Difference in Cd2+ flux around the root tips of different soybean (Glycine max L.) cultivars and physiological response under mild cadmium stress.

The purpose of the study was to compare differences in Cd2+ flux in the vicinity of root tips of 20 soybean cultivars under mild Cd stress conditions using non-invasive micro-test technology (NMT). The results indicated that Cd2+ influx to the root tips under mild Cd treatment was higher compared to controls. Cd2+ influx showed an obvious spatial distribution, with the highest Cd2+ influx measured 300 µm from the root tips, and a gradually decrease above and below this site. The cultivar Liaodou32 had a lower Cd uptake (3.40 pmol cm-2 s-1), while Liaodou23 had a relatively higher Cd uptake (66.37 pmol cm-2 s-1). Cluster analysis showed that the order of the average Cd2+ influx of the cultivars at a distance of 300 µm from the root tips was as follows: high-uptake cultivars (61.80 pmol cm-2 s-1)>medium-high-uptake cultivars (33.92 pmol cm-2 s-1)>medium-low-uptake cultivars (19.78 pmol cm-2 s-1)>low-uptake cultivars (4.84 pmol cm-2 s-1). We also analyzed physiological responses of different soybean cultivars to mild Cd stress. The results indicated that mild Cd stress could inhibit soluble protein production and root vigor among individual soybean cultivars. Moreover, stress increased SOD, CAT and POD activities and MDA content in root tissues. It should be noted that the physio-biochemical indicators of low-uptake cultivars did not change significantly after exposure to mild Cd stress compared to controls. Pearson's correlation analyses showed that all physio-biochemical indicators were significantly positively associated with influx, except of root SP and biomass. PCA analysis demonstrated that root vigor was a dominant factor causing the differences in Cd tolerance among different soybean seedling cultivars. NMT is of great significance for safe utilization of contaminated soil to distinguish the cultivars with different enrichment capacity for heavy metals from different crop cultivars.

Cadmium (II)-Induced Oxidative Stress Results in Replication Stress and Epigenetic Modifications in Root Meristem Cell Nuclei of Vicia faba.

Among heavy metals, cadmium is considered one of the most toxic and dangerous environmental factors, contributing to stress by disturbing the delicate balance between production and scavenging of reactive oxygen species (ROS). To explore possible relationships and linkages between Cd(II)-induced oxidative stress and the consequent damage at the genomic level (followed by DNA replication stress), root apical meristem (RAM) cells in broad bean (V. faba) seedlings exposed to CdCl2 treatment and to post-cadmium recovery water incubations were tested with respect to H2O2 production, DNA double-strand breaks (γ-phosphorylation of H2AX histones), chromatin morphology, histone H3S10 phosphorylation on serine (a marker of chromatin condensation), mitotic activity, and EdU staining (to quantify cells typical of different stages of nuclear DNA replication). In order to evaluate Cd(II)-mediated epigenetic changes involved in transcription and in the assembly of nucleosomes during the S-phase of the cell cycle, the acetylation of histone H3 on lysine 5 (H3K56Ac) was investigated by immunofluorescence. Cellular responses to cadmium (II) toxicity seem to be composed of a series of interlinked biochemical reactions, which, via generation of ROS and DNA damage-induced replication stress, ultimately activate signal factors engaged in cell cycle control pathways, DNA repair systems, and epigenetic adaptations.

Hormonal and gene dynamics in de novo shoot meristem formation during adventitious caulogenesis in cotyledons of Pinus pinea.

KEY MESSAGE: Several members of WOX and KNOX gene families and several plant growth regulators, basically cytokinins and auxins, play a key role during adventitious caulogenesis in the conifer Pinus pinea. Similar to Arabidopsis thaliana, Pinus pinea shoot organogenesis is a multistep process. However, there are key differences between both species, which may alter the underlying physiological and genetic programs. It is unknown if the genic expression models during angiosperm development may be applicable to conifers. In this work, an analysis of the endogenous content of different plant growth regulators and the expression of genes putatively involved in adventitious caulogenesis in P. pinea cotyledons was conducted. A multivariate analysis of both datasets was also realized through partial least squares regression and principal component analysis to obtain an integral vision of the mechanisms involved in caulogenesis in P. pinea. Analyses show that cotyledons cultured in the presence of benzyladenine during long times (2-6 days) cluster separately from the rest of the samples, suggesting that the benzyladenine increase observed during the first hours of culture is sufficient to trigger the caulogenic response through the activation of specific developmental programs. In particular, the most relevant factors involved in this process are the cytokinins trans-zeatin, dihydrozeatin, trans-zeatin riboside and isopentenyl adenosine; the auxin indoleacetic acid; and the genes PpWUS, PpWOX5, PpKN2, PpKN3 and PipiRR1. WUS is functional in pines and has an important role in caulogenesis. Interestingly, WOX5 also seems to participate in the process, although its specific role has not been determined.

