Changes in the Rate of Photosynthesis during Grain Filling and the Enzymatic Activities Associated with the Photosynthetic Carbon Metabolism in Rice Panicles

Photosynthesis is known to occur in rice panicles, but littlehas been reported about the photosynthetic or biochemical characteristicsof such panicles. The estimated gross amount of photo-syntheticallyassimilated CO₂ in a panicle is 30% of that in a flag leaf.This result and the good light-intercepting characteristicsof the panicle in the canopy suggest that photosynthesis inthe panicle may contribute significantly to grain filling. Therice panicle is composed of spikelets and of rachis-branchesincluding rachis which have estimated gross rates of photosynthesisduring the 30-day period after anthesis of 130 to 180 and 50to 100 µmol CO₂.(mg Chl)^–1.h^–1, respectively.The corresponding rate for the flag leaf is 180 to 230 µmolCO₂.(mg Chl)^–.h^–. On the basis of Chl, spikeletshave a high photosynthetic capability which is similar to thatof the flag leaf. The activities of ribulose-l,5-bisphosphate carboxylase (RuBPCase),phosphoenolpyruvate carboxylase (PEPCase), and pyruvate.P_i dikinase(PPDK) in spikelets were 129, 220, and 87 µmol.(mg Chl)^–.h^–,respectively. The activities of PEPCase and PPDK in spikeletswere considerably higher than those in the flag leaf or rachis-branches.Oxygen-insensitive photosynthesis was found only in spikelets.The K_m of NaHCO₃ for photosynthesis by slices of spikelets inan aqueous solution (0.6 mM) was considerably lower than thatfor slices of flag leaf (4.2 mM). All these results indicatethat spikelets have different photosynthetic characteristicsfrom those of the flag leaf and rachis-branches. The possibilityof C₃–C₄ intermediate photosynthesis or C₄-like photosynthesisin spikelets is discussed. ⁴Present address: Department of Biochemistry, Faculty of Science,Saitama University, Urawa, 338 Japan (Received February 14, 1990; Accepted June 12, 1990)     

Microclimate, Canopy Structure and Photosynthesis in Canopies of Three Contrasting Temperate Forage Grasses: III. Canopy Photosynthesis, Individual Leaf Photosynthesis and the Distribution of Current Assimilate

The rates of canopy and individual leaf photosynthesis and ¹⁴Cdistribution for three temperate forage grasses Lolium perennecv. S24, L. perenne cv. Reveille and Festuc'a arundinacea cv.SI70 were determined in the field during a summer growth period.Canopy photosynthesis declined as the growth period progressed,reflecting a decline in the photosynthetic capacity of successiveyoungest fully expanded leaves. The decline in the maximum photosyntheticcapacity of the canopies was correlated with a decline in theirquantum efficiencies at low irradiance. Changes in canopy structureresulted in changes in canopy net photosynthesis and dark respiration.No clear relationships between changes in the environment andchanges in canopy net photosynthesis and dark respiration wereestablished. The relative distributions of ¹⁴C in the shootsof the varieties gave a good indication of the amount of drymatter per ground area in the varieties.     

Algal 14C and total carbon metabolisms. 2. Experimental observations with the diatom Skeletonema costatum

Three sets of comparisons of net and gross inorganic carbonassimilation and ¹⁴C uptake were made with an axenic cultureof Skdetonema costatum. The comparisons showed that in the physiologicalwindow studied (10–20% of the intrinsic generation timeand gross photosynthesis/respiration ratios of 2–3), ¹⁴Cuptake into the paniculate plus the dissolved fractions approximatedto net photosynthesis. Rate constants derived from the chemicallydetermined changes were used to parameterize models that accountedfor the respiration of photosynthetic products and for the recyclingof respiratory CO₁₄. The conclusion drawn was that over thetime scale studied, the ¹⁴C technique was measuring net photosynthesis,consistent with essentially 100% recycling of respiratory CO₂.The study has shown that we now possess the basis to make arigorous analysis of net, gross CC₄ fixation and net ¹⁴C uptake,and forms the first step in the development of algorithms forthe interpretation of ¹⁴C field observations.     

