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

Abstract

Measurements of CO 2 exchange of cauliflower leaves were carried out in a field experiment which included two nitrogen fertilisation rates. Irradiance and CO 2 concentration were varied at the leaf level within a leaf cuvette and additionally a temperature treatment was applied to field grown plants moved into climate chambers. These measurements were used to estimate parameter values of a rectangular hyperbola describing cauliflower leaf CO 2 exchange as a function of irradiance and CO 2 concentration. The obtained parameter estimates were used to derive empirical regression equations with temperature and nitrogen content of the leaves as independent variables. The resulting relationships were applied within a simple photosynthesis–respiration based dry matter production model in order to derive functional relationships between light use efficiency and irradiance, leaf area index and temperature. The rectangular hyperbola was able to describe the gas exchange data as varied by irradiance and CO 2 concentration on the single leaf level with sufficient accuracy, but estimates of initial light use efficiency (about 25 μg J −1) were too high because of the bias emanating from the limited flexibility of this model. Light saturated photosynthesis rate ( P max) showed an optimum response to temperature and an increase with increasing nitrogen content of leaves. The initial slope α of the rectangular hyperbola showed no consistent responses to ambient temperature and nitrogen content of leaves. The respiration per unit leaf area β 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 needed to further substantiate the results of the gas exchange measurements. The model analysis demonstrated that LUE is independent of the light integral over a range 5–10 MJ m −2 per day photosynthetically active radiation if one assumes an adaptation of P max within the canopy and over time according to the incident irradiance. Acclimatisation within the canopy and higher leaf area indices, LAI, reduce the decrease of LUE with irradiance but a substantial decline remains even for LAI values of 4.