摘要:

ABSTRACTWe describe here an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone. The determination of xylem [ABA] and the stomatal response to xylem [ABA]are likely to involve the water flux through the plant. (1) If, as seems likely, the production of a chemical message depends on the root water status (Ψr), it will not depend solely on the soil water potential (Ψs) but also on the flux of water through the soil‐plant‐atmosphere continuum, to which are linked the difference between Ψrand Ψs. (2) The water flux will also dilute the concentration of the message in the xylem sap. (3) The stomatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water‐flux‐dependent message, with a water‐flux‐dependent sensitivity. In such a system, we have to consider a common regulation for stomatal conductance, leaf and root water potentials, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the generation and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the solution suggested that such common regulation can operate. Simulations suggest that, as well as providing control of stomatal conductance, integration of chemical and hydraulic signalling may also provide a control of leaf water potential and of xylem [ABA], features which are apparent from our experimental data. We conclude that the root message would provide the plant with a means to sense the conditions of water extraction (soil water status and resisance to water flux) on a daily timescale, while the short‐term plant response to this message would depend on the e

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00880.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTA range of approaches was used to investigate how species within a fire‐proneBanksiawoodland in South West Australia exploited inorganic soil nitrogen sources and how this changes through the development of the fire chronosequence. Nitrate and ammonium were present in soil solution throughout the chronosequence but nitrate predominated in recently burnt sites. Mean shoot nitrate reductase activities were high for all species in recently burnt sites and showed little increase when nitrate was supplied via the transpiration stream. Nitrate reductase of shoots of most species was low at sites not burnt for several years, but following transpirational induction with nitrate, developed activities similar to those at recently burnt sites. The principal amino compounds transported in the xylem were species specific, including asparagine, glutamine and citrulline‐dominated species, and changed little in relative composition across the chronosequence. Species most active in leaf nitrate reduction transported the largest amounts of nitrate in their xylem sap and proportional amounts of nitrate in xylem tended to be greatest in recently burnt sites. Most of the species examined appeared to be shoot rather than root nitrate assimilators, but marked differences were recorded in potential of leafy shoots of different species to reduce nitrate. As a general rule, shallow‐rooted herbaceous, non‐mycorrhizal or VAM‐positive species had the highest capacity to reduce nitrate, whereas woody species with ericoid mycorrhizae or combined vesicular arbuscular/ectomycorrhizal associations exhibited little capacity to reduce nitrate in roots or shoots. It seems likely that this latter group utilize ammonium or even organic forms of nitrogen rather than nitrate. Some putative nitrogen‐fixing species were active in reducing and transporting nitrate, others were virtually inactive in the

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00881.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTStudies of the variation in δ15N values for plants from a fire‐proneBanksiawoodland in South West Australia showed that pioneer herbaceous, non‐mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such speciesPtilotus polystachusdemonstrated a close correspondence between the δ15N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ15N values and moderately active in nitrate reduction and storage. Nitrogen‐fixing species had the lowest δ15N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ15N values which fell within the range of values obtained for the miscellaneous assemblage of N2‐fixing species. Consequently,15N abundance values failed to distinguish N2fixing from non‐fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00882.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTGlasshouse experiments withRicinus communisshowed that the presence/absence of a VA mycorrhizal fungus (Glomus clarum) changed the δ15N value of the host by as much as 2‰ when the plants were given urea (released as NH4+) as their only N‐source. This small change in Δ15N would create a large error in calculating sources of plant N. In particular, these results throw into doubt any models of N‐cycling which assume that soil N can be treated as a single source. The correct N‐source value for VAM‐infected NH4−‐using plants may be the δ15N of soil NH4++ 2‰. Treatment effects were also found in the distribution of δ15N and % N among plant organs. Plants with VAM had a lower N:P atom ratio and were larger in total biomass. Carbon discrimination (δ13C) was greater in the VA‐infected plants. The measured effects of VAM infection suggest that for some plants the fungus may be the primary site of N assimilation. A parallel experiment withEucalyptus globulusand the ectomycorrhizal fungusHydnangium carneumresulted in no significant differences in any of the variables measured for this host‐fungus pair when the sole N‐sources were inorganic (NO3−and NH4+released from urea). Ectomycorrhizal fungi are diverse in their physiological behaviour, and these data should not be taken as being representative of the whole group. More work is required with other types of mycorrhiza and more complex sources of N. Future work will include a water balance to partition the effects of water use and nutrient supply in determining δ13C. An on‐line combustion‐ANCA‐MS method is described for fully automated measurement of natural abundance levels of15/14N and13/12C for plant materials. This method achieves the required precision while dramat

