tereticornis and E. deglupta. In all other species, leaf water potential probably played a major role in stomatal closure. Although the stomatal conductance was reduced in the premonsoon period, net photosynthesis was not affected greatly exceptin E. urophylla and E. camaldulensis. The Amax in the different species varied from 15 to 25 µmol m-2 s-1; it was maximum in E. camaldulensis, and in all species occurred between PFD (photon flux densities) of 800 and 1000 µmol m-2 s-1. Plots of photosynthesis vs. conductance showed similar values during the premonsoon as during the postmonsoon but at lower stomatal conductance values, indicating a high water use efficiency in species other than E. tereticornis and E. deglupta. Root excavation studies inE. tereticornis in a 20-yr-old plantation were traceable up to 9.3 m depth. Roots with diameter 5.0 mm were present within 2 m below ground level. The root length density and root weight density showed large variations in different layers of the soil.Thephreatic aquifer was between 9.0 and 12.0 m during the summer season showing that the roots have access to this zone. Based on these observations, a number of conclusions are drawn and recommendations made