摘要:

Abstract.Heterotrophic assimilation of nitrate in roots and leaves in darkness is closely linked with the oxidative pentose phosphate pathway. The supply of glucose‐6‐phosphate to roots and chloroplasts in leaves in darkness is essential for assimilation of nitrite into amino acids. When green leaves are exposed to light, the key enzyme, glucoses‐phosphate dehydrogenase, is inhibited by reduction with thioredoxin. Hence the dark nitrate assimilatory pathway is inhibited under photoautotrophic conditions and replaced by regulatory reactions functioning in light. On account of direct photo‐synthetic reduction of nitrite in chloroplasts and availability of excess NADH for nitrate reduclase, the rate of nitrate assimilation is extremely rapid in light. Under dark anaerobic conditions also nitrate is equally rapidly reduced to nitrite on account of abolition of competition for NADH between nitrate reductase and mitochondrial ox

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589193

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.The osmotic behaviour of vacuolated plant cells (adaxial epidermal cells ofAllium cepabulb scales, and epidermal as well as chloroplast containing subepidermal stem base cells ofPisum sativum)was studied over a wide range of CaCl2concentrations. The following results were obtained.a. Allium cepaandPisum sativumplant cells behave as an ideal osmometer as far as plasmolytic contraction of the protoplast is concerned.b. The protoplasts of these cells could be plasmolysed to 15–45% of their original volume without the loss of membrane semi‐permeability.c. Cells plasmolysed in 1.0 kmol m−3CaCl2could be completely deplasmolysed and upon deplasmolysis the cells resumed protoplasmic streaming.d. The above findings (a‐c) indicate that during gradual plasmolysis and deplasmolysis membrane semi‐permeability is maintained.e. At very high plasmolysing concentrations vacuoles covered with the tonoplast separated from the rest of the protoplasm in some cells whereas others showed systrophy. Extruded vacuoles were able to respond to osmotic shrinkage.f. The non‐solvent space inAlliumcells of about 3% also corresponded to the protoplasm volume calculated from the protoplast geometry (mean from results of direct measurement method and subtraction method).g. Subepidermal stem base cells of water‐stress‐tolerantPisumplants had a 75% greater non‐solvent space than the control cells indicating that a water‐stress‐tolerant cell may contain a larger amount of protoplasm and/or a vacuole with a higher content of colloidal material in the vacuole.h. Water‐stress‐tolerant cells showed greater tolerance to osmotic dehydrat

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589197

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.The influence of leaf age, total leaf area and its dispersion in space on canopy photosynthesis were studied using microswards of red clover (Trifolium pratenseL.) which were established in the greenhouse. Two varieties, Renova (flowering) and Molstad (non‐flowering), were sown in separate plastic boxes at densities of 225, 400 and 625 plants per m2.Vertical distribution of photosynthetically active radiation (PAR), leaf area, leaf age and14CO2‐fixation were determined periodically. Net photosynthesis and dark respiration of canopies were measured. Maximum photosynthetic capacity of individual leaves was measured on plants taken from the intact canopy or from plants where shading of the growing leaves had been prevented.Net photosynthetic rate of canopies increased linearly with leaf area index (LAI) up to an LAI of 3.5 and then declined at higher LAI, independent of variety and sowing density. Below the optimum LAI, net photosynthesis depended mainly on interception of PAR. Decrease in canopy photosynthesis above the optimum LAI was due to a higher proportion of old leaves with decreased photosynthetic capacity, and not to an increase in respiring plant parts. It is concluded that LAI and position of leaf age categories in the canopy are more important than vertical distribution of leaf area in determining canopy photosynthesis of red clo

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589204

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.Treatment with weak acids (butyrate, isobutyrate, trimethylacetate, DMO) at a concentration of I mol m−3in apical maize root segments induced a rapid, marked hyperpolarization (ca.30 mV) of the transmembrane electrical potential, stable for at least 30 min. With butyrate, this effect increased with the increase of butyrate concentration in the medium, reaching a value ofca.75 mV at a concentration of 5 mol m−3.Both the butyrate uptake and the hyperpolarization were roughly proportional to the pH‐regulated, undissociated/dissociated acid ratio in the medium. The butyrate‐induced hyperpolarization was reduced progressively, but was still present when K+concentration in the medium was raised from 1 to 10 mol m−3.The hyperpolarization was accompanied by a significant increase of K+uptake, and was almost completely suppressed by the presence of the protonophore carbonylcyanid‐p‐trichlorometoxy‐phenylhydrazone (CCCP) and strongly reduced by erytrosin B, an inhibitor of some animal ATPases and of a K+‐activated, DCCD‐ and vanadate‐sensitive Mg2+‐ATPase from plant microsomes. The hyperpolarization effect of butyrate was additive to that of fusicoccin at low, but not at high (5 mol m−3), concentrations of the weak acid. These results suggest that the intracellular pH regulates the activity of the electrogenic proto