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00883.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThe role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidumvardurumL.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO2diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (Rd) was greater than the net photosynthesis rate (Pn) by a factor of 1.74 in the spike, whereas Rdwas much smaller, only 22.1, 65.7 and 24.8% of Pnin blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO2exchange. Results of13C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high Rdvalues of ears as well as their low diffusive conductance suggest that CO2from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO2as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear‐darkened plants, probably because of some degree of refixation (recycling) of res

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00884.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTA plastid‐localized isozytne of 3‐deoxy‐D‐arabino‐heptulosonate 7‐phosphate (DAHP) synthase, denoted DS‐Mn, has been identified in a number of higher‐plant species. Parallel characterizations were made of DS‐Mn fromSpinacia oleracealeaf tissue,Solanum tuberosumtubers, andNicotiana silvestrissuspension culture as sources of enzyme from plant materials which vary in phytogeny, developmental and tissue state, and physiological state. A highly conserved property of DS‐Mn is a transition between inactive and active states, mediated by DTT as a hysteretic activator. A procedure for isolation of DS‐Mn in the labile, inactive state is given. The process of activation appears to exhibit a higher pH optimum than the catalytic optimum. DTT‐containing preparations are very stable. The enzyme characteristically exhibits stimulation by Mn++in the range of 45–50%, relatively high affinity for erythrose‐4‐phosphate (E4P), dramatic substrate inhibition above about 0.5mol m−3E4P, sigmoid substrate saturation curves for both E4P and phosphoenolpyruvate, and inhibition byL‐arogenate (competitive against E4P and non‐competitive against PEP). DS‐Mn has a relatively high temperature optimum in the range of 45–50°C. Enzyme activity was lost when bound metal was stripped away by EDTA treatment. Reconstitution of the native‐enzyme level of activity was obtained with Ca++, and additional stimulation was achieved with Mn++. DS‐Mn control byL‐arogenate in the chloroplast is proposed as one key circuit in an overall pattern of allosteric control for the entire pathway of aromatic amino acid biosynthesis. This pattern is called sequential feedback inhibition. The potential for modulation of this control system by environmental cues

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00885.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTWhen exposed to osmotic stress,Mesembryanthemum crystallinumplants switch from C3to CAM photosynthesis. Phosphoenolpyruvate carboxylase (PEPCase) is a key enzyme in CAM plants, being responsible for the initial fixation of CO2. In C3plants the enzyme has been shown to be involved in the replenishing of TCA cycle intermediates and in the operation of stomatal guard cells. Multiple PEPCase isoforms were observed in C3‐performing leaves with four isoelectric points of 5.2, 5.5, 5.6 and 5.9 and four apparent subunit molecular masses of 105, 108, 113 and 116 kDa. In some instances, subunits of different size possessed exactly the same pI. The induction of CAM led to the predominance of a new isoform of pI 6.5 with subunit molecular mass of 108 kDa, but in addition, changes were observed in some of the isoforms present in the C3plant. PEPCase subunits were purified from the C3and CAM forms ofM. crystallinumand subjected to pep‐tide mapping. Two distinct though similar sets of maps were obtained, one from the CAM isoform (pI 6.5) and C3‐associated subunits of pi 5.9 and another for C3subunits of pI 5.2 and 5.5. It was inferred from these data that the C3isoforms expressed in the leaf were derived from at least two genes. The C3isoform (pI 5.9) showing greatest similarity to the CAM isoform in terms of peptide mapping also increased in response to salt stress. It is speculated that the CAM isoform may have evolved from this e

