Plant Growth in Polyethylene Glycol Solutions in Relation to the Osmotic Potential of the Root Medium and the Leaf Water Balance

The changes in leaf extension, plant dryweight, leaf area, netassimilation rate (E), relative growth-rate (R_W), and relativeleaf growth-rate (R_L), have been studied for four species grownfor 2 weeks in solutions of polyethylene glycol 4000 of controlledosmotic potentials. All aspects of growth were decreased bydecreasing the osmotic potential (_sol) of the root medium andthe leaf water potential (), and ceased when _/ was greater than— 10 bars in bean, cotton, maize. These plants were moresusceptible than ryegrass to water stress. Growth of bean stoppedat equal to about —6 bars, but even at —10 barsryegrass was capable of some growth. Slight decrease in fromthe values in the control plants decreased growth during thefirst week but partial recovery was apparent during the secondweek's growth in solution culture, when leaf extension, E, R_Land R_W increased in plants subjected to stress. Examinationof the water balance, water potential, osmotic potential andturgor of the leaf in relation to relative water content suggeststhat recovery was related to increased turgor and that the abilityof the plants to grow at reduced values of the osmotic potentialof the root medium and of the leaf water potential depend onthe maintenance of turgor.     

Abscisic Acid may Mediate the Rapid Thermal Acclimatization of Photosynthesis in Wheat

Exposing cold-grown (6°C) plants of a winter cultivar (Avalon)and a spring cultivar (Highbury) of wheat to a single warm night(18 h and 25°C) significantly increased the photosyntheticrate and stomatal conductance of fully-expanded leaves withoutaffecting the calculated, substomatal CO₂ pressure. Prior to the warm-night treatment net photosynthesis and stomatalconductance in Avalon were very small at high leaf temperaturesduring measurement (25–30°C). Such high temperatureinhibitions were not observed in Highbury. Exposure to the warm-nighttreatment alleviated these inhibitions in Avalon. Consequently,the temperature response of photosynthesis was similar in theleaves of the treated cultivars and showed a relatively broadoptimum. The application of exogenous abscisic acid in the transpirationstream to the leaves of treated plants caused a reduction ofsimilar proportions in the steady-state rate of photosynthesisand in stomatal conductance and essentially reversed the effectsof the warm-night treatment in a manner which depended on thecultivar. The endogenous abscisic acid content of leaves declinedby 50% during the warm-night treatment. In conjunction, thesedata suggest that changes in the content of abscisic acid inthe leaf may regulate the observed temperature acclimatizationof apparent photosynthesis in the fully-expanded leaves of cold-grownwheat. Key words: Abscisic acid, photosynthesis, stomatal conductance, temperature, acclimatizatio     

The Effect of Temperature on Photosynthesis and Carbon Fluxes in Sunflower and Rape

Sunflower (Helianthus annuusL.) and oilseed rape (Brassica napusL.) were grown at constant temperatures of 30 ?C (warm) and13 ?C (cold). Maximal rates of photosynthesis between 5 ?C and35 ?C were at higher temperatures in sunflower than rape. Photosyntheticrate over 4 h at the growth temperature declined in warm-andcold-grown rape and cold-grown sunflower, but remained constantin warm-grown sunflower. The stimulation of photosynthesis by2.0 kPa O₂ compared to 21 kPa O₂ declined with decreasing temperature.At 10 ?C in warm-grown rape photosynthesis was insensitive to2.0 kPa O₂. However, sensitivity to low O₂ continued at 10 ?Cin warm-grown sunflower. Carbohydrates accumulated in the cold,particularly fructose, glucose and sucrose in warm-grown sunflowertransferred to 13 ?C. By monitoring changes of ¹⁴C in leaves after the assimilationof ¹⁴CO₂, the rates of carbon export from leaves, pool sizesand carbon fluxes between them were estimated. The transferof warm- and cold-grown rape to 13 ?C and 30 ?C, respectively,had little effect on these parameters over 22 h. However, exportof carbon from sunflower leaves at 13 ?C was markedly less thanat 30 ?C, irrespective of the growth temperature, due to slowerexport from the transport pool. The rapid suppression of carbonexport at 13 ?C in warm-grown sunflower may be due to inhibitedtranslocation rather than reduced sink demand in the cold. It is concluded that assimilate utilisation is more depressedin the cold than is photosynthesis; this imposes a greater restrictionon biomass production in sunflower than in rape. Key words: Sunflower, rape, temperature, photosynthesis, carbon fluxes     

