A low irradiance mediated regulation of C4 metabolism during acclimation is reported for first time in Amaranthus hypochondriacus L., a NAD-ME dicot, Eleusine coracana (L.) Gaertn., an NAD-ME monocot, and Gomphrena globosa, a NADP-ME dicot. Significant decline in activities of key C4 enzymes were observed under limited irradiances in each of the species studied. When the plants were transferred to full natural irradiance, the enzyme activities were restored to originál State in 3-5 d, a similar time frame needed for acclimation to limiting irradiance. This identifies the decarboxylation reaction involving NAD-ME in Amaranthus and Eleusine or NADP-ME in Gomphrena as crucial sites of regulation under limiting irradiance.
Incorporation of labelled CO2, 3-phosphoglycerate (PGA), phosphoenolpyruvate (PEP) and pyruvate into hexane extractable rabber ffactions in the cut shoots of guayule {Parthenium argentatum Gray) was determined in order to evaluate the role of photosynthesis in providing precursors for rubber biosynthesis. DCMU inhibited the incorporation of labelled CO2 and PGA into rubber. The incorporation of i'*C02 into rubber depended on irradiance. Enzymatic activities of phosphoglyceromutase, enolase, pyruvate kinase and pyruvate dehydrogenase complex found in purified chloroplasts from the leaves indicated the chloroplast autonomy for intraplastid acetyl coenzyme A formation. The enzymes related to the biosynthesis of isopentenyl pyrophosphate (IPP) were associated with both leaf and stem extracts. Rubber producing enzyme activities, námely IPP isomerase and rubber transferase, were abundantly localized in roots and stems of guayule while the leaves exhibited low activities of these enzymes. Hence the leaves of guayule play a major role in providing precursors for rubber formation in stems and roots.