Effects of Temperature, Anoxia and Other Metabolic Inhibitors on Translocation

Partitioning and translocation of photosynthates were compared between a nonmutant genotype (Oh 43) of corn (Zea mays L.) and two starchdeficient endosperm mutants, shrunken-2 (sh2) and brittk-i (btl), with similar genetic backgrounds. Steady-state levels of "CO2 were supplied to source leaf blades for 2-hour periods, folowed by separation and identification of "C-assimilates in the leaf, kernel, and along the translocation path. An average of 14.1% of the total "C assimilated was translocated to normal kernels, versus 0.9% in sh2 kernels and 2.6% in bd kernels. Over 98% of the kernel 14C was in free sugars, and further analysis of nonmutant kernels showed 46% of this label in glucose and fructose. Source leaves of mutant plants exported significantly less total photosynthate (24.0% and 36.3% in sh2 and btl compared to 48.0% in the normal plants) and accumulated greater portions of label in the insoluble (starch) fraction. Mutant plants also showed lower percentages of photosynthate in the leaf blade and sheath below the exposed blade area. The starch-deficient endosperm mutants influence the partitioning and translocation of photosynthates and provide a valuable tool for the study of source-sink relations.A high economic yield of grain crops depends on efficient translocation of assimilates to kernel sink tissues. In the present study, maize mutants with reduced kernel capacity for starch accumulation have been used to examine the effects of sink strength on partitioning of assimilates in source leaves and their subsequent distribution within the plant. There are several advantages to using plants with mutations affecting sink metabolism for such a study. First, mechanical manipulation of source leaves or sink tissues can be avoided. Such manipulations might result in short-term decreases in phloem functioning (6) or atypical changes in the hormonal balance within the plant (9). Secondly, inhibitors are not used, avoiding other possible secondary effects (25).Two maize mutants, brittle-J (btl) and shrunken-2 (sh2), synthesize about 25% of the starch of nonmutant endosperms and consequently have shrunken endosperms at maturity (15 MATERIALS AND METHODS PLANT MATERIAL. Three corn genotypes were examined in this study; 'nonmutant' inbred (Oh 43) of maize (Zea mays L.), and two starch-deficient mutants, shrunken-2 and brittle-i in the same genetic background. We will refer to homozygous mutant sh2/sh2 plants as sh2 plants and to homozygous mutant seeds (sh2/sh2 embryos with associated sh2/sh2/sh2 endosperms) as sh2 seeds. The same type of abbreviation is used for brittle-i.Plants were grown in the field using 12-liter buried metal containers, watered as needed during the growing season, and selfpollinated. Development proceeded at rates similar to other fieldgrown inbreds. Experimental plants were transported to the laboratory for studies done hetween 20 and 23 d after pollination. "CO2 Labeling. Two-h, steady-state labeling studies were done by supplying 1'CO2 to a source leaf blade of each plant. The ear ...

Section: Resultssupporting

confidence: 91%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Source-Sink Relations in Maize Mutants with Starch-Deficient Endosperms

Koch¹,

Tsui²,

Schrader³

et al. 1982

“…The sugar supply of non-chlorophyllous tissues varies depending upon factors which affect the efficiency ofcarbon fixation by leaves and transport of the recently synthesized carbohydrates (5,7,13,21).…”

mentioning

confidence: 99%

Soluble Sugars, Respiration, and Energy Charge during Aging of Excised Maize Root Tips

