Photosynthesis, as a fundamental element in the life process, is integrated in the evolution of living systems on the basis of hydrogen cycles on various hierarchic levels. Conversion of radiant energy enables the oxidation of water, whereby free oxygen accumulates in the atmosphere. Hydrogen is (reversibly) stored in organic materials formed under reductive CO2-fixation and by the incorporation of the other elements, which are necessary for living systems. All endergonic processes in living cells are finally driven by the energy released through the clean recombination of protons and electrons with oxygen to water. Duration of the stored energy and the complexity of the systems thus produced is correlated negatively with the conversion efficiency of the radiation energy. Entropy is a unifying principle in the evolution of living systems, inclusive human societies.
To obtain a clearer understanding of the photosensitization process, we have investigated the effect of photosensitization on the photochemical and non-photochemical energy dissipation in green leaves of wheat (Triticum aestivum L.) and lettuce (Lactuca sativa L.), pretreated with 5-aminolaevulinic acid (ALA) for 2–24 h in the dark, using chlorophyll (Chl) fluorescence quenching analysis and measurement of the influence on CO2 uptake and xanthophyll cycle pigments. In response to dark pretreatment, leaves accumulated high levels of protochlorophyllide (PChlide) and non-metabolized ALA. The dark pretreatment had no effect on the intrinsic photochemical efficiency of photosystem II (PS II). Although quantitative differences exist between wheat and lettuce, exposure to actinic light caused significant effects with a similar overall response pattern in both plant species. Changes in excitation energy dissipation were obtained already by a very low photon flux density of 85 μmol photons m−2 s−1 within a few minutes CO2 uptake was almost completely suppressed in photosensitized leaves, but they were able to build up the ΔpH necessary to drive de-epoxidation of violaxanthin. Fluorescence quenching analysis revealed that a progressive increase in non-radiative energy dissipation (measured as the non-photochemical quenching of Chl fluorescence, qN) was paralled by a stronger reduction in the primary quinone electron acceptor of PS II (QA) with increasing time of ALA pretreatment in the dark. In the early stages of photosensitization, high-energy state quenching mainly contributed to qN. In the later stages, qN was dominated by a photoinhibitory component together with the photodegradation of xanthophyll cycle pigments, in particular antheraxanthin and zeaxanthin. The latter phenomenon appeared to be promoted in response to a highly increased reduction state of QA. If ALA was simultaneously applied to leaves with 4,6-dioxoheptanoic acid, which acts as a competitive inhibitor of the enzyme ALA dehydratase, photosensitization disappeared when PChlide synthesis was completely inhibited, but was enhanced when inhibition was incomplete. Electron paramagnetic resonance studies using the spin trapping technique revealed a specific ALA-mediated formation of hydroxyl and carbon-centred radicals in response to actinic light exposure of chloroplasts. On the basis of these findings, a possible role of ALA is proposed: ALA enhances the photosensitizing effect(s) triggered by PChlide.
The formation of 5-aminolevulinic acid (ALA), energy-dependent steps from ALA to protoporphyrin IX (Proto) and from Proto to protochlorophyllide (PChlide) formation, the roles of NADPH in PChlide photoreduction and geranylgeraniol hydrogenation, the source of adenylates and reduced pyridine nucleotides for the reactions of chlorophyll biosynthesis, and the compartmentation and interrelationships of porphyrin biosynthesis pathways are reviewed.
Details of the morphology and anatomy of the coleoptile of wheat plants are given; these have not been described adequately previously. The investigations focussed on the hexaploid summer wheat Triticum aestivum L. cv. Hatri, and three taxa with different ploidy levels. In darkness the longitudinal growth of the coleoptile was delayed by nearly 24 h but the final length, reached after 120 h, was double that of coleoptiles of plants cultivated under continuous light. As soon as the coleoptile has grown, the primary leaf pushes through a pore pre-formed during the meristematic stage and located 1–1·5 mm behind the apex. The pore is stabilized mechanically by anastomosing of the originally free ends of the vascular bundles, as well as by increased lignification in this region. The species investigated differ in length, f. wt and d. wt, size of epidermal cells, and especially in the size of guard cells of the coleoptiles. The number of parenchyma layers, however, shows no specificity.
The highest activity of lipoxygenase during seedling development of wheat was found in the dry caryopsis. In all other organs of light- and dark-grown plants the activity was high only in very early stages of development, strongly decreasing with increasing age. In primary leaves soluble and membrane-bound lipoxygenase could be detected only up to the 6th d of development. Hence lipoxygenase cannot be included in senescence processes. Moreover, it is not connected with the developing photosynthetic apparatus since etiolated seedlings show principally the same lipoxygenase dynamics as light grown plants.