Relationship between Leaf Structure and Gas Exchange in Wheat Leaves at Different Insertion Levels

Net photosynthesis rate (P_n), stomatal conductance to CO₂ andresidual conductance to CO₂ were measured in the last six leaves(the sixth or flag leaf and the preceding five leaves) of Triticumaestivum L. cv. Kolibri plants grown in Mediterranean conditions.Recently fully expanded leaves of well-watered plants were alwaysused. Measurements were made at saturating photosynthetic photonflux density, and at ambient CO₂ and O₂ levels. The specificleaf area, total organic nitrogen content, some anatomical characteristics,and other parameters, were measured on the same leaves usedfor gas exchange experiments. A progressive xeromorphic adaptation in the leaf structure wasobserved with increasing leaf insertion levels. Furthermore,mesophyll cell volume per unit leaf area (V_mes/A) decreasedby 52·6% from the first leaf to the flag leaf. Mesophyllcell area per unit leaf area also decreased, but only by 24·5%.However, nitrogen content per unit mesophyll cell volume increasedby 50·6% from the first leaf to the flag leaf. This increasecould be associated to an observed higher number of chloroplastcross-sections per mm² of mesophyll cell cross-sectional areain the flag leaf: values of 23000 in the first leaf and 48000in the flag leaf were obtained. P_n per unit leaf area remainedfairly constant at the different insertion levels: values of33·83±0·93 mg dm^–2 h^–1 and32·32±1·61 mg dm^–2 h^–1 wereobtained for the first leaf and the flag leaf, respectively.Residual conductance, however, decreased by 18·2% fromthe first leaf to the flag leaf. Stomatal conductance increasedby 41·7%. The steadiness in P_n per unit leaf area across the leaf insertionlevels could be mainly accounted for by an opposing effect betweena decrease in V_mes/A and a more closely packed arrangement ofphotosynthetic apparatus. Adaptative significance of structuralchanges with increasing leaf insertion levels and the steadinessin P_n per unit leaf area was studied. Key words: Photosynthesis, structure, wheat     

Growth Rate, Photosynthesis and Respiration in Relation to Leaf Area Index

This work examined three possible explanations of growth rateresponses to leaf area index (LAI) in which growth rate perunit of ground area (crop growth rate, CGR) increased to a plateaurather than decreasing above an optimum LAI at which all lightwas intercepted. Single leaf photosynthetic measurements, andwhole plant 24 h photosynthesis and respiration measurementswere made for isolated plants and plants in stands using Amaranlhushybridus, Chenopodium album, and two cultivars of Glycine maxgrown at 500 and 1000 µimol m^–2 S^–1 photosyntheticphoton flux density at 25 °C. CGR, relative growth rate(RGR), and LAI were determined from 24 h carbon dioxide exchangeand leaf area and biomass measurements. Respiration increasedrelative to photosynthesis with crowding in A. hybridus andthere was an optimum LAI for CGR. In contrast, the ratio ofrespiration to photosynthesis was constant across plant arrangementin the other species and they had a plateau response of CGRto LAI. Neither increased leaf photosynthetic capacity at highLAI nor a large change in biomass compared to the change inLAI could account for the plateau responses. It was calculatedthat maintenance respiration per unit of biomass decreased withdecreasing RGR in C. album and G. max, but not A. hybridus,and accounted for the plateau response of CGR to LAI. Sincesimilar decreases in maintenance respiration per biomass atlow RGR have been reported for several other species, a constantratio of respiration to photosynthesis may occur in more speciesthan constant maintenance respiration per unit of biomass. Amaranlhus hybridus L., Chenopodium album L., Glycine max L Merr, soybean, photosynthesis, respiration, growth, leaf area index     

Short-Term Fluctuations in the Transport of Assimilates to the Ear of Wheat Measured with Steady-State 11C-CO2-Labelling of the Flag Leaf

In order to study the regulation of photosynthate import intothe ear of wheat (Triticum aestivum L. cv. Anza), a system wasdeveloped in which the flag leaf, the major source of photosynthatefor the ear, was provided with steady-state levels of ¹¹C-CO₂for periods of several hours each time. ¹¹C is a short-livedisotope (1? c. 20?3 mm) whose breakdown products (followingpositron annihilation) include paired, high energy -rays (0?5MeV). Consequently, the movement of radioactive photosynthatethroughout the plant and into the ear could be studied in vivoduring multiple labelling sessions over the development of theear with detectors located at various positions around the plant.It was found that import into the ear was not constant in thelight during periods of rapid ear filling, even though flagleaf photosynthesis and export were constant. In more than 50%of all cases, import of ¹¹C-photosynthate into the ear tookthe form of large, regular oscillations with period lengthson the order of 70 mm. Furthermore, oscillations 180? out-of-phasewith those in the ear were observed in portions of the stembelow the point of flag leaf insertion, suggesting that regulationof oscillation may involve shifting the direction of transportbetween ear and lower plant parts. Rhythms in photosynthateimport into the ear do not appear to be synchronized to immediatefluctuations in the environment, since oscillations detectedsimultaneously in neighbouring plants were not synchronizedand had different period lengths. Key words: Triticwn aesrivum L., ¹¹CO₂, radioactive photosynthate, flag leaf, wheat ear     

