Predicting dry matter production of cauli ̄ower (Brassica oleracea L. botrytis) under unstressed conditions Part II. Comparison of light use ef®ciency and photosynthesis±respiration based modules

Six different modules for dry matter production of cauli ̄ower were parameterised and evaluated using a database of 22 cauli ̄ower crops originating from 15 independent ®eld experiments. The evaluation included a light use ef®ciency, LUE, based module assuming LUE to be constant, an LUE based module assuming a linear decrease of LUE with increasing daily photosynthetically active radiation sum, I, two photosynthesis±respiration based modules using an analytical integration of the rectangular hyperbola over the canopy, assuming either the light saturated photosynthesis rate of single leaves, Pmax, to be constant or to decrease proportionally to irradiance within the canopy. Furthermore two slightly modi®ed versions of the light interception and photosynthesis algorithms of the SUCROS model were evaluated, where the negative exponential equation for single leaf photosynthesis was replaced by the rectangular hyperbola. In order to make these modules comparable with the analytical integration approach, Pmax was also assumed to be either constant or to decrease proportionally to irradiance within the canopy. The results indicate that an estimated constant LUE (3.15 ( 0.04) g MJÿ1) is only poorly able to predict total dry matter production for cauli ̄ower (modelling ef®ciency EFˆ0.69) of an independent data set. Using a linear decline of LUE with I (LUEˆ6.66 ( 0.80)±0.36 ( 0.08)I) drastically increased the predictive value (EFˆ0.88) of the LUE approach. The descriptive and predictive value of the photosynthesis based modules was higher when assuming that Pmax declines within the canopy. Then the predictive value of the photosynthesis/respiration based approach was Scientia Horticulturae 87 (2001) 171±190 * Corresponding author. Tel.: ‡49-511-762-2919; fax: ‡49-511-762-3606. E-mail address: [email protected] (H. Kage). 0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 4 2 3 8 ( 0 0 ) 0 0 1 8 0 1 better than the simple LUE approach but not generally better than the LUE approach assuming a linear decrease of LUE with increasing daily radiation sum. # 2001 Elsevier Science B.V. All rights reserved.