Leaf chlorosis in oilseed rape plants (Brassica napus) grown on cadmium-polluted soil: causes and consequences for photosynthesis and growth

Brassica napus L. (oilseed rape) was grown from seeds on a reconstituted so il contaminated with cadmium (100 mg Cd kg(-1) dry soil), resulting in a ma rked chlorosis of the leaves which was investigated using a combination of biochemical. biophysical and physiological methods. Spectroscopic and chrom atographic analyses of the photosynthetic pigments indicated that chlorosis was not due to a direct interaction of Cd with the chlorophyll biosynthesi s pathway. In addition, mineral deficiency and oxidative stress were appare ntly not involved in the pigment loss. Leaf chlorosis was attributable to a marked decrease in the chloroplast density caused by a reduction in the nu mber of chloroplasts per cell and a change in cell size, suggesting that Cd interfered with chloroplast replication and cell division. Relatively litt le Cd was found in the chloroplasts and the properties of the photosyntheti c apparatus (electron transport, protein composition, chlorophyll antenna s ize. chloroplast ultrastructure) were not affected appreciably in plants gr own on Cd-polluted soil. Depth profiling of photosynthetic pigments by phas e-resolved photoacoustic spectroscopy revealed that the Cd-induced decrease in pigment content was very pronounced at the leaf surface (stomatal guard cells) compared to the leaf interior (mesophyll). This observation was con sistent with light transmission and fluorescence microscopy analyses, which revealed that stomata density in the epidermis was noticeably reduced in C d-exposed leaves. Concomitantly, the stomatal conductance estimated from ga s-exchange measurements was strongly reduced with Cd. When plants were grow n in a high-CO2 atmosphere (4.000 mul CO2 1(-1)). the inhibitory effect of Cd on growth was not cancelled. suggesting that the reduced availability of CO2 at the chloroplast level associated with the low stomatal conductance was not the main component of Cd toxicity in oilseed rape.