Differentiating Day from Night Effects of High Ambient [CO2] on the Gas Exchange and Growth of Xanthium strumarium L. Exposed to Salinity Stress

Sodium chloride, at a concentration of 88 mol m^-3in half strengthHoagland nutrient solution, increased dry weight per unit areaofXanthium strumarium L. leaves by 19%, and chlorophyll by 45%compared to plants grown without added NaCl at ambient (350µmol mol^-1) CO₂concentration. Photosynthesis, per unitleaf area, was almost unaffected. Even so, over a 4-week period,growth (dry weight increment) was reduced in the salt treatmentby 50%. This could be ascribed to a large reduction in leafarea (>60%) and to an approx. 20% increase in the rate ofdark respiration (Rd). Raising ambient [CO₂] from zero to 2000 µmol mol^-1decreasedRd in both control and salinized plants (by 20% at 1000, andby 50% at 2000 µmol mol^-1CO₂concentration) compared toRd in the absence of ambient CO₂. High night-time [CO₂] hadno significant effect on growth of non-salinized plants, irrespectiveof day-time ambient [CO₂]. Growth reduction caused by salt wasreduced from 51% in plants grown in 350 µmol mol^-1throughoutthe day, to 31% in those grown continuously in 900 µmolmol^-1[CO₂]. The effect of [CO₂] at night on salinized plants depended onthe daytime CO₂concentration. Under 350 µmol mol^-1day-time[CO₂], 900 µmol mol^-1at night reduced growth over a 4-weekperiod by 9% (P <0.05) and 1700 µmol mol^-1reduced itby 14% (P <0.01). However, under 900 µmol mol^-1day-time[CO₂], 900vs . 350 µmol mol^-1[CO₂] at night increasedgrowth by 17% (P <0.01). It is concluded that there is both a functional and an otiose(functionless) component to Rd, which is increased by salt.Under conditions of low photosynthesis (such as here, in thelow day-time [CO₂] regime) the otiose component is small andhigh night-time [CO₂] partly suppresses functional Rd, therebyreducing salt tolerance. In plants growing under conditionswhich stimulate photosynthesis (e.g. with increased daytime[CO₂]), elevated [CO₂] at night suppresses mainly the otiosecomponent of respiration, thus increasing growth. Consequently,in regions of adequate water and sunlight, the predicted furtherelevation of the world atmospheric [CO₂] may increase plantsalinity tolerance. Xanthium strumarium ; respiration; photosynthesis; salt stress; sodium chloride; carbon dioxide; atmosphere     

Biochemical Characteristics Associated with the Development of the Desiccation-sensitive Seeds of Machilus thunbergii Sieb. & Zucc.

Biochemical properties, i.e. endogenous abscisic acid, proline,sugars, respiration, adenosine phosphates and adenylate energycharge, and growth and moisture content were measured duringthe development of seeds of Machilus thunbergii. As dry matteraccumulated in the embryo during development, moisture content,ABA, proline, respiration and sugars all declined. At maturity,the dry mass of the seeds failed to attain a plateau beforethe period of natural seed shedding; the axis and cotyledonsreached moisture contents of 58 and 45%, respectively. Dryingof immature seeds at 73% relative humidity and 25 °C for30 d resulted in a complete loss of viability at all developmentalstages tested with the exception of mature seeds that were ableto tolerate a 5% decrease in moisture content before germinationdeclined. ABA was detected in all embryos tested, with a maximum value16.·16 µg g^-1 d. wt about midway through development.Although the presence of ABA induced no tolerance to desiccationof mature seeds, it did coincide with decreased content of waterin the developing seeds and decreased respiration. Desiccationdamage of M. thunbergii seeds occurred when moisture contentwas still high (45%) and this damage was not related to theabsence of oligosaccharides in the mature seeds. We concludethat developing embryos and mature seeds of M. thunbergii haveproperties common to many recalcitrant seeds, with seeds beingsensitive to desiccation at all stages, having a prominent ABApeak, little proline, lacking oligosaccharides, and specifically,little dormancy and a moderate rate of respiration of matureseeds (0·9 µmol O₂ min^-1 g^-1 f. wt). Adenosinetriphosphate content and energy charge decreased from stagefour to stage eight of seed development, then increased againto 103 nmol g^-1 d. wt and 0·73, respectively, in matureseeds. The moderate energy charge observed in mature seeds indicatesthat continuous metabolism is also a characteristic of recalcitrantseeds.Copyright 1995, 1999 Academic Press Machilus thunbergii, seed development, recalcitrant seed, abscisic acid, energy charge     

