The role of shoot photosynthesis as a means of supporting aerobic respiration in the roots of the seagrass Zostera marina was examined. O2 was transported rapidly (10-15 minutes) from the shoots to the root-rhizome tissues upon shoot illumination. The highest rates of transport were in shoots possessing the greatest biomass and leaf area. The rates of O2 transport do not support a simple gas phase diffusion mechanism. O2 transport to the root-rhizome system supported aerobic root respiration and in many cases exceeded respiratory requirements leading to O2 release from the subterranean tissue. Release of O2 can support aerobic processes in reducing sediments typical of Z. marina habitats. Since the root-rhizome respiration is supported primarily under shoot photosynthetic conditions, then the daily period of photosynthesis determines the diurnal period of root aerobiosis.
The response of the photosynthetic apparatus to low temperature periods differed among three hybrids of maize (Zea mays L.) grown in a phytotron. Light-saturated photosynthetic rates, leaf chlorophyll content, and mesophyll cell photosynthetic unit density all declined with increasing duration of low temperature. No single metabolic or physiological parameter appeared to control the response of the three hybrids to low temperature stress. Among all temperature treatments, net photosynthetic rate on a leaf area basis was more closely correlated with leaf chlorophyll content than with any other measured parameter. Final shoot dry weight was most highly correlated with stomatal conductance to CO(2).
ABSTRACT The contribution of light‐independent carbon fixation (LICF) to the overall carbon gain and the seasonal patterns of maximum photosynthesis (P max and LICF were characterized in a broad taxonomic range of macrophytes from Monterey Bay, California. P max and LICF rates (nmol C.g filtered seawater −1 .min −1 ) varied among species and taxonomic groups examined, and as a function of tissue type in the phaeophyte Laminaria setchellii Silva (Phaeophyceae). On average, P max values were higher in the Rhodophyta, whereas LICF rates were greater in the Phaeophyceae. LICF rates were generally less than 5% of P max in the marine macrophytes studied and, as a consequence, cannot fully compensate for respiratory carbon losses, which usually are greater than 10% of P max . All species studied possessed the highest P max and LICF rates when irradiance levels were highest and decreased during periods of low incident irradiance. Seasonal patterns of P max and LICF in most of the macrophytes from the stenothermal environment of Monterey Bay were strongly correlated with photosynthetic photon flux rather than seawater temperature. The concomitant decrease of LICF and P max rates in all species examined argues against LICF playing a major role in carbon acquisition under light‐limiting conditions as suggested previously. Rather, the strong positive correlation of P max and LICF indicates the direct coupling of photosynthate (e.g. 3‐phosphoglyceric acid) generation with production of substrates for LICF reactions. Our results also suggest that LICF might be a useful indicator of photosynthetic metabolism in marine macrophytes.
The consequences of drought stress on the organization of chlorophyll into photosynthetic units and on the chlorophyll-protein composition of mesophyll and bundle sheath chloroplasts of Zea mays L. were studied. It was found that the majority of chlorophyll lost in response to water stress occurs in the mesophyll cells with a lesser amount being lost from the bundle sheath cells. All of the chlorophyll loss could be accounted for by reduction in the lamellar content of the light-harvesting chlorophyll a/b-protein, a rather specific target for water stress. The decreased content of this chlorophyll-protein accounts for the elevated chlorophyll a/b ratios and the reduced photosynthetic unit sizes of the two cell types in stressed plants. The implications of the selective catabolism of this major membrane component are discussed.
The uptake of nitrate by phytoplankton is a central issue in biological oceanography due to its importance to primary production and vertical flux of biogenic carbon. Nitrate reductase catalyzes the first step of nitrate assimilation, the reduction of NO(3) to NO(2). A cytometric protocol to detect and quantify relative changes in nitrate reductase (NR) protein content of the marine centric diatom Skeletonema costatum is presented.Immunolabeling of NR protein was achieved with polyclonal antibodies raised against S.costatum NR. Antisera specific to a NR protein subunit and to a NR polypeptide sequence were compared, and cytometric results of NR protein abundance were related to Western analyses. Changes in cellular NR abundance and activity were followed during an upwelling simulation experiment in which S. costatum was exposed to a shift from ammonia to nitrate as major nitrogen source.NR protein could be detected in NO(3)-grown cells and at extremely low levels hardly discernible by Western Blot densiometry in NH(4)-grown cells. The protocol allowed observation of early stages of NR induction during an upwelling simulation. NR abundance increased after the nutrient shift to reach a new physiological "steady-state" 96 hrs later. NR activity exhibited diel variation with maxima at mid-day. NR abundance as estimated by both flow cytometry and Western analysis exhibited a hyperbolic relationship to NR activity. This pattern suggests post-translational activation of NR protein.The presented protocol allows the differentiation of NH(4)- versus NO(3)-grown algae as well as the monitoring of early stages in the induction of nitrate assimilatory capacities.
Cytochemical and immunocytochemical methods were used to localize photosystems I and II in barley (Hordeum vulgare L. cv Himalaya) chloroplasts. PSI activity, monitored by diaminobenzidine oxidation, was associated with the lumen side of the thylakoids of both grana and stroma lamellae. The P(700) chlorophyll a protein, the reaction center of PSI, was localized on thin sections of barley chloroplasts using monospecific antibodies to this protein and the peroxidase-antiperoxidase procedure. Results obtained by immunocytochemistry were similar to those of the diaminobenzidine oxidation: both grana and stroma lamellae contained immunocytochemically reactive material. Both the grana and stroma lamellae were also labeled when isolated thylakoids were reacted with the P(700) chlorophyll a protein antiserum and then processed by the peroxidase-antiperoxidase procedure. PSII activity was localized cytochemically by monitoring the photoreduction of thiocarbamyl nitroblue tetrazolium, a reaction sensitive to the PSII inhibitor, DCMU. PSII reactions occurred primarily on the grana lamellae, with weaker reactions on the stroma lamellae.
Wild type Gracilaria tikvahiae, a macrophytic red alga, and fourteen genetically characterized pigment mutants were analyzed for their biliprotein and chlorophyll contents. The same three biliproteins, phycoerythrin, phycocyanin, and allophycocyanin, which are found in the wild type are found in all the Mendelian and non-Mendelian mutants examined. Some mutants overproduce R-phycoerythrin while others possess only traces of phycobiliprotein; however, no phycoerythrin minus mutants were found. Two of the mutants are unique; one overproduces phycocyanin relative to allophycocyanin while the nuclear mutant obr synthesizes a phycoerythrin which is spectroscopically distinct from the R-phycoerythrin of the wild type. The phycoerythrin of obr lacks the typical absorption peak at 545 nanometers characteristic of R-phycoerythrin and possesses a phycoerythrobilin to phycourobilin chromophore ratio of 2.6 in contrast to a ratio of 4.2 found in the wild type. Such a lesion provides evidence for the role of nuclear genes in phycoerythrin synthesis. In addition, comparisons are made of the pigment compositions of the Gracilaria strains with those of Neoagardhiella bailyei, a macrophytic red alga which has a high phycoerythrin content, and Anacystis nidulans, a cyanobacterium which lacks phycoerythrin. The mutants described here should prove useful in the study of the genetic control of phycobiliprotein synthesis and phycobilisome structure and assembly.