In vivo photosynthetic electron transport does not limit photosynthetic capacity in phosphate-deficient sunflower and maize leaves

The effects of extreme phosphate (Pi) deficiency during growth on the contents of adenylates and pyridine nucleotides and the in vivo photochemical activity of photosystem II (PSII) were determined in leaves of Helianthus annuus and Zea mays grown under controlled environmental conditions. Phosphate deficiency decreased the amounts of ATP and ADP per unit leaf area and the adenylate energy charge of leaves. The amounts of oxidized pyridine nucleotides per unit leaf area decreased with Pi deficiency, but not those of reduced pyridine nucleotides. This resulted in an increase in the ratio of reduced to oxidized pyridine nucleotides in Pi-deficient leaves. Analysis of chlorophyll a fluorescence at room temperature showed that Pi deficiency decreased the efficiency of excitation capture by open PSII reaction centres (φ_e), the in vivo quantum yield of PSII photochemistry (φ_PSII) and the photochemical quenching co-efficient (q_P), and increased the non-photochemical quenching co-efficient (q_N) indicating possible photoinhibitory damage to PSII. Supplying Pi to Pi-deficient sunflower leaves reversed the long-term effects of Pi-deficiency on PSII photochemistry. Feeding Pi-sufficient sunflower leaves with mannose or FCCP rapidly produced effects on chlorophyll a fluorescence similar to long-term Pi-deficiency. Our results suggest a direct role of Pi and photophosphorylation on PSII photochemistry in both long-and short-term responses of photosynthetic machinery to Pi deficiency. The relationship between φ_PSII and the apparent quantum yield of CO₂ assimilation determined at varying light intensity and 21 kPa O₂ and 35 Pa CO₂ partial pressures in the ambient air was linear in Pi-sufficient and Pi-deficient leaves of sunflower and maize. Calculations show that there was relatively more PSII activity per mole of CO₂ assimilated by the Pi-deficient leaves. This indicates that in these leaves a greater proportion of photosynthetic electrons transported across PSII was used for processes other than CO₂ reduction. Therefore, we conclude that in vivo photosynthetic electron transport through PSII did not limit photosynthesis in Pi-deficient leaves of sunflower and maize and that the decreased CO₂ assimilation was a consequence of a smaller ATP content and lower energy charge which restricted production of ribulose, 1-5, bisphosphate, the acceptor for CO₂.     

Compartmental modelling of photorespiration and carbon metabolism of water stressed leaves

Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO₂ and O₂ concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO₂ is suggested to explain the discrepancy. The model suggests that resistance to CO₂ fixation is mainly in the carboxylation reactions, not in CO₂ transport. Using the steady state model, the kinetics of ¹⁴C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of ¹⁴C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO₂ released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.     

Sink-Regulation of Photosynthesis in Relation to Temperature in Sunflower and Rape