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589207

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.Transverse slices through developing grains ofTriticum aestivumcv. SUN 9E 16 d after anthesis were incubated in simple defined media with various radioactive labels. In some enzymic assays slices were pretreated with 2.5% Triton X‐100 or with 5% butanol to remove cellular membranes and endogenous substrates.Endogenous potassium leaked from endosperm slices into 30mol m−3sucrose while sucrose was converted partly into starch. Exogenous alkali‐ions, except Li+, stimulated conversion of sucrose to insoluble matter, specifically to starch with K+. Starch synthetase activity of Triton‐pretreated slices was stimulated by K+at both high and low substrate ADPG concentration, but was not affected by phosphate (25 mol m−3).Phosphate in the medium had no effect on incorporation of sucrose or glucose into alcohol‐insoluble material or starch in fresh slices (internal inorganic phosphate (P,) concentration was about 11 mol m−3). Three‐ to four‐fold contrasts in internal Pilevel, achieved by prolonged preincubations in different media, did not show an inhibition of starch synthesis by Pi. However, phosphate (25mol m−3) inhibited starch synthesis, that was mediated by ADPG pyrophosphorylase in butanol‐pretreated endosperm slices by 15–18%.It is concluded that starch synthesis in wheat endosperm is not regulated directly

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589214

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.We recently discussed a method for measuring optical properties of light scattering and absorbing plant tissue (Seyfried, Fukshansky&Schafer, 1983). This method has been used to measure the changes in optical properties of cotyledons between 360 and 1000 run during the first 7d of development. The seedlings were either etiolated or grown under continuous white light, the latter either herbicide‐treated (SAN 9789 = Norflurazon) or untreated. Some of the observed changes in seedlings grown under white light are due to chlorophyll accumulation. This accumulation leads to an increase in absorption coefficients at all wavelengths except in the 750 to 850 nm region. Reflectance, transmittance, and the scattering coefficient decreased markedly. Other changes seem to be independent of light conditions since they occur in much the same way under all treatments. These are a generally decreasing reflectance and scattering coefficient and an even stronger decrease of reflectance from the upper face of the cotyledon as compared to the reflectance from the lower face, in particular in the blue region of the spectrum. The observed changes are discussed in terms of light gradients and the resulting problems forin vivospectroscop

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589223

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.The effect upon potential maximum nitrogen uptake rate of root morphology and nitrogen availability in soil was investigated using a simple nutrient transport model. Parameter values appropriate to an ecological or an agricultural context were introduced from the literature. The model predicted that the maximum uptake rate of nitrate was morphology‐dependent only at extremely low concentrations. For ammonium, this was so for all realistic concentrations, assuming a high potential maximum uptake rate. The important concentration range for ammonium was two orders of magnitude greater than that for nitrate. With a lower potential maximum uptake rate of ammonium, root morphology was important below 15/igNg′ soil, the concentration range in this case being a single order of magnitude greater than that for nitrate. The effects of root hairs were to decrease the threshold concentration for morphology‐dependence, and to minimize root dry weight per unit volume of soil needed to maintain maximum nitrogen uptake rate. The effects of simultaneous mass flow of solution were negligible. The possible significance of these effects upon plant growth are discussed in relation to nitrogen availab

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589228

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.The influence of insulating dead leaves on water balance inEspeletia timotensisCuatr., an Andean caulescent giant rosette plant, was studied under field and laboratory conditions. Removal of the dead leaf layer surrounding the stem changed the pattern of diurnal stem temperature variation and produced transient and permanent effects on water balance. The pattern of liquid water flow resistance increase at low stem temperatures suggested that much of the water flow in the stem was through living membranes, probably those of the pith cells. The pith was determined to be an important source of stored water for daily transpirational needs. The lethal effects of dead leaf removal were attributed to one or more of the following causes: (1) inhibition of pith recharge by subfreezing stem temperatures; (2) embolisms in stem xylem; (3) freezing injury to pith tissue. The results suggested that an insulating layer of marcescent leaves and the presence of an internal water reservoir closer to the rosette than the soil water are important adaptations for maintenance of a favourable water balance in tropical alpine habitats where freezing temperatures occur regularly but last only a few hours.

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589230

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.Two anatomical variants ofPanicum maximumJacq. were observed to accumulate an unusually large number of starch grains in the bundle sheath chloroplasts when grown under controlled environmental conditions in a nutrient medium containing a low level of nitrate nitrogen (20 mg N dm−3as KNO3). When these plants were placed under dark conditions the chloroplasts were destarched, but exhibited a marked distortion of the thylakoid membranes. Under a higher level of nitrate nitrogen supply (200 mg N dm−3as KNO3) the number of starch grains was markedly reduced compared to that observed above in both plant variants. When the nitrogen was supplied as ammonium nitrogen (200 mg N dm−3as NH4Cl) there was again a high level of starch in the bundle sheath chloroplasts, the level being only slightly lower than that observed at the low KNO3supply. An unusually large number of starch grains accumulated in the bundle sheath chloroplasts in the absence of added phosphorus in the nutrient medium, in the presence of the higher nitrate nitrogen level. It is suggested that the increased starch accumulation results from a reduced trans‐location of Calvin cycle intermediates out of the chloroplasts into the cytoplasm and that both nitrate nitrogen and phosphorus may play an important role in this process. A good correlation between high net photosynthetic activity and low bundle sheath starch content was observed. Nutrient medium requirements favouring low starch content in chloroplasts also favoured high net photosyntheti

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589234

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY

摘要:

Abstract.The boundary layer resistance of model leaves was measured in still air, at a range of leaf‐to‐air temperature differences. The results were compared to those calculated from standard formulae for natural convection. The agreement between observed and calculated was only satisfactory when Grashof numbers exceeded about 105. At the lower Grashof numbers, which often prevail in nature, the observed rates of heat transfer considerably exceeded those calcula

ISSN:0140-7791    

DOI:10.1111/1365-3040.ep11589240

出版商:Blackwell Publishing Ltd    

年代:1983

数据来源: WILEY