Relationship between thylakoid electron transport and photosynthetic CO2 uptake in leaves of three maize (Zea mays L.) hybrids

The introduction of a more efficient means of measuring leaf photosynthetic rates under field conditions may help to clarify the relationship between single leaf photosynthesis and crop growth rates of commercial maize hybrids. A large body of evidence suggests that gross photosynthesis (A_G) of maize leaves can be accurately estimated from measurements of thylakoid electron transport rates (ETR) using chlorophyll fluorescence techniques. However, before this technique can be adopted, it will first be necessary to determine how the relationship between chlorophyll fluorescence and CO2 assimilation is affected by the non-steady state PPFD conditions which predominate in the field. Also, it must be determined if the relationship is stable across different maize genotypes, and across phenological stages. In the present work, the relationship between ETR and A_G was examined in leaves of three maize hybrids by making simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, both under controlled environment conditions and in the field. Under steady-state conditions, a linear relationship between ETR and A_G was observed, although a slight deviation from linearity was apparent at low A_G. This deviation may arise from an error in the assumption that respiration in illuminated leaves is equivalent to respiration in darkened leaves. The relationship between chlorophyll fluorescence and photosynthetic CO2 assimilation was not stable during fluctuations in incident PPFD. Since even minor (e.g. 20%) fluctuations in incident PPFD can produce sustained ( > 20 s) departures from the mean relationship between ETR and A_G, chlorophyll fluorometry can only provide an accurate estimate of actual CO2 assimilation rates under relatively stable PPFD conditions. In the field, the mean value of ETR / A_G during the early part of the season (4.70 ± 0.07) was very similar to that observed in indoor-grown plants in the vegetative stage (4.60 ± 0.09); however, ETR / A_G increased significantly over the growing season, reaching 5.00 ± 0.09 by the late grain-filling stage. Differences in ETR / A_G among the three genotypes examined were small (less than 1% of the mean) and not statistically significant, suggesting that chlorophyll fluorometry can be used as the basis of a fair comparison of leaf photosynthetic rates among different maize cultivars.     

Conversion of photosynthetic products in the light in CO2-free O2 and N2 in leaves of Zea mays L. and Phaseolus vulgaris L.

Summary After 10 min illumination of segments of bean (Phaseolus vulgaris L.) or maize (Zea mays L.) leaves in air with ¹⁴CO₂, the atmosphere was changed to CO₂-free O₂ or N₂ and conversion of photosynthetic products in the light was investigated. The experiments have shown that after the ¹⁴CO₂ assimilation period the bean leaves contain the pool of weakly fixed ¹⁴C (WF-¹⁴C) which is converted into stable products during the subsequent period of illumination in CO₂-free N₂. In O₂ atmosphere the WF-¹⁴C pool is initially the main source of CO₂ evolved. The marked decrease in radioactivity of sucrose and starch during illumination of bean leaves in O₂ atmosphere indicates that these compounds were also the source of CO₂ evolved in the light. The total amount of previously fixed ¹⁴C remained almost on the same level during illumination of maize leaves in N₂ as well as in O₂. However, oxygen changed the distribution of ¹⁴C in photosynthetic products, which is suggested to be the consequence of the photorespiration process in maize.Abbreviation WF-¹⁴C weakly fixed ¹⁴C     

CO2 gas exchange and phosphoenolpyruvate carboxylase activity in leaves of Zea mays L.

Important gas exchange characteristics of C4 plants depend on the properties of phophoenolpyruvate carboxylase (PEPC), the enzyme catalysing the primary fixation of CO2 during C4 photosynthesis. In this study, the relationship between intracellular resistance for CO2 fixation (r_i) at high photosynthetically active photon flux densities (PPFD) and maximum PEPC activity in vitro (V_pm) was examined in leaves of Zea mays L. The analysis allowed the estimation of the Michaelis constant K_p of the enzyme for CO2 (or the equivalent number for bicarbonate) in vivo. At low PPFD (below 100 mol m-2 s-1) the initial slopes of the curves describing net CO2 uptake rate A as a function of intercellular CO2 concentration c_i increased with increasing PPFD. The increase (i. e. a decrease in r_i) was interpreted as due to a reversible activation of PEPC by light. Including this assumption into a model of C4 photosynthesis enabled us to reproduce A(c_i) response curves measured at low levels of PPFD. Fitting the model to experimental data resulted in values for K_I, the PPFD at which PEPC reaches half of its full activation, of about 200 mol m-2 s-1. Similar results were derived from the dependence of r_i on PPFD. The analysis of the relationships between r_i and V_pm and between r_i and PPFD, as well as fitting of the model to gas exchange data all gave rise to estimates for the resistance for CO2 transfer within mesophyll cells that are comparable with those known from C3 plants.     

Plastids undifferentiated, a nuclear mutation that disrupts plastid differentiation in Zea mays L

Photosynthetic development in any plant requires the intracellular co-ordination of chloroplast and nuclear gene expression programs. In this report, we investigate the role of a nuclear gene in photosynthetic development by examining C4 photosynthetic differentiation in a yellow mutant of maize (Zea mays L.). The plastids undifferentiated (pun) mutation disrupts plastid biogenesis in both bundle sheath and mesophyll cells, at an early developmental stage and in a light-independent manner. Chloroplast thylakoids are disrupted in the mutant and both membrane-associated and soluble chloroplast-encoded proteins accumulate at much reduced levels. The observed plastid morphology is consistent with a general defect in chloroplast biogenesis that is most likely exerted at the post-translational level. Despite aberrant chloroplast development, nuclear photosynthetic genes are expressed normally in pun mutants. Thus, neither functional chloroplasts nor the Pun gene product are required to establish nuclear photosynthetic gene expression patterns in maize.     

Some relationships between contents of photosynthetic intermediates and the rate of photosynthetic carbon assimilation in leaves of Zea mays L.

The relationship between the gas-exchange characteristics of attached leaves of Zea mays L. and the contents of photosynthetic intermediates was examined at different intercellular partial pressure of CO₂ and at different irradiances at a constant intercellular partial pressure of CO₂. (i) The behaviour of the pools of the C₄-cycle intermediates, phosphoenolpyruvate and pyruvate, provides evidence for light regulation of their consumption. However, light regulation of phosphoenolpyruvate carboxylase does not influence the assimilation rate at limiting intercellular partial pressures of CO₂. (ii) A close correlation between the pools of phosphoenolpyruvate and glycerate-3-phosphate exists under many different flux conditions, consistent with the notion that the pools of C₄ and C₃ cycles are connected via the interconversion of glycerate-3-phosphate and phosphoenolpyruvate. (iii) The ratio of triose-phosphate to glycerate-3-phosphate is used as an indicator of the availability of ATP and NADPH. Changes of this ratio with CO₂ and with irradiance are compared with results obtained in C₃ leaves and indicate that the mechanism of regulation of carbon assimilation by light in leaves of C₄ plants may differ from that in C₃ plants. (iv) The behaviour of the ribulose-1,5-bisphosphate pool with CO₂ and irradiance is contrasted with the behaviour of these pools measured in leaves of C₃ plants.Abbreviations P _i intercellular CO₂ pressure - RuBP ribulose-1,5-bisphosphate - PEP phosphoenolpyruvate - triose-P triose phosphates - PGA glycerate-3-phosphate     

Genetic structure of the R-Navajo allele in maize,Zea mays L.

Summary Mutational and recombinational analyses carried out with the R-nj allele in maize to elucidate the genetic mechanism involved in unique pattern formation and origin of occasional self-coloured kernels in this stock revealed that R-nj represents a complex with two closely linked discrete components. The self-colour (Sc) component is responsible for anthocyanin production and the navajo (Nj) component regulates the time of onset and termination of pigment synthesis restricting the pigmentation to the crown region of the kernel. The probable gene order in the R region of the R-nj:Illinois isolate is: G-Sc-Nj-K.     

Interrelationships between Potassium, Calcium, and Magnesium Nutrition of Zea mays L.

The uptake of potassium, calcium, and magnesium ions by maizeand the interrelationships among the cations have been investigatedat 48 K: Ca: Mg ratios in culture solutions. Calcium was foundto stimulate K⁺ and Mg⁺⁺ uptake at certain cation ratios butinhibit it at others. Potassium did the same for Ca⁺⁺ uptake,and Mg⁺⁺ for Ca⁺⁺ and K⁺. The uptake of Mg⁺⁺ was generally enhancedby K⁺. The sum of the cations in the plants expressed in meqwas fairly constant for treatments of the same K⁺ concentrationat the low to moderate levels of K⁺, but at considerably higher(> 24 meq l^–1) K⁺ levels the constancy was not dependenton K⁺ concentration. Potassium depressed, but Mg⁺⁺ stimulatedphosphorus accumulation. Calcium stimulated phosphate absorptionat certain cation ratios but had no effect at others. The plantyield increased with increasing K⁺ up to 24 meq l^–1 ofK⁺ after which the yield tended to fall with further increasein K⁺. The yield was also increased by Ca⁺⁺. Magnesium increasedthe yield at certain cation ratios and either depressed it orwas without effect at others.     

Purification and characterization of proteolytic enzymes from normal and opaque-2Zea mays L. developing endosperms

Purification and characterization of proteases from developing normal maize endosperm and high lysine opaque-2 maize endosperm have been carried out with a view to understand their role in storage protein modification. At day 15, normal maize endosperm had two types of proteolytic enzymes, namely, protease I and protease II, while at day 25 protease n disappeared and in place protease III appeared. However, in opaque-2 maize endosperm at both the stages only one type of enzyme (protease I) was present. These proteases had many properties in common-optimum pH and temperature were respectively, 5.7and 40°C; their activity was inhibited to the extent of 75 –93 % by p-chloromercuribenzoate; trypsin inhibitor inhibited the activity more at early stages of endosperm development; all proteases cleaved synthetic substrates p-tosyl-L-arginine methylesler and N-benzoyl-L-tyrosine ethyl ester and poly-L-glutamic acid. TheKm values of day 15 and 25 normal maize endosperm proteases ranged from 2.73–3.30, while for opaque-2 maize endosperm protease I it was 3.33 mg azocasein per ml assay medium. These enzymes, however, differed with respect to proteolytic activity towards poly-L-lysine. Only normal maize endosperm protease III at day 25 followed by protease II at day 15 showed high activity towards this homopolypeptide suggesting thereby their role in determining the quality of normal maize endosperm protein. Part of Ph.D. thesis submitted by the first author     

Nitrogen-metabolizing enzymes of Diplodia maydis, a Zea mays L. stalk rot causing fungus.

The nitrogen source available to Diplodia maydis in vivo is reported to affect the severity of stalk rot in maize. Nitrate and (or) ammonium salts were tested for their effect on the type of nitrogen metabolism found in Diplodia maydis in vitro. The level of glutamate dehydrogenase remained essentially constant on either nitrogen salt but nitrate reductase was induced by growth on nitrate salts and was not extractable on ammonium salts. Properties of nitrate reductase reported here are similar to those reported for the higher plant and Neurospora crassa enzymes. Thr relationship of nitrogen metabolism in Diplodia maydis to Zea mays L. stalk rot is discussed.     

Duplicated plastid and triplicated cytosolic isozymes of triosephosphate isomerase in maize (Zea mays L.)

We studied electrophoretic variation and inheritance of triosephosphate isomerase (TPI) isozymes in maize (Zea mays L.). In contrast to most diploid plants, in maize, TPI exists as multiple isozymes in both the plastid and cytosolic subcellular compartments. Phenotypes result from the overlay of two independent sets of isozymes and allozymes, representing the plastid (encoded by the nuclear genes Tpi1 and Tpi2) and cytosolic (encoded by Tpi3, Tpi4, and Tpi5) systems. All possible intragenic and intergenic dimeric enzymes are formed between polypeptides within each subcellular compartment. No heterodimers are formed between plastid and cytosolic polypeptides. Extensive surveys of accessions of land races and inbred lines revealed 22 allelic variants for the five loci. Most alleles have been formally validated by segregation analysis. We describe two null alleles at Tpi4, distinguished by their relative abilities to form intergenic heterodimers with polypeptides specified by Tpi3 and Tpi5. Linkage analyses and crosses with B-A translocation stocks were effective in determining the chromosome locations of all five loci. Duplicated genes for both the plastid and cytosolic isozymes were localized to genomic regions that possess numerous other redundant sequences. We placed Tpi1 on the long arm of chromosome 7, approximately 23 centimorgans (cM) distal to g11; we localized its duplicate--Tpi2--17 cM distal to v4 on the long arm of chromosome 2. The triplicate loci encoding cytosolic TPIs reside on chromosomes 3 and 8. Tpi4 is approximately equidistant (11 cM) from d1 and Lg3, near the centromere of chromosome 3. Tpi3 and Tpi5 are located on distal ends of the most poorly marked maize chromosome; Tpi3 is 29 cM distal to Idh 1 on 8L, and Tpi5 is on 8S or near the centromere on 8L. In contrast to most duplicated maize sequences, which often occur in parallel linkages on different chromosomes, Tpi3 and Tpi5 provide an example of intrachromosomal gene duplication. Several of the Tpi loci are located in sparsely mapped regions of the genome, and Tpi1 is the first isozyme marker for chromosome 7.     

A possible role for C4 photosynthetic enzymes in tolerance of Zea mays to NaCl

Treatment of 14-day-old maize cultivars (Hybrid351 and Giza2) with 250 mM NaCl significantly reduced shoot fresh and dry weights and protein content during the subsequent 12 days. The magnitude of reduction was more pronounced in Giza than Hybrid. Both cultivars contained converging levels of protein for the enzymes phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), pyruvate phosphate dikinase (PPDK) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) under normal conditions; however, NaCl led to increase these levels in Hybrid and decrease them in Giza. Moreover, NaCl significantly inhibited the activities of PEPC, MDH and PPDK in both cultivars during the first 2 days, thereafter the inhibition nullified only in Hybrid; nonetheless, Rubisco was the least affected enzyme in both cultivars. In addition, NaCl slightly increased V _max of PEPC, MDH and PPDK in Hybrid with no change in K _m; nevertheless V _max dropped in Giza with an increase in K _m of only PEPC and MDH. Also K _cat, K _cat/K _m and V _max/K _m of all enzymes were lower in treated Giza than in treated Hybrid. The increased V _max of all enzymes in only Hybrid by NaCl confirms that they were synthesised more in Hybrid than in Giza. However, the decreased V _max in Giza concomitant with the increased K _m points to an interference of salinity with synthesis of enzymes and their structural integrity. This would lead to a noncompetitive inhibition for the enzymes. These findings declare that maize tolerance to NaCl was larger in Hybrid compared to Giza due to a role for C4 enzymes.     

Microbial population,fungal biomass and CO2 evolution in maize (Zea mays L.) field soils

Summary CO₂ evolution, fungal biomass and microbial population of two maize field soils differing in agricultural systemsviz., permanent agriculture on plain lands in valleys and ‘slash and burn’ type of shifting agriculture, were estimated at monthly intervals for one crop cycle. The results showed significant positive correlation among CO₂ evolution, fungal biomass, microbial population, organic C and total N. There was significant positive correlation between bacterial population and moisture content in both the agricultural systems. Microbial population and CO₂ evolution were always higher in the soils of permanent agriculture as compared to that of ‘slash and burn’ type of shifting agriculture.     

Seed development and vivipary in Zea mays L.

Seed development was investigated in kernels of developing wild-type and viviparous (vp-1) Zea mays L. Embryos and endosperm of wild-type kernels began to dehydrate at approx. 35 d after pollination (DAP); viviparous embryos did not desiccate but accumulated fresh weight via coleoptile growth in the caryopses. Concentrations of endogenous abscisic acid (ABA) in the embryo were relatively high early in development, being approx. 150 ng·g^-1 fresh weight at 20 DAP. The ABA content declined thereafter, falling to approx. 50 ng·g^-1 at 30 DAP. Endosperm ABA content was always low, being less than 20 ng·g^-1. There were no differences between wild-type and vp-1 tissues. Immature kernels did not germinate when removed from the ear until late in development. The ability to germinate was correlated with decreasing moisture content in the endosperm at the time of removal; premature drying of immature kernels resulted in greatly increased germination following imbibition. Excised embryos germinated precociously when removed from the endosperm as early as 25 DAP. Such germination could be prevented by treatment with 10^-5 M ABA or by lowering the solute potential (_s) of the medium with 0.3 M mannitol. Treatment of excised embryos with ABA led to internal ABA concentrations comparable to those in embryos in which germination was inhibited in situ. Mannitol treatment did not have this effect, although water-deficit stress of excised embryos resulted in substantial ABA production. Germinated vp-1 embryos were less sensitive to growth inhibition by ABA or mannitol than germinating wild-type embryos. The vp-1 seedlings were not wilty and their transpiration rates were reduced in response to ABA or water shortage.Abbreviations and symbols ABA abscisic acid - DAP days after pollination - FW fresh weight - vp-1 viviparous genotype - _s solute potential     

Possible Linkage between NADH-Oxidation and Proton Secretion in Zea mays L. Roots

A possible involvement of two different systems in proton translocationand the correlation of this factor to growth rates were measuredsimultaneously by means of a pH stat and an optical system.Ferricyanide, which can accept electrons at the plasmalemma,led to an immediate increase of net H⁺ -efflux but also decreasedroot growth rate. The reduced form, ferrocyanide, inhibitednet H⁺ -effluxwithout changing the growth rate. Thus, corn rootgrowth was not determined by proton secretion exclusively. Vanadatestrongly inhibited net H⁺ -efflux by the roots but did not preventthe stimulating effect of fcrricyanide. Moreover, the extentof enhancement of net H⁺ -effluxby ferricyanide was exactlythe same in vanadate pretreated as in untreated roots. Alcoholswere used to try to increase the intracellular NADH level throughthe action of the cytoplasmic alcohol dehydrogenase presentin the roots and coleoptiles. Alcohols, known to be substratesfor alcohol dehydrogenase such as propan- 1-ol, ethanol andbutan-l -ol increased net H⁺ -effluximmediately but methanoland secondary alcohols which are not substrates had no effecton proton secretion. The K_m values of alcohol dehydrogenasefor the alcohols correspond only partly to their effects onproton secretion. However, the specificlty observed suggeststhat increased H⁺ -efflux arose from reduction of endogenousNAD by ADH and consequent increased membrane NADH-oxidasc activitytrans locating protons and electrons out of the cells. Decreased oxygen concentrations slowed proton secretion at valuesfar higher than are necessary to saturate cytochrome c oxidase.The results of these experiments suggest two distinct systemscontributing to proton efflux. Key words: ADH, proton transport, redox chain     

Correlations between Gravitropic Curvature and Auxin Movement across Gravistimulated Roots of Zea mays

