Non-foliar photosynthesis – a strategy of additional carbon acquisition

https://mediatum.ub.tum.de/doc/1306470/file.pdf

Physiological responses of chickpea genotypes to terminal drought in a mediterranean-type environment

Contents

Summary 17

I. Introduction 17
II. Oxygen diffusion and the barriers to gas exchange 18
III. Metabolic indicators of seed hypoxia 19
IV. High-resolution mapping of oxygen distribution in the developing seed 20
V. The effect of environmental factors on steady-state oxygen concentrations 23
VI. Gene expression and metabolism under low oxygen 24
VII. Mechanisms of oxygen sensing and balancing in seeds 25
VIII. Might low internal oxygen levels be advantageous for seed development? 27

Acknowledgements 27


References 28



Summary
Recent applications of oxygen-sensitive microsensors have demonstrated steep oxygen gradients in developing seeds of various crops. Here, we present an overview on oxygen distribution, major determinants of the oxygen status in the developing seed and implications for seed physiology. The steady-state oxygen concentration in different seed tissues depends on developmental parameters, and is determined to a large extent by environmental factors. Photosynthetic activity of the seed significantly diminishes hypoxic constraints, and can even cause transient, local hyperoxia. Changes in oxygen availability cause rapid adjustments in mitochondrial respiration and global metabolism. We argue that nitric oxide (NO) is a key player in the oxygen balancing process in seeds, avoiding fermentation and anoxia in vivo. Molecular approaches aiming to increase oxygen availability within the seed are discussed.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2008.02752.x

The oxygen status of the developing seed.

Soybean (Glycine max) yields high levels of both protein and oil, making it one of the most versatile and important crops in the world. Light has been implicated in the physiology of developing green seeds including soybeans but its roles are not quantitatively understood. We have determined the light levels reaching growing soybean embryos under field conditions and report detailed redox and energy balance analyses for them. Direct flux measurements and labeling patterns for multiple labeling experiments including [U-(13)C(6)]-glucose, [U-(13)C(5)]-glutamine, the combination of [U-(14)C(12)]-sucrose + [U-(14)C(6)]-glucose + [U-(14)C(5)]-glutamine + [U-(14)C(4)]-asparagine, or (14)CO2 labeling were performed at different light levels to give further insight into green embryo metabolism during seed filling and to develop and validate a flux map. Labeling patterns (protein amino acids, triacylglycerol fatty acids, starch, cell wall, protein glycan monomers, organic acids), uptake fluxes (glutamine, asparagine, sucrose, glucose), fluxes to biomass (protein amino acids, oil), and respiratory fluxes (CO2, O2) were established by a combination of gas chromatography-mass spectrometry, (13)C- and (1)H-NMR, scintillation counting, HPLC, gas chromatography-flame ionization detection, C:N and amino acid analyses, and infrared gas analysis, yielding over 750 measurements of metabolism. Our results show: (i) that developing soybeans receive low but significant light levels that influence growth and metabolism; (ii) a role for light in generating ATP but not net reductant during seed filling; (iii) that flux through Rubisco contributes to carbon conversion efficiency through generation of 3-phosphoglycerate; and (iv) a larger contribution of amino acid carbon to fatty acid synthesis than in other oilseeds analyzed to date.

The role of light in soybean seed filling metabolism

Specific morphological and biochemical characteristics of seeds can cause oxygen deficiency within maternal and embryonic tissues. In this study, optical sensors were used to measure O(2) profiles across developing seeds of Vicia faba and Pisum sativum and developmental and environmental modulations of internal O(2) levels were studied. In addition, the metabolic state of developing embryos was analysed by monitoring adenylate energy charge, adenylate nucleotides and the levels of nucleotide sugars. Within the seed coat O(2) concentration decreased sharply to approximately 3% towards the inner border. Lowest O(2) levels were detected within the endospermal cavity between the seed coat and embryo. It is probable that low seed coat permeability provides an hypoxic environment for legume embryo development. The O(2) concentration in embryonic tissue changed during development with the lowest levels in the early stages. Measured in darkness, the levels were below 3%, but increased upon illumination indicating that photosynthesis significantly contributes to internal O(2) levels. Only in very young embryos were ATP levels and energy charge low. Otherwise they were maintained at a constant higher value. ADP-glucose and UDP-glucose did not show large fluctuations. Throughout embryo development fermentative activity did not play a major role. Obviously, specific mechanisms prevent seed tissues from becoming anoxic during development. The possible role of low oxygen on seed metabolism and on the control of seed development in legumes is discussed.