A laser ablation technique maps differences in elemental composition in roots of two barley cultivars subjected to salinity stress.

In saline soils, high levels of sodium (Na+ ) and chloride (Cl- ) ions reduce root growth by inhibiting cell division and elongation, thereby impacting on crop yield. Soil salinity can lead to Na+ toxicity of plant cells, influencing the uptake and retention of other important ions [i.e. potassium (K+ )] required for growth. However, measuring and quantifying soluble ions in their native, cellular environment is inherently difficult. Technologies that allow in situ profiling of plant tissues are fundamental for our understanding of abiotic stress responses and the development of tolerant crops. Here, we employ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to quantify Na, K and other elements [calcium (Ca), magnesium (Mg), sulphur (S), phosphorus (P), iron (Fe)] at high spatial resolution in the root growth zone of two genotypes of barley (Hordeum vulgare) that differ in salt-tolerance, cv. Clipper (tolerant) and Sahara (sensitive). The data show that Na+ was excluded from the meristem and cell division zone, indicating that Na+ toxicity is not directly reducing cell division in the salt-sensitive genotype, Sahara. Interestingly, in both genotypes, K+ was strongly correlated with Na+ concentration, in response to salt stress. In addition, we also show important genetic differences and salt-specific changes in elemental composition in the root growth zone. These results show that LA-ICP-MS can be used for fine mapping of soluble ions (i.e. Na+ and K+ ) in plant tissues, providing insight into the link between Na+ toxicity and root growth responses to salt stress.

Phytochemical analysis and effect of the essential oil of Psidium L. species on the initial development and mitotic activity of plants.

The use of allelopathic compounds is an alternative for weeds control, since they present low toxicity when compared with the synthetic herbicides, that may cause several damages, as the contamination of the environment. Our objective was to determine the chemical composition and allelopathic properties of the essential oils of Psidium cattleianum, P. myrtoides, P. friedrichsthalianum, and P. gaudichaudianum on the germination and root growth of Lactuca sativa and Sorghum bicolor, and to evaluate their action on the cell cycle of root meristematic cells of L. sativa. The main compound found in all the studied species was (E)-caryophyllene (P. cattleianum-23.4 %; P. myrtoides-19.3%; P. friedrichsthalianum-24.6% and P. gaudichaudianum-17.0%). The different essential oils were tested at different concentrations on L. sativa and S. bicolor, reducing germination, germination speed index, and root and shoot growth of lettuce and sorghum seedlings. The cytotoxicity and aneugenic potential of these oils were evidenced by the reduction of the mitotic index and increase of the frequency of chromosomal alterations in L. sativa. The essential oils of the species of Psidium studied have potential to be used in weeds control.

Supraoptimal Cytokinin Content Inhibits Rice Seminal Root Growth by Reducing Root Meristem Size and Cell Length via Increased Ethylene Content.

Cytokinins (CKs), a class of phytohormone, regulate root growth in a dose-dependent manner. A certain threshold content of CK is required for rapid root growth, but supraoptimal CK content inhibits root growth, and the mechanism of this inhibition remains unclear in rice. In this study, treatments of lovastatin (an inhibitor of CK biosynthesis) and kinetin (KT; a synthetic CK) were found to inhibit rice seminal root growth in a dose-dependent manner, suggesting that endogenous CK content is optimal for rapid growth of the seminal root in rice. KT treatment strongly increased ethylene level by upregulating the transcription of ethylene biosynthesis genes. Ethylene produced in response to exogenous KT inhibited rice seminal root growth by reducing meristem size via upregulation of OsIAA3 transcription and reduced cell length by downregulating transcription of cell elongation-related genes. Moreover, the effects of KT treatment on rice seminal root growth, root meristem size and cell length were rescued by treatment with aminoethoxyvinylglycine (an inhibitor of ethylene biosynthesis), which restored ethylene level and transcription levels of OsIAA3 and cell elongation-related genes. Supraoptimal CK content increases ethylene level by promoting ethylene biosynthesis, which in turn inhibits rice seminal root growth by reducing root meristem size and cell length.

