Mt. Devi et As. Raghavendra, FORMATION OF PHOTORESPIRATORY GLYCOLATE AND GLYOXYLATE IN LEAF-DISKS OF C3-C4 INTERMEDIATES - REDUCED SENSITIVITY TO EXTERNAL BICARBONATE, Photosynthetica, 30(3), 1994, pp. 455-463
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.