Die Wirkung unterschiedlicher Lichtintensitäten während der Anzucht auf die CO2-Kompensationslage, die Glykolsäure-Oxidase- und Ribulosebiphosphat-Carboxylase-Aktivitäten bei Sinapis alba

Abstract

Sinapis alba (white mustard) plants were grown under conditions of strong (100 W · m -2 ) and weak (3 W · m -2 ) light and the effect of the light intensity during growth on the CO 2 -compensation point and the activities of glycolic acid oxidase and ribulose bisphosphate carboxylase were studied. The CO 2 -compensation concentration was determined according to two methods: firstly in a closed system with an infrared CO 2 -analyzer, and secondly according to the curve of net photosynthesis as a function of C0 2 -concentration over the range of 400 vpm CO 2 to the CO 2 -compensation point (Fig. 4). The intensity of light during growth was observed to exert an unmistakable effect on the CO 2 -compensation point. In stronglight leaves the CO 2 -compensation point was lower than in weak-light leaves. Plants grown in strong light had a compensation value of 54 ± 3 vpm (first method) or 59 ± 4 vpm (second method), whereas values of 63 ± 4 vpm or 79 ± 7 vpm, respectively, were obtained with plants grown in weak light. The differences obtained were statistically significant at the 99% confidence level according to both methods. The in vitro activities and Michaelis-Menten constants of ribulose bisphosphate carboxylase from leaves of Sinapis alba grown for 16 days in strong light and for 26 days in weak light are presented in Table 1. Based on fresh weight or leaf area, the measurements indicated a 3.3- or 7.4-fold higher activity of this enzyme in the strong-light plants. The glycolate oxidase activities were 2- or 4-fold higher in the strong-light plants than in the weak-light plants, when based on fresh weight or leaf area, respectively (Table 3). The pH-curve and the influences of the preparation time, the presence of flavine mononucleotide and the use of different buffer on the enzyme activities were also tested (Fig. 1). The higher activities of ribulose bisphosphate carboxylase and glycolate oxidase in the strong-light leaves indicated a higher activity of the glycolate pathway and, therefore, a greater production of CO 2 in these cells. The lower CO 2 -compensation point due to the influence of strong light thus appears to contradict the findings of the enzyme measurements. This discrepancy is explained in terms of a greater re-fixation of the CO 2 produced in the glycolate pathway. In strong-light leaves the re-fixation of CO 2 is increased by the low carboxylation resistance of the enzymatic carboxylation system, as well as by the leaf anatomy. The palisade layers of strong-light leaves have a 4-fold greater volume than do those of weak-light leaves. The longer path lenghts from the CO 2 -producing reaction sites to the stomata and the open air and the more comapct arrangement of these palisade cells also contribute to the increased re-fixation of CO 2 produced in light. The CO 2 -compensation point can therefore not be valued as an exact indicator of the activity of the glycolate pathway formation of CO 2 in the leaves.