© 2010 19 th World Congress of Soil Science, Soil Solutions for a Changing World 5 1 – 6 August 2010, Brisbane, Australia. Published on DVD. Assessing macroscopic salinity models for predicting canola response to salinity under bud stage Vahidreza. Jalali A and Mehdi. Homaee A A Tarbiat Modares University, Faculty of Agriculture, Department of Soil Science. Tehran, Iran. 14115-335, Email mhomaee@modares.ac.ir Abstract Plant response to salinity varies during growth stages. Canola is more sensitive to salinity at earlier growth stages but becomes resistant at germination stage. Earlier growth stages including bud stage are very important parts of plant life, because their survival in these stages will determine their final yield. Few macroscopic models have been proposed to quantify the plant response to average root zone salinity during the whole growth period. Since plant response to salinity varies during different growth stages, developing appropriate models for quantitative characterization of plant response to salinity at each growth stage seems to be crucial. To determine the effect of salinity on canola at bud growth stage, an extensive experiment was conducted with a natural saline loamy sand soil, using some salinity treatments including one non-saline water (tap water) and 8 natural saline waters of 3 to 17 dS/m. The Maas and Hoffman (1977), van Genuchten and Hoffman (1984), Dirksen et al. (1993), and Homaee et al. (2002) function were used as macroscopic models to predict relative transpiration (T a /T p ). To compare the models and their efficiency, some statistics were used. The results showed that the calculated statistical parameters were same for Homaee et al. (2002) and Dirksen et al. (1993) model, but since input parameters for Homaee et al. (2002) model are easier to obtain, it is recommended to be used for simulating canola response to salinity at bud growth stage. Key Words Canola, salinity, relative transpiration, bud stage. Introduction The adverse effects of salinity are generally most pronounced in arid and semi-arid regions because of insufficient annual rainfall to flush out accumulated salts from the root zone (Bresler et al. 1982). Plants in response to salinity, represents various resistances with respect to its phonologic stages. Most plants are resistant at germination stage. However, at seedling or earlier growth stages, plants usually become more sensitive to salinity but their tolerance increases with age. Salt tolerance of various plants has been extensively studied by different researchers (e.g. mer et al. 2000. Keshta et al. 1999 and Francois 1994) with the main conclusion that plant response to salinity highly depends on their phonologic stages. Plant water consumption is low at primary growth stage but increases with plant growth and reaches its maximum at bud and rosette stages. Measurement of transpiration or evapotranspiration is necessary for determining plant water demand. Several researchers (e.g. deWit 1958; Hanks 1984) showed a linear relationship between plant growth and transpiration or evapotranspiration rate (Homaee and Schmidhalter 2008). Water absorption by plants decreases with increasing salinity. Water movement in unsaturated soils is described with the Richard’s equation (Richards 1931). Including the root extraction term S, it reads: S ) h ( K z h ) h ( K z t − ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + ∂ ∂ ∂ ∂ = ∂ ∂θ (1) where Ө is volumetric water content (L 3 /L 3 ), t is the time (T), h is the soil water pressure head (L), z is gravitational head, as well as the vertical coordinate (L) taken positive upward, k is the soil hydraulic conductivity (L/T), and S is the water extraction rate by plant roots (L 3 /L -3 /T). Feddes et al. (1978) introduced a macroscopic sink term depending on soil water pressure head h only as: max ) ( S h S α = (2) where S max is the maximum rate of water uptake and α(h) is a dimensionless function of pressure head. Analogously, one may introduce a soil salinity reduction term, ) ( o h α , instead of ) (h α in Eq. (2). This salinity function can be put in the form of the Maas and Hoffman (1977) equation. Written in terms of the soil solution osmotic head o h , this gives (Homaee et al. 2002): ) h (h 360 a 1 ) α(h o o o − − = ∗ (3)