Cytological Effects of Bleaching Agent (Quneex) on Plant Cells and Plant DNA.

BACKGROUND AND OBJECTIVE: There have been a number of reported drawbacks and efficacy issues regarding the use of bleaching agents in the plant industry. This study was conducted to determine the cytological effects of the bleaching agent (Quneex) on the plant cells and plant DNA using the Allium cepa assay. MATERIALS AND METHODS: It was subjected sixteen root meristems of A. cepa to different concentrations of the bleaching agent (0.1, 0.2, 0.3, 0.4 and 0.5%) with different periods of time (6, 12 and 24 h). Recovery was done for 6, 12 and 24 h after exposure. RESULTS: The mitotic index significantly decreased with time and also decreased with increase in the concentration of the bleaching agent. Abnormal chromosomal changes reflecting mutagenesis including stickiness, laggards, bridges, C-metaphase, star-metaphase, binucleation, polyploidy, disturbance and multinucleation were observed in the different concentrations and periods of time. After recovery, a slow increase in the mitotic index was observed. All treatments with or without recovery for 12 and 24 h resulted in reduction in the amount of DNA. CONCLUSION: Bleaching agents similar to Quneex containing sodium hypochlorite have mutagenic properties that can be potentially hazardous to the environment and also to humans. Thus, there is a need to regulate the use and disposal of such chemicals into the environment particularly to the sewers, to prevent contamination of potable water, plant and biodiverse aquatic animals.

Polyphenolic profile and antioxidant activity of meristem and leaves from "chagual" (Puya chilensis Mol.), a salad from central Chile.

The Bromeliaceae Puya chilensis Mol. is a native monocotyledonous food plant that can be found in central Chile. It is traditionally known as chagual. The tender basal part of the leaves, just starting from the meristem, are consumed as a salad. The aim of this work was to describe the phenolic content and composition of the meristem and leaves of chagual, as well as their antioxidant capacity and inhibitory activity against metabolic syndrome-associated enzymes. Samples of chagual, including two cultivated and three wild growing plants, were analyzed and compared for composition and bioactivity. From the phenolic enriched extract of the plant (PEE), 26 compounds were tentatively identified by HPLC-DAD-ESI-MSn, including 12 hydroxycinnamic acids and 14 flavonoids. The main compounds were identified as diferuloyl hexaric acid isomers and 5-p-Coumaroylquinic acid. The compounds were quantified in both meristem and leaves. The PEE content was up to ten times higher in the meristem than in the leaves, ranging from 0.18 to 124.08 mg/g PEE. The samples inhibited α-glucosidase, but did not show effect on α-amylase and pancreatic lipase. This is the first report on the polyphenol composition and bioactivity of the edible components of the chagual food plant.

A phosphoinositide map at the shoot apical meristem in Arabidopsis thaliana.

BACKGROUND: In plants, the shoot apical meristem (SAM) has two main functions, involving the production of all aerial organs on the one hand and self-maintenance on the other, allowing the production of organs during the entire post-embryonic life of the plant. Transcription factors, microRNA, hormones, peptides and forces have been involved in meristem function. Whereas phosphatidylinositol phosphates (PIPs) have been involved in almost all biological functions, including stem cell maintenance and organogenesis in animals, the processes in meristem biology to which PIPs contribute still need to be delineated. RESULTS: Using biosensors for PI4P and PI(4,5)P2, the two most abundant PIPs at the plasma membrane, we reveal that meristem functions are associated with a stereotypical PIP tissue-scale pattern, with PI(4,5)P2 always displaying a more clear-cut pattern than PI4P. Using clavata3 and pin-formed1 mutants, we show that stem cell maintenance is associated with reduced levels of PIPs. In contrast, high PIP levels are signatures for organ-meristem boundaries. Interestingly, this pattern echoes that of cortical microtubules and stress anisotropy at the meristem. Using ablations and pharmacological approaches, we further show that PIP levels can be increased when the tensile stress pattern is altered. Conversely, we find that katanin mutant meristems, with increased isotropy of microtubule arrays and slower response to mechanical perturbations, exhibit reduced PIP gradients within the SAM. Comparable PIP pattern defects were observed in phospholipase A3ß overexpressor lines, which largely phenocopy katanin mutants at the whole plant level. CONCLUSIONS: Using phospholipid biosensors, we identified a stereotypical PIP accumulation pattern in the SAM that negatively correlates with stem cell maintenance and positively correlates with organ-boundary establishment. While other cues are very likely to contribute to the final PIP pattern, we provide evidence that the patterns of PIP, cortical microtubules and mechanical stress are positively correlated, suggesting that the PIP pattern, and its reproducibility, relies at least in part on the mechanical status of the SAM.