Metabolic changes of the Antarctic green alga Prasiola crispa subjected to water stress investigated by in vivo 31P NMR

The energy status and the phosphate metabolism of Prasiola crispduring and after desiccation stress was investigated by in vivo³¹P NMR. The effect of desiccation was simulated by additionof the nonionic osmoticum PEG 200 (polyethylene glycol). Photosynthesisand respiration were effectively inhibited under these conditions.The most notable changes in the in vivo ³¹P NMR spectra werean increase in the cytoplasmic inorganic phosphate signal afterPEG stress, a decrease in the polyphosphates and a lowfieldshift of the core polyphosphate signal followed by an appearanceof extracellular inorganic phosphate. Cytoplasmic pH remainedalmost constant during stress. After a return to control conditions,photosynthesis and respiration recovered within 4 h as wellas the concentrations of the phosphorus metabolites. An as yetunassigned phosphate signal increased in the phosphodiesterregion of the NMR spectra. Simultaneousty, the polyphosphatesignal recovered in intensity and chemical shift. It is suggestedthat phosphate metabolism and complexation of cations to polyphosphatesmay play an important role in the distinct desiccation toleranceof P. crispa. Key words: In vivo ³¹P NMR, Prasiola crispa, desiccation tolerance, polyphosphates     

A global assessment of mesozooplankton respiration in the ocean

Published data on the biomass and specific respiration ratesof mesozooplankton in the oceans across all latitudes were combinedto assess their community respiration on a global basis. Mesozooplanktonbiomass was higher in boreal/anti-boreal and polar waters, intermediatein equatorial waters and lowest in the subtropical gyres. Specificrespiration rates were the highest in equatorial waters anddecreased rapidly poleward. Global community respiration ofmesozooplankton in the upper 200 m of the oceans integratedover all latitudes was 10.4 ± 3.7 (SE) Gt C year^–1(n = 838). Below the epipelagic zone, mesozooplankton respirationliving in the mesopelagic (200–1000 m) and bathypelagic(below 1000 m) zones was estimated as 2.2 ± 0.4 (n =57) and 0.40 ± 0.2 (n = 12) Gt C year^–1, respectively.Thus, global depth-integrated mesozooplankton respiration was13.0 ± 4.2 Gt C year^–1 (17–32% of globalprimary production), which is 3–8-fold higher than thevalues assigned to mesozooplankton respiration in recent estimatesof total respiration in the ocean. Thus, it appears that mesozooplanktonrepresent a major, but neglected component of the carbon cyclein the ocean.     

Metabolism of Barley Leaves Inoculated with Erysiphe graminis Merat

Inoculating Proctor barley leaves with Erysiphe graminis decreasedrates of photosynthesis, after an initial lag period, and increasedrespiration. Increasing the area inoculated progressively decreased ratesof photosynthesis, but the effects cannot be attributed to asimple loss of leaf area. When less than 30 per cent of a leafwas inoculated, decreases were equivalent to area losses greaterthan those inoculated; when more than 30 per cent was inoculatedthe photo-synthetic losses were equivalent to area losses lessthan those inoculated. Although the relative effects of E. graminis on photosynthesisand respiration were of the same order, the absolute effectson photosynthesis were greater than those on respiration. Inoculating30 per cent of a leaf decreased photosynthesis by 5–6mg CO₂/dm²/hr from 12.9 in the uninoculated controls to 7.3.Respiration increased by 0.6 mg CO₂/dm/hr, from 1.7 to 2.3-     

Algal 14C and total carbon metabolisms. 1. Models to account for the physiological processes of respiration and recycling

