Predicting dry matter production of cauliflower (Brassica oleracea L. botrytis) under unstressed conditions - I. Photosynthetic parameters of cauliflower leaves and their implications for calculations of dry matter production

Measurements of CO2 exchange of cauliflower leaves were carried out in a fi eld experiment which included two nitrogen fertilisation rates. Irradiance and CO2 concentration were varied at the leaf level within a leaf cuvette a nd additionally a temperature treatment was applied to field grown plants m oved into climate chambers. These measurements were used to estimate parame ter values of a rectangular hyperbola describing cauliflower leaf CO2 excha nge as a function of irradiance and CO2 concentration. The obtained paramet er estimates were used to derive empirical regression equations with temper ature and nitrogen content of the leaves as independent variables. The resu lting relationships were applied within a simple photosynthesis-respiration based dry matter production model in order to derive functional relationsh ips between light use efficiency and irradiance, leaf area index and temper ature. The rectangular hyperbola was able to describe the gas exchange data as var ied by irradiance and CO2 concentration on the single leaf level with suffi cient accuracy, but estimates of initial light use efficiency (about 25 mug J(-1)) were too high because of the bias emanating from the limited flexib ility of this model. Light saturated photosynthesis rate (P-max) showed an optimum response to temperature and an increase with increasing nitrogen co ntent of leaves. The initial slope alpha of the rectangular hyperbola showe d no consistent responses to ambient temperature and nitrogen content of le aves. The respiration per unit leaf area beta increased exponentially with increasing temperature, resulting in a Q(10) value of 1.86. Because only a limited number of plants was evaluated in this study, additional work is ne eded to further substantiate the results of the gas exchange measurements. The model analysis demonstrated that LUE is independent of the light integr al over a range 5-10 MJ m(-2) per day photosynthetically active radiation i f one assumes an adaptation of P-max within the canopy and over time accord ing to the incident irradiance. Acclimatisation within the canopy and highe r leaf area indices. LAI, reduce the decrease of LUE with irradiance but a substantial decline remains even for LAI values of 4. (C) 2001 Elsevier Sci ence B.V. All rights reserved.