Phenolic acid allelochemicals induced morphological, ultrastructural, and cytological modification on Cassia sophera L. and Allium cepa L.

The allelopathic potential of leaf aqueous extract (LAE) of Calotropis procera on growth behavior, ultrastructural changes on Cassia sophera L., and cytological changes on Allium cepa L. was investigated. LAE at different concentrations (0.5, 1, 2, and 4 %) significantly reduced the root length, shoot length, and dry biomass of C. sophera. Besides, the ultrastructural changes (through scanning electron microscopy, SEM) induced in epidermal cells of 15-day-old seedlings of Cassia leaf were also noticed. The changes induced were shrinking and contraction of epidermal cells along with the formation of major grooves, canals, and cyst-like structures. The treated samples of epidermal cells no longer seem to be smooth as compared to control. LAE at different concentrations induces chromosomal aberrations and variation in shape of the interphase and prophase nucleus in A. cepa root tip cells when compared with control groups. The mitotic index in treated onion root tips decreased with increasing concentrations of the extracts. The most frequent aberrations were despiralization at prophase with the formation of micronuclei, sticky anaphase with bridges, sticky telophase, C-metaphase, etc. The results also show the induction of ghost cells, cells with membrane damage, and cells with heterochromatic nuclei by extract treatment. Upon HPLC analysis, nine phenolic acids (caffeic acid, gentisic acid, catechol, gallic acid, syringic acid, ellagic acid, resorcinol, p-coumaric acid, and p-hydroxy benzoic acid) were identified. Thus, the phenolic acids are mainly responsible for the allelopathic behavior of C. procera.

Effect of aluminum on metabolism of organic acids and chemical forms of aluminum in root tips of Eucalyptus camaldulensis Dehnh.

Eucalyptus (Eucalyptus camaldulensis) has relatively high resistance to aluminum (Al) toxicity than the various herbaceous plants and model plant species. To investigate Al-tolerance mechanism, the metabolism of organic acids and the chemical forms of Al in the target site (root tips) in Eucalyptus was investigated. To do this, 2-year old rooted cuttings of E. camaldulensis were cultivated in half-strength Hoagland solution (pH 4.0) containing Al (0, 0.25, 0.5, 1.0, 2.5 and 5.0mM) salts for 5weeks; growth was not affected at concentrations up to 2.5mM even with Al concentration reaching 6000µgg(-1) DW. In roots, the citrate content also increased with increasing Al application. Concurrently, the activities of aconitase and NADP(+)-isocitrate dehydrogenase, which catalyze the decomposition of citrate, decreased. On the other hand, the activity of citrate synthase was not affected at concentrations up to 2.5mM Al. (27)Al-NMR spectroscopic analyses were carried out where it was found that Al-citrate complexes were a major chemical form present in cell sap of root tips. These findings suggested that E. camaldulensis detoxifies Al by forming Al-citrate complexes, and that this is achieved through Al-induced citrate accumulation in root tips via suppression of the citrate decomposition pathway.

Effects of silicon on the distribution of cadmium compartmentation in root tips of Kandelia obovata (S., L.) Yong.

The Effects of silicon (Si) on the distribution of cadmium (Cd) compartmentation in root tips of Kandelia obovata (S., L.) Yong were investigated by pot experiments. Cd concentrations in the apoplastic saps and symplastic fractions of the root tips of K. obovata seedlings were decreased at both Si-supplied treatments. Si addition reduced the concentrations of BaCl(2-)extractable cell-wall-Cd in root tips, but increased the concentrations of Na(3)citrate-extractable cell-wall-Cd and HCl-extractable cell-wall-Cd in root tips. The total root-tip contents of Cd were mainly distributed in the apoplast and most of the Cd in the apoplast was bound to the cell wall. Our experiment found that Si increased the ratio of apoplast Cd (>87.08%) and reduced the ratio of Cd in the symplast (<12.92%). This suggested that Si enhanced binding of Cd to the cell walls and restricted the apoplastic transport of Cd.

