Mark R. Schmitt

Microstegium vimineum, a shade adapted C4 grass☆

Abstract Microstegium vimineum, a C4 plant of the family Poaceae, is shown to be extremely adaptable to growing under shade conditions. The capacity for dry matter production of the species was similar from 18% to 100% full sunlight. Even at 5% full sunlight substantial growth occurred (17% of that at full sunlight). In comparison, growth of the C4 species Digitaria sanguinalis and Sporobolus airoides declined rapidly below full sunlight with no growth occurring at 5% full sunlight. A number of measurements on leaves of M. vimineum grown under 5% of full sunlight versus full sunlight reflects the ability of this species to adapt to low light. Under low light there was a 2-fold increase in leaf area per leaf with an approx. 2-fold decrease in leaf thickness. The soluble protein/ chlorophyll ratio increased about 3.5-fold in the low light-grown plants due to an increase in chlorophyll/leaf area and a decrease in soluble protein/leaf area. Under both light regimes the plants had Kranz anatomy and high levels of phosphoenolpyruvate (PEP) carboxylase which are known features of C4 plants. The results indicate that there are no inherent limitations in the C4 mechanism or in the efficiency of energy utilization for carbon assimilation which prevents C4 plants from adapting to low light conditions.

Activity and quantity of ribulose bisphosphate carboxylase- and phosphoenolpyruvate carboxylase-protein in two Crassulacean acid metabolism plants in relation to leaf age, nitrogen nutrition, and point in time during a day/night cycle

Activity of ribulose 1,5-bisphosphate (RuBP) carboxylase in leaf extracts of the constitutive Crassulacean acid metabolism (CAM) plant Kalanchoe pinnata (Lam.) Pers. decreased with increasing leaf age, whereas the activity of phosphoenolpyruvate (PEP) carboxylase increased. Changes in enzyme activities were associated with changes in the amount of enzyme proteins as determined by immunochemical analysis, sucrose density gradient centrifugation, and SDS gel electrophoresis of leaf extracts. Young developing leaves of plants which received high amounts of NO3-during growth contained about 30% of the total soluble protein in the form of RuBP carboxylase; this value declined to about 17% in mature leaves. The level of PEP carboxylase in young leaves of plants at high NO3-was an estimated 1% of the total soluble protein and increased to approximately 10% in mature leaves, which showed maximum capacity for dark CO2 fixation. The growth of plants at low levels of NO3-decreased the content of soluble protein per unit leaf area as well as the extractable activity and the percentage contribution of both RUBP carboxylase and PEP carboxylase to total soluble leaf protein. There was no definite change in the ratio of RuBP carboxylase to PEP carboxylase activity with a varying supply of NO3-during growth. It has been suggested (e.g., Planta 144, 143–151, 1978) that a rhythmic pattern of synthesis and degradation of PEP carboxylase protein is involved in the regulation of β-carboxylation during a day/night cycle in CAM. No such changes in the quantity of PEP carboxylase protein were observed in the leaves of Kalanchoe pinnata (Lam.) Pers. or in the leaves of the inducible CAM plant Mesembryanthemum crystallinum L.

Dependence of rates of apparent photosynthesis on tissue phosphorus concentrations in Myriophyllum spicatum L.

Abstract The relationship between internal phosphorus concentration and apparent photosynthesis was examined in 10-cm shoot tips of the submersed macrophyte Myriophyllum spicatum L. A pseudo-Michaelis-Menten type relationship was observed in both laboratory and field-grown plants. Curves fit to the data showed phosphorus compensation points (tissue phosphorus concentrations at which the predicted rates of apparent photosynthesis = 0) of 0.024 and 0.023% for laboratory and field-grown plants respectively. Critical concentrations (tissue phosphorus concentration resulting in a predicted rate of apparent photosynthesis equal to 90% of a “mean saturated” photosynthetic rate) were determined to be 0.29 and 0.32%, respectively. Literature values for tissue phosphorus concentrations frequently fall in the range between Gerloffs critical concentration of 0.07% and the 0.3% critical concentration derived in the current study, indicating that data originally interpreted as showing phosphorus sufficiency may indeed indicate suboptimal levels of phosphorus.

Glycerate kinase from leaves of C3 plants

D-Glycerate-3-kinase (EC 2.7.1.31) in six C3 species, including dicots (Pisum sativum, Spinacea oleracea, Antirrhinum majus) and monocots (Secale cereale, Hordeum vulgare, Avena sativa), ranged in activity from 44 to 353 mumol X mg chl-1 X h-1. Studies with protoplast extracts of these species indicate that the enzyme is localized in the chloroplasts. Glycerate kinase was partially purified from Secale (rye, 288-fold) and Pisum (pea, 252-fold) chloroplasts by DEAE-cellulose chromatography, sucrose gradient centrifugation, and chromatofocusing. The enzymes from both species showed similar physical (Mr = 41,000, pI = 4.6-4.7) and kinetic (Km ATP = 655 to 692 microM, Km D-glycerate = 180-188 microM) properties. Activity of the enzyme was essentially insensitive to variations in assay pH from 6.4 to 9.0 and to energy charge variations from 0.4 to 1.0. Rye glycerate kinase was able to utilize UTP and GTP but less effectively than ATP. Neither ADP nor pyrophosphate served as an energy source. Mn2+, Co2+, Ca2+, and Sr2+ could function as metal cofactors, although to a lesser extent than Mg2+. Millimolar levels of sulfate were found to significantly inhibit the enzyme while similar concentrations of other anions (Cl-, NO-3, NO-2, and acetate) had little or no effect.