Legume embryos develop in a hypoxic environment

The nutritional economy of the developing fruit of Pisum sativum L. (cv. Greenfeast) was studied in terms of intake of translocate, incorporation of C and N into dry matter, transpiration, and CO 2 exchanges of the fruit with its external and internal atmospheres. The environmental conditions were 12-hr days (22 C, 850 μeinsteins m −2 sec −1 at fruit level); 12-hr nights of 15 C. Between 6 and 30 days after anthesis, pod photosynthesis resulted in small gains of CO 2 from the external atmosphere, and assimilated most of the CO 2 respired by the fruit during the day. From then until maturity (40 days) the fruit lost CO 2 during the day. Night losses of CO 2 increased with fruit age. The gas cavity of the fruit contained 0.15 to 1.5% (v/v) CO 2 . Lower levels were maintained in the day than at night. CO 2 levels were influenced by fruit age, radiant flux, and temperature. Labeled CO 2 injected into the gas cavity was fixed by the pod but not by seeds in the light, and by neither pod nor seeds in darkness. Dark-to-light or light-to-dark transfer of a fruit promoted rapid changes in CO 2 and O 2 levels of the gas space, consistent with a shift in the assimilation-respiration balance of the pod. The fruit transpired 27.6 cm 3 H 2 O per gram dry matter accumulated. Daytime ventilation was greatest 12 to 15 days after anthesis and declined as pod photosynthesis became increasingly involved in the retrieval of CO 2 respired by pod and seeds. Most, 69% by weight, of the translocate from the parent plant was converted to dry matter of seeds; nearly half, 45%, to useful seed reserves (sugar plus starch-protein-oil, 45:20:1). Illumination resulted in a fruit requiring 16% less translocate than if laying down an equal amount of dry matter in darkness.

Significance of Photosynthetic and Respiratory Exchanges in the Carbon Economy of the Developing Pea Fruit

The effects of CO 2 concentration and illumination on net gas exchange and the pathway of 14 CO 2 fixation in detached seeds from developing fruits of Lupinus albus (L.) have been studied. Increasing the CO 2 concentration in the surrounding atmosphere (from 0.03 to 3.0% [v/v] in air) decreased CO 2 efflux by detached seeds either exposed to the light flux equivalent to that transmitted by the pod wall (500 to 600 micro-Einsteins per square meter per second) in full sunlight or held in darkness. Above 1% CO 2 detached seeds made a net gain of CO 2 in the light (up to 0.4 milligrams of CO 2 fixed per gram fresh weight per hour) but 14 CO 2 injected into the gas space of intact fruits (containing around 1.5% CO 2 naturally) was fixed mainly by the pod and little by the seeds. Throughout development seeds contained ribulose-1,5-bisphosphate carboxylase activity (EC 4.1.1.39), especially in the embryo (up to 99 micromoles of CO 2 fixed per gram fresh weight per hour) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) in both testa (up to 280 micromoles of CO 2 fixed per gram fresh weight per hour) and embryo (up to 355 micromoles of CO 2 fixed per gram fresh weight per hour). In kinetic experiments the most significant early formed product of 14 CO 2 fixation in both light and dark was malate but in the light phosphoglyceric acid and sugar phosphates were also rapidly labeled. 14 CO 2 fixation in the light was linked to the synthesis of sugars and amino acids but in the dark labeled sugars were not formed.

Carbon Dioxide Fixation in the Carbon Economy of Developing Seeds of Lupinus albus (L.).

Thenutritional economy ofthedeveloping fruit ofPisumsativum L. (cv. Greenfeast) wasstudied intermsofintake oftranslocate, incorporation ofCandNinto drymatter, transpiration, andCO2exchanges ofthe fruit withitsexternal andinternal atmospheres. Theenvironmental conditions were12-hr days(22C,850yeinsteins m-2sec-' atfruit level); 12-hr nights of15C. Between 6and30daysafter anthesis, podphotosynthesis resulted in small gains ofCO2fromtheexternal atmosphere, andassimilated most oftheCO2respired bythefruit during theday. Fromthenuntil maturity (40days) thefruit lost CO2during theday.Night losses ofCO2increased withfruit age. Thegascavity ofthefruit contained 0.15to1.5%(v/v) CO2.Lower levels weremaintained inthedaythanatnight. CO2levels wereinfluenced byfruit age, radiant flux, andtemperature. Labeled CO2injected into thegascavity wasfixed bythepodbutnotbyseeds inthelight, and byneither podnorseeds indarkness. Dark-to-light orlght-to-dark transfer ofafruit promoted rapid changes inCO2and02levels ofthegas space, consistent withashift intheassimilation-respiration balance of thepod. Thefruit transpired 27.6cm3H20pergramdrymatter accumulated. Daytime ventilation wasgreatest 12to15daysafter anthesis anddeclined aspodphotosynthesis becameincreasingly involved intheretrieval ofCO2respired bypodandseeds. Most, 69%byweight, ofthe trndocate fromtheparent plant wasconverted todrymatter ofseeds; nearly hlf,45%,tousefl seedreserves (sugar plus starch-protein-oil, 45:20:1). Mumination resulted inafrit requiring 16%less trandocate thaniflaying downanequal amountofdrymatter indarkness.

Alastair M. Flinn

Papers

Significance of Photosynthetic and Respiratory Exchanges in the Carbon Economy of the Developing Pea Fruit

Carbon Dioxide Fixation in the Carbon Economy of Developing Seeds of Lupinus albus (L.).

Significance ofPhotosynthetic andRespiratory Exchanges inthe CarbonEconomyoftheDeveloping PeaFruit1