A consistent set of equations has been written to describe thenet rate of algal ¹⁴CO₂ uptake (and where appropriate respirationand photosynthesis) which take into account separately complicationsdue to respiration of the labelled photosynthetic products andthe recycling of respiratory CO₂. Written specifically intothe equations is the concept of ‘new’ and ‘old’carbon, the coefficient q is used in the respiration model toallow for the differential respiration of organic material fromthe ‘new’ and ‘old’ carbon pools. Analyticalintegrals have been found for respiration and recycling models,and the behaviour of the models studied over periods of 12 h(i.e. up to 70% of the intrinsic generation time). The rateconstant for respiration has a greater effect on the behaviourof the recycling than the respiration model. Over short timecourses (up to 30% of the intrinsic generation time), the effectsof respiration and recycling on net ¹⁴CO₂ uptake are quite distinct,especially at high P/R ratios, and not complicated by assumptionsover the value of q. Although the value of q will have a time-dependentsecondary effect on the modelled total carbon-specific respirationrate, this was found not to give rise to major problems of interpretation.Beyond 50% of the intrinsic generation time, the separate treatmentof respiration and recycling in the models becomes less satisfactory.It was concluded that the present equations, which are not constrainedby mass balance considerations, would not be appropriate fora model that combines the two processes. The pattern of recyclingat low P/R values is identified as one of the major uncertaintiesin producing models of ¹⁴C uptake. The effect of the releaseof dissolved organic material can be anticipated in a generalway. The models have been used to define an experimental strategyto establish the separate effects of respiration and recyclingon the time course of net ¹⁴C uptake. The initial rates givethe clearest resolution of the two processes and it would appearthat with photosynthetic rates in the region of 1 day^–1,incubation periods up to 3–6 h would be suitable to determinethe importance of recycling in controlling net ¹⁴C uptake. Withthe present models, only in the absence of recycling could theeffect of respiration be studied and the value of q established.     

Effect of the Photosynthetic Light Period on the Carbon Budget of Young Tomato Leaves

This study was carried out to elucidate the carbon budget inyoung tomato plants in photosynthetic light periods of 8, 12and 16 h after being acclimated to an 8-h light period. Thephotosynthetic rate was nearly constant for 16 h: thereforethe amount of ¹⁴C fixed was proportional to the light period(13·77 mg C per 8 h, 20·2 mg C per 12 h, 30·5mg C per 16 h). The amounts exported, lost by respiration and accumulated whenexpressed as percentages of the carbon fixed in the day differedlittle between the light periods. The leaf continued to exportcarbon at a nearly constant rate during the light periods and,for example, exported approximately twice as much during a 16-hperiod as in an 8-h light period, even though it was not acclimatedto the long light period. The amount of starch breakdown affected the amount of carbonexported and carbon lost by respiration at night, although itwas not sufficient to account for these losses altogether. Thepossible roles of carbon accumulation and respiration are alsodiscussed. Carbon budget, steady state feeding, ¹⁴C, photosynthesis, respiration, translocation, carbon metabolism, tomato, Lycopersicon esculentum Mill     

Effect of Nitrogen Fertilizer on Photosynthesis of Several Varieties of Winter Wheat

The rates of gross photosynthesis of the flag leaf and the nextleaf below (second leaf) in crops of winter wheat were estimatedfrom the ¹⁴C uptake of the leaves after exposure to short pulsesof ¹⁴CO₂. The photosynthetic rates of both leaves during thegrain-filling period decreased with increase in nitrogen fertilizerbecause the intensity of photosynthetically active radiationwas less at the surface of the leaves in the dense crops withadditional nitrogen. In addition, the rate of photosynthesisat saturating light intensity was slightly decreased by nitrogen.The effects of nitrogen, in decreasing the rate of photosynthesisper unit area of leaf and in increasing the leaf-area indexof the top two leaves, were such that the photosynthetic productivityper unit area of land of the flag leaf was increased by nitrogenbut the productivity of the second leaf was unaffected. Applying180 kg N ha^–1 increased the productivity of the top twoleaves by a factor of 2.3 but increased grain yield by only1.8. The photosynthetic productivity of the second leaf duringthe grain-filling period was about half that of the flag leaf. There was no difference in photosynthetic rate per unit areaof leaves of Cappelle-Desprez and Maris Huntsman which couldaccount for the larger yield of the latter cultivar. There wasa slight indication that the leaves of the semi-dwarf cultivarsMaris Fundin and Hobbit photosynthesized faster than those ofMaris Huntsman. Triticum aestivum L., winter wheat, photosynthesis, nitrogen fertilizer     

