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Activities of tricarboxylic acid (TCA) cycle enzymes in seedlings of barnyard grass (Echinochloa phyllopogon (Stapf.) Koss) and rice (Oryza sativa L.) germinated under aerobic and anaerobic conditions were investigated. In E. phyllopogon, development of TCA-cycle enzyme activities during 10 d of anoxia generally paralleled those in air, although at lower rates. After 5 d, E. phyllopogon seedlings germinating under N2 exhibited 50-80% of the activity of seedlings grown in air, except for 2-oxoglutarate dehydrogenase (EC 1.2.4.2) and fumarate reductase (EC 1.3.1.6) which exhibited only 25-35% of aerobic activity. In anaerobically germinated rice, development of TCA-cycle enzyme activities also paralleled those in air except for aconitase (EC 4.2.1.3), isocitrate dehydrogenase (EC 1.1.1.41), and 2-oxoglutarate dehydrogenase. Those enzymes did not increase in activity under anoxia. Development of maximum enzyme activities generally occurred more rapidly and persisted longer in E. phyllopogon compared to rice. The data indicate that mitochondria of E. phyllopogon function better during anaerobiosis than those of rice and this factor may contribute to the successful "biochemical strategy" of this weed in rice paddies throughout the world.

Four populations of Mollugo verticillata L. were compared on the basis of their photosynthetic products, photosynthetic rates, enhancement under low oxygen concentration, and CO2 compensation points. In addition, pulse-chase labeling experiments were conducted using one of the four populations. Depending on the plant population, C4 acids ranged from 40% to 11% of the primary products under short-term exposure to (14)CO2. These compounds were also metabolized during pulse-chase experiments. All four populations had significantly different photosynthetic rates and those rates were correlated with the amounts of labelled C4 acids produced and C4-acid turnover. Three populations of M. verticillata had similar compensation points (40 μl/l) and degrees of photosynthetic enhancement under low [O2] (20%), the fourth population was much lower in both characteristics (CO2 compensation, 25 μl/l; low-O2 enhancement, 12%). The results verify the intermediate nature of photosynthesis in this species, and illustrate populational differences in its photosynthetic and photorespiratory carbon metabolism.

Plastid development in the primary leaf of Echinochloa crus-galli (L.) Beauv. var. oryzicola (Vasing.) Ohwi was followed during 5 d of anoxic germination and growth. Plastids develop slowly from simple spheroidal proplastids into larger pleomorphic plastids with several stromal membranes and many peripheral membrane vesicles. A small prolamellar body is present at 96 h with perforated (pro)thylakoids extending into the stroma. Changes in starch grains and plastoglobuli are evidence of carbohydrate and lipid metabolism. Plastid division is indicated by dumbbell plastid profiles after 4 d of anoxia. These results demonstrate that plastids not only maintain their integrity during anaerobic germination but also show developmental changes involving an increase in internal membrane complexity, although to a lesser extent than in etiolated shoots.

The activities of several enzymes, including ribulose-1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured as a function of leaf age in Z. mays. Mature leaf tissue had a RuDP-carboxylase activity of 296.7 μmol CO2 g(-1) fresh weight h(-1) and a PEP-carboxylase activity of 660.6 μmol CO2 g(-1) fresh weight h(-1). In young corn leaves the activity of the two enzymes was 11 and 29%, respectively, of the mature leaves. In senescent leaf tissue, RuDP carboxylase activity declined more rapidly than that of any of the other enzymes assayed. On a relative basis the activities of NADP malic enzyme (EC 1.1.1.40), aspartate (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2), and NAD malate dehydrogenase (EC 1.1.1.37) exceeded those of both PEP and RuDP carboxylase in young and senescent leaf tissue. Pulse-chase labeling experiments with mature and senescent leaf tissue show that the predominant C4 acid differs between the two leaf ages. Labeling of alanine in senescent tissue never exceeded 4% of the total (14)C remaining during the chase period, while in mature leaf tissue alanine accounted for 20% of the total after 60 s in (12)CO2. The activity of RuDP carboxylase during leaf ontogeny in Z. mays parallels the development of the activity of this enzyme in C3 plants.

Ribulose diphosphate (RuDP) and (PEP) phosphoenolpyruvate carboxylase enzyme activities were studied in young, mature, and senescent Portulaca oleracea leaves. While the absolute amount of both the C3 (RuDP) and C4 (PEP) carboxylase is less in senescent leaves than in mature leaves, RuDP carboxylase activity is reduced to a lesser degree. In senescent leaves, PEP carboxylase activity equals 10% of that in mature tissue, but RuDP carboxylase is 27% of that in mature leaves. The same ontogenetic series was also used to determine photorespiration rates and responses to several gas treatments. Young and mature leaves were unaffected by changes in the light regime or oxygen concentrations, and exhibited typical C4-plant light/dark (14)CO2 evolution ratios. Senescent leaves, on the other hand, have photorespiration ratios similar to C3-plants. In addition, senescent leaves were affected by minus CO2, 100% O2 and N2 in a manner expected of C3-plants, but not C4-plants. These results are discussed in terms of a relative increase in activity of the C3 cycle in later developmental stages in this plant.

The photosynthetic products of Portulaca oleracea differ greatly depending on leaf age and length of exposure to (14)CO2. Mature leaves of P. oleracea fix (14)CO2 primarily into organic and amino acids during a 10-s exposure period. Less than 2% of the (14)CO2 fixed appears in phosphorylated compounds. In contrast, incorporation into amino acids can account for over 60% of the total (14)CO2 fixed by young leaves in an equal time period, and incorporation into alanine alone can account for up to one half of this amount. Senescent leaves display a quantitative shift of primary products toward phosphorylated compounds with a concomitant reduction of the label residing in malate and asparate. About 8 times more phosphoglyceric acid is produced in senescent leaves than in mature leaves. The aspartate/ malate ratio is not constant and depends on the length of time the leaves are exposed to (14)CO2 and the age of the leaves under study. It appears as if the stage of leaf development is one of the most important factors determining the operation of a particular enzyme system in C4 plants.