Stimulation of the rate of photosynthesis at 2·0 kPaO₂ in comparison with 21 kPa O₂ and carbohydrate accumulationover 4h were measured during exposure of sunflower (Helianthusannuus L.) and rape (Brassica napus L.), grown at 30 °Cand 13 °C, to temperatures between 7 °C and 35 °C.The effect of reducing source: sink ratio by shading on theresponse of photosynthetic rate to temperature was also determined.Stimulation of photosynthesis by 2·0 kPa O₂ in comparisonwith 21 kPa O₂ decreased over 4 h at cool temperatures in sunflowerplants grown at 30 °C but not in rape grown at 30 °C.Stimulation did not decrease over 4 h in plants grown at 13CC. Sucrose was the main carbohydrate accumulated over 4 h;its accumulation increased with decreasing temperature. Starchaccumulation either decreased or remained the same with decreasingtemperature. In plants grown at 30 °C more carbohydrateaccumulated between 8 °C and 21 °C in sunflower thanin rape, but more carbohydrate accumulated at 30 °C in rapethan in sunflower. In plants grown at 13 °C much less carbohydrateaccumulated between 13 °C and 23 °C than in plants grownat 30 °C. Photosynthetic rate in plants grown at 30 °Cexposed to between 20 °C and 35 °C over 32 h (14 h light-10h dark-8 h light), declined over 32 h at 20 °C and 25 °Cin sunflower and at 20 °C in rape. This fall over 32 h,especially at 20 °C in sunflower, was significantly reducedby shading the rest of the plant. Shading had little effecton photosynthetic rate above 25 °C. The work confirms thatlow temperature imposes a sink-limitation on photosynthesiswhich occurs at higher temperatures in sunflower than in rape.This limitation may be relieved by decreasing the source:sinkratio. Key words: Sunflower, rape, photosynthesis, carbohydrates, sink demand, temperature     

Limitation to Photosynthesis in Water-stressed Leaves: Stomata vs. Metabolism and the Role of ATP

Decreasing relative water content (RWC) of leaves progressivelydecreases stomatal conductance (g_s), slowing CO₂ assimilation(A) which eventually stops, after which CO₂ is evolved. In somestudies, photosynthetic potential (A_pot), measured under saturatingCO₂, is unaffected by a small loss of RWC but becomes progressivelymore inhibited, and less stimulated by elevated CO₂, below athreshold RWC (Type 1 response). In other studies, A_pot andthe stimulation of A by elevated CO₂ decreases progressivelyas RWC falls (Type 2 response). Decreased A_pot is caused byimpaired metabolism. Consequently, as RWC declines, the relativelimitation of A by g_s decreases, and metabolic limitation increases.Causes of decreased A_pot are considered. Limitation of ribulosebisphosphate (RuBP) synthesis is the likely cause of decreasedA_pot at low RWC, not inhibition or loss of photosynthetic carbonreduction cycle enzymes, including RuBP carboxylase/oxygenase(Rubisco). Limitation of RuBP synthesis is probably caused byinhibition of ATP synthesis, due to progressive inactivationor loss of Coupling Factor resulting from increasing ionic (Mg²⁺)concentration, not to reduced capacity for electron or protontransport, or inadequate trans-thylakoid proton gradient (pH).Inhibition of A_pot by accumulation of assimilates or inadequateinorganic phosphate is not considered significant. DecreasedATP content and imbalance with reductant status affect cellmetabolism substantially: possible consequences are discussedwith reference to accumulation of amino acids and alterationsin protein complement under water stress.     

Photosynthesis, Respiration, and Carbon Assimilation in Water-Stressed Maize at Two Oxygen Concentrations

Photosynthesis decreased with decreasing leaf water potentialas a consequence of stomatal closure and possibly non-stimataleffects of severe stress. Assimilation ceased at c. 16x 10⁵Pa. Photo-respiration, in 21% O₂, was small in relation to assimilationin unstressed leaves and decreased as leaf water potential fellbut it was much larger in proportion to photosynthesis at severestress. Decreasing the O₂ content to 1.5% increased photosynthesisslightly and decreased photo-respiration but did not changethe stress at which assimilation stoped. Dark respiration wasinsensitive to both O₂ and stress. Less ¹⁴C accumulated in stressedleaves but in 21% O₂ a greater proportion of it was in aminoacids, particularly glycine and serine. 1.5% O₂ decreased the¹⁴C in glycine to 10% and in serine to 50% of their levels in21% O₂. In both O₂ concentrations the proportion of ¹⁴C in serineincreased only at the most severe stress. Gas exchange measurementsand changes in the ¹⁴C flux to glycine are interpreted as theresult of glycolate pathway metabolism increasing as a proportionof assimilation in stressed leaves in high O₂. The small absoluterate of photorespiration in high O₂ and at low leaf water potentialmay be due to slow rates of glycine decarbodylation as wellas efficient fixation of any CO₂ produced. Serine is synthesizedby an O₂-sensitive pathway and an O₂-insensitive pathway, whichis most active at severe stress. Synthesis of alanine competeswith that of glycine and serine for a common precursor suppliedby the photo-synthetic carbon reduction cycle. The relativespecific radioactivities of aspartate and alanine suggest thatthey are derived from a common precursor pool, probably pyruvatefrom 3-PGA. The amounts of 3-PGA, aspartate, malate, alanine,and sucrose decreased with increasing water stress as a consequenceof slower assimilation and pool filling. Other amino acids,glycine, serine, glutamate, and proline, accumulated at lowwater potential possibly due to increased synthesis and slowerrates of consumption. Changes in pool sizes, carbon fludes,and specific activities of metabolites are related to the mechanismof C₄ photosynthesis and current concepts of glycolate pathwaymetabolism.     