Anatomical changes in roots of wheat seedlings (Triticum aestivum L. cv. Hatri) following oxygen deficiency in the rooting medium were investigated. The response of the plant to stress was tested at a very early developmental stage when the first adventitious roots had just emerged. In order to analyze the adaptation of different roots, respiration rates of the roots 1–3 and 4–n were compared with the respiration rates of the total root system. Oxygen deficiency was induced either by flushing nutrient solution with nitrogen or flooding of sand. In contrast to plants grown in well aerated media, both stress variants led to a significant increase of the intercellular space of the root cortex in seminal and first adventitious roots. Radial cell enlargement of cortical cells near the root tip, cell wall thickenings in flooded sand cultures and an increase in phloroglucinol-stainable substances were found to be further indicators of low oxygen supply. The roots 4–n which were promoted in growth under hypoxia showed higher respiration rates; hence the total root respiration was not restricted.
Exogenous linolenic acid is able to inhibit photosynthetic electron transport of isolated wheat chloroplasts if the molar ratio linolenic acid to chlorophyll is greater than 10:1. Addition of serum albumin after incubation with linolenic acid does not induce restoration of activity, but linolenic acid treatment performed in the presence of serum albumin does not result in an inhibition of the electron transport rate. Application of linolenic acid to wheat chloroplasts isolated from senescing leaves has been shown to be less effective than to chloroplasts from younger leaves. These results do not support the view that loss of activity of chloroplasts during senescence is caused by liberation of free unsaturated fatty acids.
In dry and 16h soaking caryopses of 9 selected evolutionary wheat forms the contents of AMP, ADP and ATP were estimated and their energy charge calculated. The data reflect no clear quantitative relations between adenylate content and genome or dry weight. However, in those caryopses with the smaller endosperm reserves the de novo synthesis of adenylates starts earlier. The results are discussed in relation to the importance of the adenylates for the short-term and long-term regulation of the energy metabolism in plants.
Preliminary results indicated the existence of a DCMU-resistant photosynthetic electron transport. Additional experiments were made to characterise the photochemical activity in isolated chloroplasts as well as the gas exchange and pigment content of primary leaves of Triticum aestivum, treated with DCMU during greening and in the fully greened state, respectively. In the light etiolated Triticum leaves are able to develop an active photosynthetic apparatus in the presence of DCMU but as in green treated leaves — the photochemical activity and chlorophyll content are reduced. Analyzing the electron transport in chloroplasts of Triticum aestivum and 8 selected evolutionary forms of wheat under influence of different concentrations of DCMU, we found a resistant electron transport component in every case. It is discussed as a possible bypath via cyt b559 regulating the direct electron transfer from the acceptor side of PS II to DCPIP.
Preliminary results indicated the existence of a DCMU-resistant photosynthetic electron transport. Additional experiments were made to characterise the photochemical activity in isolated chloroplasts as well as the gas exchange and pigment content of primary leaves of Triticum aestivum, treated with DCMU during greening and in the fully greened state, respectively. In the light etiolated Triticum leaves are able to develop an active photosynthetic apparatus in the presence of DCMU but as in green treated leaves — the photochemical activity and chlorophyll content are reduced. Analyzing the electron transport in chloroplasts of Triticum aestivum and 8 selected evolutionary forms of wheat under influence of different concentrations of DCMU, we found a resistant electron transport component in every case. It is discussed as a possible bypath via cyt b559 regulating the direct electron transfer from the acceptor side of PS II to DCPIP.
The transition from the heterotrophic to the autotrophic phase of development was characterized in seedlings of Triticum aestivum L. and Phaseolus vulgaris L. in the course of normal development as well as after illumination of etiolated seedlings. The activity of NADP-GADPH increases (induced by light) to the same extent as the efficiency of the photosynthetic apparatus is developed. After initial stimulation, the activity of NAD-GADPH (as well as of the G6PDH/6PGDH system) decrease, an ‘enzymatic compensation point’ indicating the autotrophic phase of development reached. Experimental delays of the developmental course by NaCl or CCC treatment shift the entry into the autotrophic phase of nutrition, the same ATP contents being ensured by a complex regulative mechanism between photosynthesis and respiration.
Zusammenfassung Salicornia-Pflanzen salzreicher Boden zeichnen sich gegenuber Pflanzen weniger haliner Standorte durch prostraten Wuchs, verringertes Frisch- und Trockengewicht und verminderten Stickstoffgehalt aus. Die Chlorophyllbildung ist stark herabgesetzt, dafur kommt es aber zur vermehrten Ausbildung von Betacyanen. Durch eine zusatzliche Stickstoffdungung uber wochentliche Bespruhung von Versuchsparzellen derartiger Salicornieta mit 1 % Harnstoff in Meerwasser lassen sich diese Mangelerscheinungen — bei Aufrechterhaltung aller ubrigen Milieufaktoren — nahezu oder vollig aufheben. Die hervorragende Bedeutung des Stickstoffs vor allem fur die Zunahme des Chlorophyllgehaltes und fur die Verminderung der Betacyansynthese wird unter besonderer Berucksichtigung des Halophytenproblems diskutiert, und es wird die Schlusfolgerung gezogen, das nicht ausschlieslich der erhohte Salzgehalt und ein Mangel an Nahrstoffen im Substrat, sondern auch deren erschwerte Ausbeute seitens des Wurzelsystems der Pflanze die Ursache fur das beschriebene Verhalten der Pflanzen auf stark halinen Standorten darstellt.