Saglio¹,

Pradet²

1980

206

123

Oxygen uptake and energy charge were monitored during aging of excised maize root tips and related to the soluble sugar content and exogenous sugar supply.Oxygen uptake declined imsediately after excision to 50 to 30% of its initial value after 8 and 24 hours of aging at 25 C. There was also a sharp decline of the total sugar content (glucose, fructose, and sucrose). Starch content was very low at the time of excision and aimost neglgible 5 hours later. During the same period, the respiratory quotient declined from I to 0.75 and then remained stable.The addition of exogenous sugars induced a rapid rise of the respiratory rate which stabilized at a level correlated to the external sugar concentration. Addition of 0.2 molar glucose was necessary to restore the respiratory rate to the initial, also the maximum, level. These results indicate that metabolic activity of root tips is hily reliant on sugar import and carbohydrate reserves at the time of excision cannot compensate for the cessation of import. The control of respiration by substrate supply is in good agreement with the failure for dinitrophenol to stimulate oxygen uptake in aged sugar-depleted root tips.The energy charge remained constant at about 0.9, irrespective of the presence or absence of glucose and in spite of a large decline of respiratory activity in aged, sugar-depleted tissues.In normoxia as defined by Pradet and Bomsel (16), the respiratory rate of tissues is not limited by the 02 partial pressure.Under such conditions, most of the biological energy provided to the cells comes from sugar oxidation through respiratory pathways. The sugar supply of non-chlorophyllous tissues varies depending upon factors which affect the efficiency ofcarbon fixation by leaves and transport of the recently synthesized carbohydrates (5, 7, 13, 21).The question arises as to how metabolism adjusts to such fluctuation. Stimulation of root respiratory rate by light after darkness has been reported previously (6, 9). Hatrick and Bowling (10), using sunflower and barley, reported evidence for a complete dependence of root respiration on the rate of assimilate translocation from the shoot, suggesting that roots of young herbaceous plants have no reserves which might reduce the effect of fluctuations in the rate of translocation of photosynthetic sugars.The present study is part of an effort to understand the factors which limit and control the metabolic activity of roots. Our approach was to relate 02 uptake, taken as a measure of the metabolic activity, to soluble sugar and adenine nucleotide content of the tissues at various times after excision. The data reported demonstrate the influence of exogenous sugars on the metabolic activity of root tips. The significance ofenergy charge as parameter of normoxic cellular metabolism is discussed. MATERIALS AND METHODSPrimary root tips, 0.5 cm, were cut from maize seedlings (Zea mays L. INRA 402) germinated for 3 days at 25 C between sheets of filter paper soaked with 2 mm CaCl2.Determination of Gas Exchange. Measureme...

“…Assimilate unloading from seed-coat tissues of French bean (13), soybean (22), pea (28), and broad bean (24) is believed to be an energydependent process and is markedly inhibited by sulfhydryl reagents. Sugar and amino acid unloading from legume seedcoats is also sensitive to metabolic inhibitors (13,22,24,28); however, the active site of these inhibitors is difficult to establish because chemical inhibitors induce many secondary effocts (4 Sugar and Amino Acid Determinations. Sugars were extracted from the agar solute traps and the reducing sugars and sucrose present were quantified as previously described (15).…”

mentioning

confidence: 99%

Assimilate Unloading from Maize (Zea mays L.) Pedicel Tissues

Porter¹,

Knievel²,

Shannon³

1987

ABSTRACrSugar, amino acid, and "C-assimilate release from attached maize (Zea mays L.) pedicels was studied following treatment with several chemical inhibitors. In the absence of these agents, sugar release was nearly linear over a 7-hour period. At least 13 amino acids were released with glutamine comprising over 30% of the total. Release was not affected by potassium concentration, 10-minute pretreatments with p-chloromercuribenzene sulfonic acid (PCMBS) or dithiothreitol, and low concentrations of CaC12. Three hours or more exposure to PCMBS, dinitrophenol, N-ethylmaleimide, or 2,4,6-trinitrobenzene sulfonic acid strongly inhibited "C-assimilate, sugar, and amino acid release from the pedicel. These treatments also reduced 'C-assimilate movement into the kernel bases.It is, therefore, likely that reduced unloadings caused by these relatively long-term exposures to chemical inhibitors, was related to reduced translocation of assimilates into treated kernels. Whether this effect is due to disruption of kernel metabolism and sieve element function or reduced assimilate unloading and subsequent accumulation of unlabeled assimilates within the pedicel tissues cannot be determined at this time.Assimilate unloading processes are difficult to understand because unloading mechanisms apparently differ among crop species and among the various sink tissues within a plant (21). In growth sinks, such as root tips and expanding leaves, there is evidence that unloading is via the symplast (5, 19). Within stem tissues, unloading occurs into the apoplast from which assimilates may be taken up for storage or reloaded into the phloem (7,11). In reproductive sinks, such as legume seeds, unloading occurs from the maternal seed coat tissue into the apoplast which surrounds the developing embryo (21). Assimilates must then be taken up into the embryo. Since no vascular or plasmodesmatal connections exist between the maternal tissues and the developing endosperm cells in maize, a similar assimilate pathway to that of legumes must be followed in maize kernels (3,20). We conclude from available evidence (3,12, 15,16,20) PA. Plant culture and experimental conditions for these greenhouse studies have been described (15). Plants which were 21 or 22 d postpollination were utilized for all experiments. Kernels from these plants were in the linear phase of dry matter accumulation and had attained less than 50% oftheir final dry weight. Kernel water content ranged from 55 to 65% by weight. The experiments reported herein were initiated between 9:00 and