[Genoprotective activities of the oils from leaves and fruits of Fagus orientalis Lipsky].

The antimutagenic activities of the oils obtained from leaves and fruits of Fagus orientalis have been shown in experiments with spontaneous and mutagen- and ageing-induced variability. The aberrations of chromosomes in the meristematic cells of the Allium cepa L., Vicia faba L., Triticum aestivum L., and marrow cells of Vistar rats as well as Arabidopsis thaliana gene mutations have been mobilized as experimental tests.

The cysteine pairs in CLV2 are not necessary for sensing the CLV3 peptide in shoot and root meristems.

Receptor-like proteins (RLPs) are involved in both plant defense and developmental processes. Previous genetic and biochemical studies show that the leucine-rich repeat (LRR) receptor-like protein CLAVATA2 (CLV2) functions together with CLAVATA1 (CLV1) and CORYNE (CRN) in Arabidopsis to limit the stem cell number in shoot apical meristem, while in root it acts with CRN to trigger a premature differentiation of the stem cells after sensing the exogenously applied peptides of CLV3p, CLE19p or CLE40p. It has been proposed that disulfide bonds might be formed through two cysteine pairs in the extracellular LRR domains of CLV1 and CLV2 to stabilize the receptor complex. Here we tested the hypothesis by replacing these cysteines with alanines and showed that depletions of one or both of the cysteine pairs do not hamper the function of CLV2 in SAM maintenance. In vitro peptide assay also showed that removal of the cysteine pairs did not affect the perception of CLV3 peptides in roots. These observations allow us to conclude that the formation of disulfide bonds is not needed for the function of CLV2.

Protein extraction for proteome analysis from cacao leaves and meristems, organs infected by Moniliophthora perniciosa, the causal agent of the witches' broom disease.

Preparation of high-quality proteins from cacao vegetative organs is difficult due to very high endogenous levels of polysaccharides and polyphenols. In order to establish a routine procedure for the application of proteomic and biochemical analysis to cacao tissues, three new protocols were developed; one for apoplastic washing fluid (AWF) extraction, and two for protein extraction--under denaturing and nondenaturing conditions. The first described method allows a quick and easy collection of AWF--using infiltration-centrifugation procedure--that is representative of its composition in intact leaves according to the smaller symplastic contamination detected by the use of the hexose phosphate isomerase marker. Protein extraction under denaturing conditions for 2-DE was remarkably improved by the combination of chemically and physically modified processes including phenol, SDS dense buffer and sonication steps. With this protocol, high-quality proteins from cacao leaves and meristems were isolated, and for the first time well-resolved 1-DE and 2-DE protein patterns of cacao vegetative organs are shown. It also appears that sonication associated with polysaccharide precipitation using tert-butanol was a crucial step for the nondenaturing protein extraction and subsequent enzymatic activity detection. It is expected that the protocols described here could help to develop high-level proteomic and biochemical studies in cacao also being applicable to other recalcitrant plant tissues.

Exogenous application of glycinebetaine increases chilling tolerance in tomato plants.

Tomato (Lycopersicon esculentum Mill. cv. Moneymaker) plants are chilling sensitive, and do not naturally accumulate glycinebetaine (GB), a metabolite that functions as a stress protectant. We reported previously that exogenous GB application enhanced chilling tolerance in tomato. To understand its protective role better, we have further evaluated various parameters associated with improved tolerance. Although its effect was most pronounced in younger plants, this benefit was diminished 1 week after GB application. When administered by foliar spray, GB was readily taken up and translocated to various organs, with the highest levels being measured in meristematic tissues, including the shoot apices and flower buds. In leaves, the majority of endogenous GB was found in the cytosol; only 0.6-22.0% of the total leaf GB was localized in chloroplasts. Immediately after GB application, levels of H(2)O(2), catalase activity and expression of the catalase gene (CAT1) were all higher in GB-treated than in control plants. One day after exposure to chilling stress, the treated plants had significantly greater catalase activity and CAT1 expression, although their H(2)O(2) levels remained unchanged. During the following 2 d of this chilling treatment, GB-treated plants maintained lower H(2)O(2) levels but had higher catalase activity than the controls. These results suggest that, in addition to protecting macromolecules and membranes directly, GB-enhanced chilling tolerance may involve the induction of H(2)O(2)-mediated antioxidant mechanisms, e.g. enhanced catalase expression and catalase activity.

