Evaluation of Artificial Light for Plants on the Basis of Transpiration Model

Abstract

To develop a method for evaluating artificial light for plants, a mathematical model of transpiration rate in cucumber plants was presented by using parameters representing light, air temperature, humidity and spectral dependence of transpiration rate. The transpiration rate (E, mg⋅dm^-2⋅min^-1) was divided into two components ofE₁in darkness andE₂under radiation, as written byE=E₁+E₂. E₁was given byf₁ (T, H) , whereT (°C) andH (%) are air temperature and relative humidity, respectively.E₂was expressed asE₂=L⋅f₂ (T, H) , where L is the parameter representing the light condition and the plant response to the light. The wavelength region of 400 to 700 nm was divided into three regions of B (400≤λB≤500 nm), G (500≤λG≤600 nm) andR (600 <λR≤700 nm), correspond-ing to the spectral dependence of leaf conductance for water vapor in Cucurbita maximaL, and blue (b), green (g) and red (r) fluorescent lights were used for the examination; most of the spectral energy of those lights distributed inB, GandR, respectively. The light condition was evaluated by using the total light intensity (Q, nE⋅cm^-2⋅sec^-1) in the region of 400 to 700 nm and the relative light intensities ofI_B, I_G and I_R (I_B+I_G+I_R=1) inB, G andRfor representing the spectral energy distribution. The spectral dependence of transpiration rate was similar to that of leaf conductance and was represented by the parameters ofK_B, K_GandK_R (K_B+K_G+K_R=1) which characterized the transpiration rate affected by the respective lights inB, GandR. Thus, L was given byL=Q⋅ (K_B⋅I_B+K_G⋅I_G+K_R⋅I_R) , andf₂ (T, H) =E₂/Lwas determined by measuringE₂ under the respective light and air conditions. Therefore, E=f₁ (T, H) +Q⋅ (K_B⋅IB+K_G⋅I_G+K_R⋅I_R) ⋅f₂ (T, H) was presented as a model. From the fact that the transpiration rates simulated from the model were close to measured ones under two typical lights from white fluorescent and incandescent lamps at different air temperatures and humilities, it was demonstrated that the parameters representing the light can be used to evaluate artificial light for plants.