Enzyme Activities and Products of CO2 Fixation in Various Photosynthetic Organs of Wheat and Oat

Activities of key enzymes of the Calvin cycle and C. metabolism, rates of CO2 frixation, and the initial products of photosynthetic '4CO2 fLxation were determined in the podwall seed coat (fruiting structures), and the subtending leaf (leaf below a receme) of Brassica campestris L. cv 'Toria.' Compared to activities of ribulose-1,5-bisphosphate carboxylase and other Calvin cycle enzymes, eg. NADP-glyceraldehyde-3-phosphatedehydrogenase and ribulose-5-phosphate kinase, the activities of phosphoenol pyruvate carboxylase and other enzymes of C4 metabolism, viz. NADP-malate dehydrogenase, NADP-malic enzyme, glutamate pyruvate transaminase, and glutamate oxaloacetate rns se, were generally much higher in seed than in podwall and leaf. Podwall and leaf were comparable to each other. Pulse-chase experiments showed that in seed the major product of '4CO2 assimilation was malate (in short time), whereas in podwall and leaf, the label initially appeared in 3-PGA. With time, the label moved to sucrose. In contrast to legumes, Brassica pods were able to fix net CO2 during light. However, respiratory losses were very high during the dark period.

Section: Resultsmentioning

confidence: 75%

mentioning

confidence: 99%

Photosynthetic Carbon Fixation Characteristics of Fruiting Structures of Brassica campestris L.

Singal¹,

Sheoran²,

Singh³

1987

“…In the same way, Hubick and Farquhar (7) reported in barley less carbon isotope discrimination in dry matter of heads than of leaves. This variation in b'3C along plant parts may be caused by differences in the ratio of assimilation rate to CO2 diffusive conductance (5), although variations in the photosynthetic metabolism of the plant parts may also be involved (2,15,16). With regard to the former possibility, although the CO2 conductance is basically stomatal in the leaf, stomatal conductance is probably much less important in awns and even more in ear bracts (1).…”

Section: Differences In 613c Among Plant Partsmentioning

confidence: 99%

Carbon Isotope Ratios in Ear Parts of Triticale

Araus¹,

Santiveri²,

Bosch-Serra³

et al. 1992

Four triticale (xTriticosecale Wittmack) genotypes were grown under rainfed conditions with limited irrigation support in Lleida in northeast Spain. For each variety, samples consisting of 10 tillers with half-sterilized spikes were taken three times from anthesis to maturity. Carbon isotope ratios (513C) were then determined in water extracts from ear bracts (glumes, paleas, and lemmas), awns and flag leaves, and in powdered kernels. For the half-sterilized spikes, carbon isotope analysis was carried out separately in bracts and awns from fertile and nonfertile spikelets. The 613C in the water-soluble fraction of awns, glumes, and glumells from fruitless spikelets was significantly higher than that from fertile spikelets sampled at mid-grain filling. Differences in 613C among sterile and fertile spikelets were not significant in samples taken a few days after anthesis or at maturity. These results are in accordance with some degree of refixation by awns and ear bracts of the CO2 respired by grains during grain filling. There was progressively higher 63C from flag leaf blades to awns, glumes, and glumells. This variation in 613C along plant parts may be caused by differences in the ratio of assimilation rate to C02-diffusive conductance. Values of 613C of mature kernels were between the values at anthesis and mid-grain filling for the water-soluble fraction of flag leaves and inner bracts and were fairly similar to those of glumes and awns. Conservation of respired CO2 by an efficient recycling mechanism in fruit could provide a significant source of C for yield productivity (2). Cereal crops have received the greatest attention in this regard (4, 9, 16). For example, Kriedemann (9) studied the refixation of respired carbon dioxide in whole ears by measuring CO2 evolution of ears in a C02-free environment in the light and dark, reporting that evolution was lower in the light and suggesting that refixation was taking place. However, evidence of bracts and awns refixing the intemal CO2 released from growing grains remains to be elucidated.Carbon isotope ratios of plant tissue have been proposed as an indicator not only of water use efficiency or photosynthetic metabolism (5) but also of recycling of respired CO2 in

“…The PPDK activity of C3 leaves is only 1 to 5% of that found in C4 maize leaves so that detection by the activity assay can be difficult (19,28).…”

mentioning

confidence: 99%

Pyruvate, Pi Dikinase in Bundle Sheath Strands as Well as in Mesophyll Cells in Maize Leaves

Aoyagi

Nakamoto²

1985

Mesophyll protoplasts and bundle sheath strands were isolated from maize leaves. Light microscopic observation showed the preparations were pure and without cross contamination. Protein blot analysis of mesophyll and bundle sheath cell soluble protein showed that the concentration of pyruvate orthophosphate dikinase (EC 2.7.9 The C4 pathway is characterized by the initial assimilation of CO2 and C4 acids in mesophyll cells and subsequent decarboxylation and reduction of the carbon by the RPP2 pathway in the bundle sheath cells (4, 7). Previous studies have provided evidence for the differential localization ofcertain enzymes involved in the C4 pathway (7). For example, PEPC, PPDK, and NADPdependent MDH are predominantly present in mesophyll cells and RuBPC and NADP-dependent ME are predominantly present in bundle sheath cells.