Aluminum fractions in root tips of slash pine and loblolly pine families differing in Al resistance.

Aluminum (Al) distribution among several cellular fractions was investigated in root tips of seedlings of one Al-resistant and one Al-sensitive family of slash pine (Pinus elliottii Engelm.) and loblolly pine (Pinus taeda L.) grown in nutrient solution containing 100 microM AlCl3 (pH 4) for 167 h. Aluminum present in 5-mm-long root tips was fractionated into cell-wall-labile (desorbed in 0.5 mM citric acid), cell-wall-bound (retained after filtering disrupted cells through 20-microm mesh) and symplasmic (filtrate following cell disruption) fractions. When averaged across both species, 12% of Al absorbed by root tips appeared in the symplasmic fraction and 88% in the apoplasmic fraction (55% as cell-wall-labile, and 33% as cell-wall-bound). On a fresh mass basis, total Al in root tips was lower in loblolly pine than in slash pine, lower in the Al-resistant slash pine family than in the Al-sensitive slash pine family, and lower in the Al-resistant families than in the Al-sensitive families across species. Although the data support the hypothesis that Al-resistant plants limit Al uptake to root apices, they do not exclude other mechanisms of Al resistance. Differential Al resistance between the species and between slash pine families may also be associated with the size of the total non-labile and cell-wall-labile Al fractions, respectively. We were unable to identify the basis for differential Al resistance in loblolly pine.

A subclass of myosin XI is associated with mitochondria, plastids, and the molecular chaperone subunit TCP-1alpha in maize.

The role and regulation of specific plant myosins in cyclosis is not well understood. In the present report, an affinity-purified antibody generated against a conserved tail region of some class XI plant myosin isoforms was used for biochemical and immunofluorescence studies of Zea mays. Myosin XI co-localized with plastids and mitochondria but not with nuclei, the Golgi apparatus, endoplasmic reticulum, or peroxisomes. This suggests that myosin XI is involved in the motility of specific organelles. Myosin XI was more than 50% co-localized with tailless complex polypeptide-1alpha (TCP-1alpha) in tissue sections of mature tissues located more than 1.0 mm from the apex, and the two proteins co-eluted from gel filtration and ion exchange columns. On Western blots, TCP-1alpha isoforms showed a developmental shift from the youngest 5.0 mm of the root to more mature regions that were more than 10.0 mm from the apex. This developmental shift coincided with a higher percentage of myosin XI /TCP-1alpha co-localization, and faster degradation of myosin XI by serine protease. Our results suggest that class XI plant myosin requires TCP-1alpha for regulating folding or providing protection against denaturation.

Viewing the difference between the diploid and the polyploid in the light of the upland cotton aneuploid.

The aneuploidy of Gossypium hirsutum L. (upland cotton) aneusomatics were obtained by induced parthenogenesis. These aneuploids could grow and set seeds normally. In the process of meiosis there appeared large quantities of heteromorphic pairs and multivalent chromosomes and many cases of cytomixis and multisperm fertilization occurred. The aneuploids produced offsprings through sexual propagation. We explored penetratingly the questions how and why these aneuploids could survive. Through this research, we found that the upland cotton possessed an immense latent capacity to adapt to adverse environments. More importantly, in the case of the upland cotton, we discovered that the genetic pattern of the polyploid differs in some respects from that of the diploid.

In situ localisation of cytokinins in the shoot apical meristem of Sinapis alba at floral transition.

In plants of Sinapis alba L. induced to flower by one long day (LD), previous work showed that the phloem sap feeding the shoot apex is enriched in cytokinins of the isopentenyladenine (iP)-type between 9 and 25 h after start of the LD [P. Lejeune et al. (1994) Physiol Plant 90:522-528]. We have checked the hypothesis that the cytokinin content of the shoot apical meristem (SAM) should increase in response to floral induction by one LD using histoimmunolocalisation techniques and rabbit antiserum against isopentenyladenosine or zeatin riboside. The free bases iP and zeatin are present only in apical tissues containing dividing cells. At 30 h after the start of an inductive LD, a markedly increased iP immune reaction is observed in SAM tissues while the level of zeatin is not modified. Our results are in line with the data obtained by analysis of phloem sap.