Respiratory Loss of Recently Assimilated Carbon in Wheat

A series of experiments was undertaken to assess the amountof respiration associated with the growth of wheat at differentstages. Plants (or in some cases just the flag leaf) were labelledwith ¹⁴CO₂ and the amount of ¹⁴CO₂ respired during the subsequent48 or 72 h was measured. The evolution of ¹⁴C, expressed asa percentage of the amount initially assimilated (referred toas the R/A value) was used as a measure of the overall efficiencyof dry matter production. Respiratory ¹⁴CO₂ evolution from labelledplants was most rapid in the first 12 h after labelling, thereafterdeclining rapidly. Evolution was also more rapid following labellingsat the end of the light period (dusk) than at the beginningof it (dawn). The R/A values were greatest (42 and 50 per centrespectively for dawn- and dusk-labelled plants) for young plantsand least (13 and 28 per cent respectively) for plants duringmid grain filling. When flag leaves, as distinct from wholeplants, were labelled, R/A values were lower still (9 and 21per cent respectively), indicating that flag leaf assimilatewas used efficiently in grain production. The calculated minimum R/A for the formation of grain material(10 per cent protein, 90 per cent starch) was 6.2 per cent.That the experimentally determined values were greater thanthis is attributed to the turnover of carbon in enzymes, toother maintenance processes, and possibly to the operation ofthe pentose phosphate pathway of glucose oxidation. R/A valueswere lower in those plants labelled at the beginning than thoseat the end of the photoperiod. This was considered to be a consequenceof refixation of respiratory ¹⁴CO₂ during the light. The higherR/A values found for young plants were considered to be a consequenceof the greater percentage of ¹⁴C translocated to the roots (rootsbeing unable to refix respired CO₂) and of greater turnoverof enzymes associated with more active metabolism. Triticum, wheat, respiration, carbon assimilation, carbon loss, grain-filling     

Effect of Nitrogen on Growth, Yield and Photorespiratory Activity in Spring Wheat

Spring wheat plants growing in pots in controlled environmentrooms were given extra nitrogen after flag leaf emergence. Theeffect of nitrogen on growth, yield, the activity of ribulose1,5–bisphosphate carboxy–lase/oxygenase and thedistribution of¹⁴C in photorespiratory intermediates and indifferent parts of the plants was determined. Extra nitrogenincreased the movement of ¹⁴C to the ear and increased grainyield by 29 per cent, mainly because of an increase in grainnumber. Though extra nitrogen delayed senescence of the leaves,the growth of the ear in the later stages was not increasedin proportion to the extra green area. The relative inefficiencyof leaf area with extra nitrogen, which has also been foundin the field, was not due to a reduction in photosynthesis perunit leaf area. Nor was there evidence of an increase in photorespirationas reflected by a greater flow of carbon into the photorcspiratorymetabolites glycine and serine, or an increase in the activityof ribulose 1,5–bisphosphate oxygenase relative to thecarboxylase. We suggest that there may be an increase in theloss of carbon in dark respiration. Triticum aesttvum, nitrogen, growth, yield, photorespiration     

Maintenance Respiration and Carbon Balance of Plants at Low Levels of Sodium Chloride Salinity

The daily course of carbon influx and efflux was measured inyoung plants of Phaseolus vulgaris, Xanthium strumarium, Zeamays, and Atriplex halimus, exposed to low levels of salinity(NaCl) and varying daytime light intensities. Maintenance respiration(R_M) was calculated. In Phaseolus, Xanthium, and Atriplex, R_Mrose with increasing salinity, approximately up to those levelsof salinity above which apparent signs of toxicity appear. Athigher levels of salinity R_M declined. There was no responseof R_M to salinity in Zea. At the levels of salinity tested,salinity did not affect the ratio of growth respiration to photosynthesis. At –5 x 10⁵ Pa of NaCl salinity, the increase in R_M inXanthium was calculated to account for 24% of the growth reductioncaused by salt. The remainder could be ascribed to reduced photosynthesis.The increase of R_M is considered to be indicative of an adaptivemechanism, not present in the very salt-sensitive Zea.     