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00886.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTA model of maize stomatal behaviour has been developed, in which stomatal conductance is linked to the concentration of abscisic acid ([ABA]) in the xylem sap, with a sensitivity dependent upon the leaf water potential (Ψ1). It was tested against two alternative hypotheses, namely that stomatal sensitivity to xylem [ABA] would be linked to the leaf‐to‐air vapour pressure difference (VPD), or to the flux of ABA into the leaf. Stomatal conductance (gs) was studied: (1) in field‐grown plants whose xylem [ABA] and Ψ1depended on soil water status and evaporative demand; (2) in field‐grown plants fed with ABA solutions such that xylem [ABA] was artificially raised, thereby decreasinggsand increasing Ψ1and leaf‐to‐air VPD; and (3) in ABA‐fed detached leaves exposed to varying evaporative demands, but with a constant and high Ψ1. The same relationships betweengs, xylem [ABA] and Ψ1, showing lower stomatal sensitivity to [ABA] at high Ψ1, applied whether variations in xylem [ABA] were due to natural increase or to feeding, and whether variations in Ψ1, were due to changes in evaporative demand or to the increased Ψ1observed in ABA‐fed plants. Conversely, neither the leaf‐to‐air VPD nor the ABA flux into the leaf accounted for the observed changes in stomatal sensitivity to xylem [ABA]. The model, using parameters calculated from previous field data and the detached‐leaf data, was tested against the observations of both ABA‐fed and droughted plants in the field. It accounted with reasonable accuracy for changes ings(r2ranging from 0.77 to 0.81). These results support the view that modelling of stomatal behaviour requires consideration of both chemical and hydraulic aspects

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00887.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThis investigation determined whether thylakoid proteins would be degraded more rapidly or not in senescing wheat (Triticum aestivumL. em. Thell.) leaves concurrently exposed to high temperatures. Excised leaves were pulse‐labelled with [35S]‐methionine for a 12 h period, and then incubated at 22,32 or 42°C for 0, 1, 2, or 3 d, before extracting a thylakoid enriched membrane sample. After electrophoretic separation, two prominent [35S]‐labelled protein bands were chosen for further analyses. Band A contained the D‐1 thylakoid protein and band B contained thylakoid proteins of the light harvesting complex (LHCII) associated with photosystem II (PSII). Total protein, [35S]‐labelled protein, band A protein, and band B protein within the thylakoid enriched membrane samples were measured. Unlabelled thylakoid enriched membrane samples, extracted from leaves given similar treatments, were used to measure uncoupled whole‐chain and photosystem II (PSII) electron transport and chlorophyll fluorescence. Accentuated decline in whole‐chain and PSII electron transport, increasing Fovalues, and decreasing Fmaxvalues were a result of high temperature injury in leaves treated at 42°C. None of the thylakoid enriched membrane protein fractions were degraded more rapidly in high‐temperature treated leaves. Degradation of the total [35S]‐labelled membrane proteins and band B was inhibited by the 42°C treatment. The results indicate that high temperature stress may disrupt some aspects

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00888.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTStomatal control of crown transpiration was studied inAnacardium excelsum, a large‐leaved, emergent canopy species common in the moist forests of Central and northern South America. A construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35‐m‐tall individual. Stomatal conductance at the single leaf scale, and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively. This permitted the sensitivity of transpiration to a marginal change in stomatal conductance to be evaluated using a dimensionless coupling coefficient (1‐ω) ranging from zero to 1, with 1 representing maximal stomatal control of transpiration. Average stomatal conductance varied from 0.09 mol m−2s−1during the dry season to 0.3 mol m−2s−1during the wet season. Since boundary layer conductance was relatively low (0.4 mol m−2s−1), 1‐ω ranged from 0.46 during the dry season to only 0.25 during the wet season. A pronounced stomatal response to humidity was observed, which strongly limited transpiration as evaporative demand increased. The stomatal response to humidity was apparent only when the leaf surface was used as the reference point for measurement of external vapour pressure. Average transpiration was predicted to be nearly the same during the dry and wet seasons despite a 1 kPa difference in the prevailing leaf‐to‐air vapour pressure difference. The patterns of stomatal behaviour and transpiration observed were consistent with recent proposals that stomatal responses to humidity are based on sensing th

ISSN:0140-7791    

DOI:10.1111/j.1365-3040.1993.tb00889.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY