Effect of light intensity on the formation of carotenoids in normal and mutant maize leaves

Carotenoid composition in leaves of normal, lycopenic and ζ-carotenic mutants of Zea mays were investigated. In lycopenic leaves, in addition to lycopene, phytoene, phytofluene, δ- and γ-carotene, trace amounts of α- and β-carotene and antheraxanthin were identified. Low light promoted accumulation of α- and β-carotene; high light brought about an increase in antheraxanthin content. In the leaves of the ζ-carotenic mutant, phytoene, phytofluene and ζ-carotene were synthesized. Illumination of low intensity stimulated carotenoid synthesis to a slight extent. Relative amounts of carotenoid components were essentially the same as in etiolated material, except for a small increase in cis-ζ-carotene. Under high intensity illumination, carotenoids were rapidly destroyed.     

Effect of light intensity on ammonia assimilation in maize leaves

The effect of light on the metabolism of ammonia was studied by subjecting detached maize leaves to 150 or 1350 mol m^–2 s^–1 PAR during incubation with the leaf base in 2 mM ¹⁵NH₄Cl. After up to 60 min, leaves were extracted. Ammonia, glutamine, glycine, serine, alanine, and aspartate were separated by isothermal distillation and ion exchange chromatography. ¹⁵N enrichments were analyzed by emission spectroscopy. The uptake of ammonium chloride did not influence CO₂ assimilation (8.3 and 17.4 mol m^–1 s^–1 at 150 and 1350 mol m^–2 s^–1 PAR, respectively). Leaves kept at high light intensity contained more serine and less alanine than leaves from low light treatments. Within 1 h of incubation the enrichment of ammonia extracted from leaves rose to approximately 20% ¹⁵N. In the high light regime the amino acids contained up to 15% ¹⁵N, whereas in low light ¹⁵N enrichments were small (up to 6%). The kinetics of ¹⁵N incorporation indicated that NH₃ was firstly assimilated into glutamine and then into glutamate. After 15 min ¹⁵N was also found in glycine, serine and alanine. At high light intensity nearly half of the ¹⁵N was incorporated in glycine. On the other hand, at low light intensity alanine was the predominant ¹⁵N sink. It is concluded that light influences ammonia assimilation at the glutamine synthetase reaction.     

Effects of High Temperature on Chlorophyll Fluorescence Induction and the Kinetics of Far Red Radiation-Induced relaxation of apparent F0 in maize leaves

Temperature dependence (25–50 °C) of chlorophyll (Chl) fluorescence induction, far-red radiation (FR)-induced relaxation of the post-irradiation transient increase in apparent F₀, and the trans-thylakoid proton gradients (pH) was examined in maize leaves. Temperatures above 30 °C caused an elevation of F₀ level and an enhancement of F₀ quenching during actinic irradiation. Millisecond delayed light emission (ms-DLE), which reflects the magnitude of pH, decreased strikingly above 35 °C, and almost disappeared at 50 °C. It indicates that the heat-enhanced quenching of F₀ under actinic irradiation could not be attributed mainly to the mechanism of pH-dependent quenching. The relaxation of the post-irradiation transient increase in apparent F₀ upon FR irradiation could be decomposed into two exponential components (₁ = 0.7–1.8 s, ₂ = 2.0–9.9 s). Decay times of both components increased with temperature increasing from 25 to 40–45 °C. The bi-phasic kinetics of FR-induced relaxation of the post-irradiation transient increase in apparent F₀ and its temperature dependence may be related to plastoquinone (PQ) compartmentation in the thylakoid membranes and its re-organisation at elevated temperature.     

The ultrastructural development of plastids in leaves of maize plants exposed to continuous illumination

Summary Chloroplast differentiation in relation to increasing leaf age has been investigated in maize plants exposed to continuous illumination. In the young leaves the proplastids differentiate into chloroplasts containing well organized grana as well as prolamellar bodies. In the older leaves, while plastids differentiate, the prolamellar bodies are no longer detectable. Chloroplast ability to build up prolamellar bodies does not seems so much a light dependent process as it is affected by cell differentiation rate.Supported by a grant of C.N.R.     

Effects of red and blue light on the composition and morphology of maize kernels grown in vitro

Growth and development of plants are known to be affected by exposure to red and blue light. Mechanisms by which light quality influences gene expression in maize (Zea mays L.) embryos have not been explored. Maize kernels can be cultured in vitro allowing experimental manipulation of environmental factors during seed development. We used the in vitro kernel culture system to investigate the response of developing maize seeds, which normally develop without exposure to light, to controlled light quality. Kernels grown under red light accumulated more dry weight than those grown in darkness, whereas kernels grown under blue light accumulated less. Reciprocal color shift experiments showed that light quality during the first week in culture had more influence on kernel weight than during the subsequent three weeks in culture. Soluble sugars were higher in both light treatments than in darkness. Blue-grown kernels had higher amino acid and lower lipid levels than red-or dark-grown kernels. Embryo morphology was markedly affected by red light, under which the upper shoot axis was longer than under blue light or in darkness. Embryo morphology was influenced by light quality during the later stages of development rather than the first week. We suggest, based on these results, that gene expression in the embryo and endosperm of developing maize seeds is sensitive to light quality, and the mechanism and time dependence of this effect warrant further study. In vitro maize kernel culture affords a convenient system for such light quality experiments.     

Effects of water stress on the chlorophyll content,nitrogen level and photosynthesis of leaves of two maize genotypes

The dynamics of leaf chlorophyll level, nitrogen content, photosynthesis and stomatal conductance were followed in detail in two cultivars of maize (Zea mays) during a short period of water stress, applied at tasseling, and during the subsequent recovery phase. Plants used in the experiment were grown in sand-nutrient solution culture under field weather conditions. Water stress reduced chlorophyll levels, stomatal conductance and photosynthesis, but the nitrogen content of the leaves was not affected. It is concluded that the stress-induced loss of chlorophyll is not mediated by a lack of nitrogen. Considerable differences were observed between genotypes in the rate of post-stress recovery of chlorophyll level. Recovery, upon rewatering, of stomatal conductance and photosynthesis preceded that of chlorophyll level. Losses of up to 40% of leaf chlorophyll content were insufficient to affect rates of photosynthesis measured at mid-day.     

Glycolate oxidase isoforms are distributed between the bundle sheath and mesophyll tissues of maize leaves

Glycolate oxidase (EC 1.1.3.15) activity was detected both in the bundle sheath (79%) and mesophyll (21%) tissues of maize leaves. Three peaks of glycolate oxidase activity were separated from maize leaves by the linear KCl gradient elution from the DEAE-Toyopearl column. The first peak corresponded to the glycolate oxidase isoenzyme located in the bundle sheath cells, the second peak had a dual location and the third peak was related to the mesophyll fraction. The mesophyll isoenzyme showed higher affinity for glycolate (Km 23 micromol x L(-1)) and a higher pH optimum (7.5-7.6) as compared to the bundle sheath isoenzyme (Km 65 micromol x L(-1), pH optimum 7.3). The bundle sheath isoenzyme was strongly activated by isocitrate and by succinate while the mesophyll isoenzyme was activated by isocitrate only slightly and was inhibited by succinate. It is concluded that although the glycolate oxidase activity is mainly attributed to the bundle sheath, conversion of glycolate to glyoxylate occurs also in the mesophyll tissue of C4 plant leaves.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Diurnal changes in the flux of calcium toward meristems and transpiring leaves in tomato and maize plants

Tomato (Lycopersicon esculentum Mill. cv. Moneymaker) and maize (Zea mays L. cv. spec.) plants were supplied with ⁴⁵Ca-labeled nutrient solutions for a period of 8 or 16 h in the dark, in the light, or in a light-dark régime. Plant parts were analyzed for ⁴⁵Ca content. The partitioning of ⁴⁵Ca between mature leaves and meristems was shown to be affected by the presence of light. The transport of ⁴⁵Ca to meristems was higher in a dark period than in a comparable light period. Experiments with excised tomato shoots yielded similar distribution patterns of ⁴⁵Ca over leaves and meristems, thus excluding root pressure as the main driving force for the enhanced import of ⁴⁵Ca into the meristems in the dark. Results are discussed in terms of cation-exchange transport and competition between the various calcium sinks.Abbreviations DM dry material - IAA indole-3-acetic acid - TIBA 2,3,5-trijodbenzoic acid - CEC cation exchange capacityContribution No. 1693 of the Radiation Protection Programme of the Commission of the European Communities     

Activation of the maize transposable element Suppressor-mutator (Spm) in tissue culture

Summary Previous experiments have revealed that the maize transposable element Activator (Ac) may become active during tissue culture. The objective of the present study was to determine whether a second transposable element, Suppressor-mutator (Spm), could also be activated in tissue culture and detected in regenerated maize plants. Approximately 500 R₁ progeny of 143 regenerated plants (derived from 49 embryo cell lines) were crossed as males onto an Spm-responsive tester stock. Spm activity was observed in two R₁ progeny of a single regenerated plant. This plant had been regenerated from Type II (friable embryogenic) callus of an A188 × B73 genetic background after 8 months in culture; the absence of Spm activity in four other plants regenerated from this same callus demonstrates that Spm activity was not present before culturing. Approximately 20 Spm-homologous DNA sequences were detected in each of the inbreds used to initiate the tissue cultures; it is presumed that one of these became active to give rise to Spm activity.     

Cleavage polyembryony in maize

Summary Two types of cleavage polyembryony are described in the inbred line VIR 17 of maize. Suspensorial embryony was observed to occur spontaneously. Typical cleavage of the zygotic proembryo occurred spontaneously, but could also be induced by treating the developing caryopses with 2,4-Dichlorophenoxyacetic acid (2,4-D) on the second day after pollination. 2,4-D was active as a decorelative factor also evoking the expression of totipotency in individual proembryonal cells.     

Identification of indole-3-butyric acid as an endogenous constituent of maize kernels and leaves

Indole-3-butyric acid (IBA) was identified by thin layer chromatography, gas-liquid chromatography and gas chromatography-mass spectrometry in kernels and leaves of corn (Zea mays) var. Hazera 224. Free and ester conjugated IBA were present in dry and germinating corn kernels and leaves. This is the first report of IBA in a monocotyledonous plant and, as far as we know, the first evidence for the presence of conjugated IBA.     

Flow cytometric identification of two different rhodamine-123-stained mitochondrial populations in maize leaves

Summary. Flow cytometric analysis of mitochondria isolated from maize leaves revealed two distinct rhodamine-123-stained fluorescence populations distinguishable by their main fluorescence channel. Further microscopic observation of mitochondria stained with Janus Green B and rhodamine-123 revealed the occurrence of differently sized mitochondrial particles. It was shown by pulsed-field gel electrophoresis that the DNA from the isolated mitochondria ranged in size from 45 to 100 kb. These results suggest that different types of mitochondria with different physiological status, mass, and genomic DNA size probably coexist and carry out different physiological functions throughout the whole process of maize leaf growth and development. Correspondence: Lijia Li, Key Laboratory for Plant Developmental Biology of the Ministry of Education, College of Life Sciences, Wuhan University, Wuhan 430072, People’s Republic of China.     

Promotive effect of jasmonates on the senescence of detached maize leaves

Promotion of senescence of detached maize leaves by jasmonates was investigated. Senescence of detached maize leaves was promoted by linolenic acid, the precursor of biosynthesis of jasmonic acid, and retarded by inhibitors of lipoxygenase, the first enzyme in the biosynthetic pathway of jasmonic acid. Results support a role of endogenous jasmonates in the regulation of senescence of detached maize leaves. Silver thiosulfate, an inhibitor of ethylene action, was found to inhibit methyl jasmonate, linolenic acid- and abscisic acid-promoted senescence of detached maize leaves. It seems that jasmonate-promoted senescence is mediated through an increase in ethylene sensitivity in detached maize leaves.Abbreviations ABA abscisic acid - MJ methyl jasmonate - STS silver thiosulfate     

Improved frequency of transformation in rice and maize by treatment of immature embryos with centrifugation and heat prior to infection with Agrobacterium tumefaciens

The efficiency of transformation was improved by treating immature embryos with heat and centrifugation before infection with Agrobacterium tumefaciens in rice and maize. Because the effects were detected both in the levels of transgene expression after co-cultivation and in the number of independent transgenic plants obtained per embryo, conditions were first optimized based on the transgene expression, and then transformants were produced. The optimal conditions varied considerably depending on species and genotypes, but reasonably good parameters were identified for Japonica rice, Indica rice or maize. As a general tendency, the effect of centrifugation was greater than that of heat in Japonica rice, whereas that of heat was greater than that of centrifugation in Indica rice and maize A188, and the combination of the treatments was the most effective in all of the genotypes tested. The frequency of transformation was improved several fold in rice and maize. In addition, transformation of certain genotypes of maize, which were not transformable before, and transformation of maize with a less efficient vector, which could not transform maize before, became possible by these pre-treatments. In the highest case, 18 independent transgenic plants were obtained from a single immature embryo of Japonica rice. Although nothing is known about the mechanism, these pre-treatments seemed to render cells of rice and maize more competent for transformation mediated by A. tumefaciens.