FORMATION OF PHOTORESPIRATORY GLYCOLATE AND GLYOXYLATE IN LEAF-DISKS OF C3-C4 INTERMEDIATES - REDUCED SENSITIVITY TO EXTERNAL BICARBONATE

Accumulation of glycollate or glyoxylate on irradiation of leaf discs in the presence of alpha-hydroxypyridinemethane sulfonate (alpha-HPMS) or glycidate, respectively, was studied in C3-C4 intermediates of Alt ernanthera species (A. ficoides, A. tenella) and Parthenium hysteropho rus in comparison with the pattern in C3 or C4 species. The levels of these two photorespiratory metabolites were reduced in the intermediat es (< 75 % of that in C3) while being very low in the C4 species (abou t 10 % of that in C3). The inhibitory effect of bicarbonate on the gly collate or glyoxylate accumulation was pronounced in the C3 species (6 0% inhibition), moderate in the intermediates (about 45 %), and very l ow (< 2 %) in the C4 plants. The negligible effect of bicarbonate on t hese photorespiratory metabolites in the C4 species is expected to be due to their C4 acid-based CO2-concentrating mechanism. In the presenc e of 5 mM bicarbonate, the levels of glycollate and glyoxylate in the C3 species were similar to those in the C3-C4 intermediates. We specul ate that a high internal CO2 pool, possibly due to an efficient CO2 re cycling/refixation mechanism, may be an additional reason besides the partial reduction in photorespiratory enzymic capacity for reduced lev els of photorespiratory glycollate/glyoxylate in C3-C4 intermediates.