Photosynthesis by Flag Leaves of Wheat in Relation to Protein, Ribulose Bisphosphate Carboxylase Activity and Nitrogen Supply

The effects of nitrate supply on the composition (cell numbers,protein and chlorophyll contents) of flag leaves of winter wheatgrown with two amounts of N fertilizer and of spring wheat grownin the glasshouse under controlled nitrate supply are describedand related to photosynthesis. Nitrogen deficiency decreasedthe size of leaves, mainly by reducing cell number and, to asmaller extent, by decreasing cell volume. Protein content perunit leaf area, per cell and per unit cell volume was largerwith abundant N. Total soluble protein, ribulose bisphosphatecarboxylase-oxygenase (RuBPc-o) protein and chlorophyll changedin proportion irrespective of nitrogen supply and leaf age.Photosynthesis per unit area of flag leaf and carboxylationefficiency in both winter and spring wheat were proportionalto the amount of total soluble protein up to 7.0 g m^–2and to the amount of RuBPc-o protein up to 4.0 g m^–2.However, photosynthesis did not increase in proportion to theamount of total soluble or RuBPc-o protein above these amounts.In young leaves with a high protein content the measured ratesof photosynthesis were lower than expected from the amount andactivity of RuBPc-o. Carboxylation per unit of RuBPc-o protein,measured in vitro, was slightly greater in N-deficient leavesof winter wheat but not of spring wheat. RuBPc-o activity perunit of RuBPc-o protein was similar in winter and spring wheatleaves and remained approximately constant with age, but increasedin leaves showing advanced senescence. RuBPc-o protein fromN-deficient leaves migrated faster on polyacrylamide gels thanprotein from leaves with high N content. Regulation of the rateof photosynthesis in leaves and chloroplasts with a high proteincontent is discussed. The conductance of the cell to the fluxof CO₂ from intercellular spaces to RuBPc-o active sites iscalculated, from cell surface areas and CO₂ fluxes, to decreasethe CO₂ partial pressure at the active site by less than 0.8Pa at an internal CO₂ partial pressure of 34 Pa. Thus the decreasein partial pressure of CO₂ is insufficient to account for theinefficiency of RuBPc-o in vivo at high protein contents. Otherlimitations to the rate of photosynthesis are considered. Key words: Wheat, photosynthesis, nitrogen, ribulose, bisphosphate carboxylase     

The Control of Water and Carbon Dioxide Flux in Water-stressed Sugar Beet

Changes in leaf and canopy water potential of sugar beet growingin soil of decreasing water content depended on soil water potentialand were independent of water flux from the plant when thiswas varied by changing the water vapour content of the air.The calculated hydraulic conductance of the plant increasedas flux increased and decreased as leaf water potential decreasedand as the plant aged. The conductances to water vapour of individualleaves and of the canopy decreased as leaf water potential decreasedand increased with increasing humidity of the air. The lattereffect was independent of changes in leaf water potential. Theconductances were not affected by the rate of evaporation orleaf temperature. The rate of photosynthesis was directly relatedto leaf conductance except in severely stressed, mature leavesin which leaf water potential had a more direct effect on photosynthesis.Stomatal conductances, transpiration, and photosynthesis weregreater in young leaves than mature leaves on the same plantand at the same leaf water potential. These results are discussedin relation to current agricultural irrigation practices usedfor sugar beet.