We compared the kinetics of auxin redistribution across the caps of primary roots of 2-day-old maize (Zea mays, cv Merit) seedlings with the time course of gravitropic curvature. [³H] indoleacetic acid was applied to one side of the cap in an agar donor and radioactivity moving across the cap was collected in an agar receiver applied to the opposite side. Upon gravistimulation the roots first curved upward slightly, then returned to the horizontal and began curving downward, reaching a final angle of about 67°. Movement of label across the caps of gravistimulated roots was asymmetric with preferential downward movement (ratio downward/upward = ca. 1.6, radioactivity collected during the 90 min following beginning of gravistimulation). There was a close correlation between the development of asymmetric auxin movement across the root cap and the rate of curvature, with both values increasing to a maximum and then declining as the roots approached the final angle of curvature. In roots preadapted to gravity (alternate brief stimulation on opposite flanks over a period of 1 hour) the initial phase of upward curvature was eliminated and downward bending began earlier than for controls. The correlation between asymmetric auxin movement and the kinetics of curvature also held in comparisons between control and preadapted roots. Both downward auxin transport asymmetry and downward curvature occurred earlier in preadapted roots than in controls. These findings are consistent with suggestions that the root cap is not only the site of perception but also the location of the initial redistribution of effectors that ultimately leads to curvature.     

玉米(Zea mays L.)叶脉发育的研究

玉米的叶脉在单子叶植物中有一定的代表性,叶脉由四级组成,粗细不同的一、二、三级叶脉均从叶基向叶尖方向延伸,属叶片的纵向叶脉,四级脉横向与一、二、三级叶脉连接,是横向的叶脉,各级叶脉有各自的形成方式,由于它们有规律的分布,从而构成了叶片的输导网络,各级叶脉的发生和发育与叫片的生长有直接的关系。     

Salinity Induced Changes in Zea mays L. (Ganga-2)

Histochemical studies of shoot-tips have been carried out withZea mays L. plants grown under the influence of sodium chloridesalinity. Qualitative studies of the shoot-tips alter NaCl treatmentshowed a low concentration of starch grains in the cells. Changesin nucleolar and cytoplasmic RNA were assessed and are discussed. Key words: Salinity, Polysaccharides, RNA, Shoot apex, Zea mays L.     

Leaf structure in relation to solute transport and phloem loading in Zea mays L.

Small and intermediate (longitudinal) vascular bundles of the Zea mays leaf are surrounded by chlorenchymatous bundle sheaths and consist of one or two vessels, variable numbers of vascular parenchyma cells, and two or more sieve tubes some of which are associated with companion cells. Sieve tubes not associated with companion cells have relatively thick walls and commonly are in direct contact with the vessels. The thick-walled sieve tubes have abundant cytoplasmic connections with contiguous vascular parenchyma cells; in contrast, connections between vascular parenchyma cells and thin-walled sieve tubes are rare. Connections are abundant, however, between the thin-walled sieve tubes and their companion cells; the latter have few connections with the vascular parenchyma cells. Plasmolytic studies on leaves of plants taken directly from lighted growth chambers gave osmotic potential values of about-18 bars for the companion cells and thin-walled sieve tubes (the companion cell-sieve tube complexes) and about-11 bars for the vascular parenchyma cells. Judging from the distribution of connections between various cell types of the vascular bundles and from the osmotic potential values of those cell types, it appears that sugar is actively accumulated from the apoplast by the companion cell-sieve tube complex, probably across the plasmalemma of the companion cell. The thick-walled sieve tubes, with their close spatial association with the vessels and possession of plasmalemma tubules, may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream. The transverse veins have chlorenchymatous bundle sheaths and commonly contain a single vessel and sieve tube. Parenchymatic elements may or may not be present. Like the thick-walled sieve tubes of the longitudinal bundles, the sieve tubes of the transverse veins have plasmalemma tubules, indicating that they too may play a role in retrieval of solutes entering the leaf apoplast in the transpiration stream.