The Acquisition and Utilization of Carbon in Early Spring by Kiwifruit Shoots

Measurements of net photosynthetic rate (at 1450µ molm^-2s^-1photosynthetically active radiation) of leaves, of leafand stem respiration, and of shoot growth of potentially-fruitinglaterals on kiwifruit (Actinidia deliciosa ) were used to estimateweekly shoot carbon balances over the first 10 weeks of shootgrowth (budburst to anthesis). Consistent differences in therate of shoot elongation, of internode expansion and of increasein basal diameter were found among shoots. Faster-growing (long)shoots acquired carbon by photosynthesis at a faster rate evenin the first few weeks after budburst, but the amount of carbonrequired to sustain this growth resulted in shoot carbon deficitswhich were approx. seven times greater than those of the slower-growing(short) shoots. It was estimated that the transition from shootcarbon deficit to carbon surplus occurred 3–4 weeks afterbudburst, irrespective of shoot growth rate. As a result ofsubsequent rapid increases in shoot photosynthetic rate, longshoots had a shoot carbon surplus of 4.4 g C week^-1in the weekbefore anthesis, approx. three times that of the short shoots.Defoliation (66%) of shoots 1 week after budburst, and subsequentremoval of later-emerging leaves to maintain the level of defoliation,had the effect of slowing shoot growth in the carbon deficitperiod, particularly for the long shoots. However, the durationof shoot expansion in the defoliated shoots was longer, resultingultimately in shoots which were longer than the control shoots.Linkages among early carbon balance dynamics of shoots, shootlength at anthesis, and fruit growth are discussed. Actinidia deliciosa ; kiwifruit; shoot growth; carbon acquisition; respiration; photosynthesis     

Flag Leaf Photosynthesis of Triticum aestivum and Related Diploid and Tetraploid Species

Rates of net photosynthesis of the flag leaves of 15 genotypesof wheat and related species were measured throughout theirlife, using intact leaves on plants grown in the field. At thestage when rates were maximal, they were in general highestfor the diploid species, intermediate for the tetraploidspeciesand lowest for Triticum aestivum (means of 38, 32 and 28 mgCO₂ dm^–2 h^–1 respectively). Rates were stronglynegatively correlated with leaf area, leaf width and the meanplan area per mesophyll cell and positvely correlated with stomatalfrequency and number of veins per mm of leaf width. The differencesamong species in these attributes were mainly related to ploidylevel. It was not possible to determine the relative importanceof each anatomical feature, though the changes in stomatal frequencyhad only slight effects on stomatal conductance and the observeddifferences in rates of photosynthesis were much greater thanwould be expected from those in stomatal conductance alone. There was genetic variation in rates of light dependent oxygenevolution of isolated protoplasts and intact chloroplasts butno difference attributable to ploidy. The mean rate, 91 µmolO₂ mg^–1 chlorophyll h^–1, equivalent to 3.9 mg CO₂mg^-1chlorophyll h^-1 was considerably less than the rate of photosynthesisin comparable intact leaves, which was 7.2 mg CO₂ mg^–1chlorophyll h^–1. The total above-ground dry matter yields were least for thewild diploids T. urartu and T. thauodar and the wild tetraploidT. dicoccoides, but the other wild diploids produced as muchdry matter as the hexaploids. The prospects of exploiting differences in photosynthetic ratein the breeding of higher yielding varieties are discussed. Triticum aestivum L., wheat, Aegilops spp, photosynthesis, stomatal conductance, stomatal frequency, polyploidy     

Optimal Nitrogen Content and Photosynthesis in Cauliflower (Brassica oleracea L. botrytis). Scaling up from a Leaf to the Whole Plant

A simple model of photosynthesis is described which is dependenton leaf area, organic nitrogen content and distribution withinthe canopy as well as on the light and temperature environments.The model is parameterized using a cauliflower crop as an example.The optimized protein-N profile within the canopy is calculatedwith respect to daily growth rate. By comparison with measuredprotein-N contents, the amount of super-optimal N-uptake, i.e.the N-uptake which does not increase productivity, is assessedfor two different nitrogen and light treatments. The amountof super-optimal N accumulated in cauliflower depends on N-supplyand can exceed 80 kg N ha^-1. Copyright 2000 Annals of BotanyCompany Brassica oleracea L. botrytis, cauliflower, nitrogen, photosynthesis, respiration, model, optimization