Cytokinins enhance the metabolic activity of in vitro-grown catmint (Nepeta nuda L.).

The phytohormones cytokinins are essential mediators of developmental and environmental signaling, primarily during cell division and endophytic interactions, among other processes. Considering the limited understanding of the regulatory mechanisms that affect the growth and bioactivity of the medicinal plant Nepeta nuda (Lamiaceae), our study aimed to explore how cytokinins influence the plant's metabolic status. Exogenous administration of active cytokinin forms on in vitro N. nuda internodes stimulated intensive callus formation and de novo shoot regeneration, leading to a marked increase in biomass. This process involved an accumulation of oxidants, which were scavenged by peroxidases using phenolics as substrates. The callus tissue formed upon the addition of the cytokinin 6-benzylaminopurine (BAP) acted as a sink for sugars and phenolics during the allocation of nutrients between the culture medium and regenerated plants. In accordance, the cytokinin significantly enhanced the content of polar metabolites and their respective in vitro biological activities compared to untreated in vitro and wild-grown plants. The BAP-mediated accumulation of major phenolic metabolites, rosmarinic acid (RA) and caffeic acid (CA), corresponded with variations in the expression levels of genes involved in their biosynthesis. In contrast, the accumulation of iridoids and the expression of corresponding biosynthetic genes were not significantly affected. In conclusion, our study elucidated the mechanism of cytokinin action in N. nuda in vitro culture and demonstrated its potential in stimulating the production of bioactive compounds. This knowledge could serve as a basis for further investigations of the environmental impact on plant productivity.

Elevated CO2 concentration around alfalfa nodules increases N2 fixation.

Nodule CO2 fixation via PEPC provides malate for bacteroids and oxaloacetate for N assimilation. The process is therefore of central importance for efficient nitrogen fixation. Nodule CO2 fixation is known to depend on external CO2 concentration. The hypothesis of the present paper was that nitrogen fixation in alfalfa plants is enhanced when the nodules are exposed to elevated CO2 concentrations. Therefore nodulated plants of alfalfa were grown in a hydroponic system that allowed separate aeration of the root/nodule compartment that avoided any gas leakage to the shoots. The root/nodule compartments were aerated either with a 2500 microl l(-1) (+CO2) or zero microl l(-1) (-CO2) CO2-containing N2/O2 gas flow (80/20, v/v). Nodule CO2 fixation, nitrogen fixation, and growth were strongly increased in the +CO2 treatment in a 3-week experimental period. More intensive CO2 and nitrogen fixation coincided with higher per plant amounts of amino acids and organic acids in the nodules. Moreover, the concentration of asparagine was increased in both the nodules and the xylem sap. Plants in the +CO2 treatment tended to develop nodules with higher %N concentration and individual activity. In a parallel experiment on plants with inefficient nodules (fix-) the +CO2 treatment remained without effect. Our data support the thesis that nodule CO2 fixation is pivotal for efficient nitrogen fixation. It is concluded that strategies which enhance nodule CO2 fixation will improve nitrogen fixation and nodule formation. Moreover, sufficient CO2 application to roots and nodules is necessary for growth and efficient nitrogen fixation in hydroponic and aeroponic growth systems.

Effect of foliar feeding on nitrogen assimilation in alfalfa plants at insufficient molybdenum supply.

The influence of foliar feeding on the nitrogen assimilation in alfalfa plants under conditions of Mo shortage was studied. It was established that foliar fertilization with 0.3% solution of Agroleaf® resulted in increase of nitrogen fixation and nitrogen assimilation in the absence of Mo. Insufficient molybdenum supply leads to significant reduction of plant Mo content and nitrogen-fixing activity, while stress induced amino acids as alanine, GABA, threonine, proline and serine increased repeatedly. The negative effect of Mo deficiency on the enzyme activities related to the primary nitrogen assimilation (NR, GS, GOGAT) and plant growth diminished due to the foliar absorbed nutrients.

Response of inoculated foliar fed pea plants (Pisum sativum L.) to reduced Mo supply.

The application of nutrients to the roots and leaves of inoculated pea plants grown under conditions of reduced Mo supply was studied. Pea plants (Pisum sativum L.) were grown on liquid nutrient solution excluding Mo from the media until the 35th day under glasshouse conditions. Plants were inoculated with the bacterial suspension of Rhizobium leguminosarum Bv. Vicae, strain D293 at approximately 10(8) cells per cm3. The foliar fertilizer Agroleaf was applied at 0.3% concentration. Changes in the root nodulation and the activities of the enzymes connected with nitrogen assimilation pathway (nitrate reductase--NR-NADH: EC 1.6.6.1; glutamine synthetase--GS: EC 6.3.1.2; glutamate synthase--NADH-GOGAT: EC 1.4.1.14 and nitrogenase--NG: EC 1.7.99.2) were observed. It was established that the foliar application of nutrients reduced the inhibitory effect on the root nodulation and nitrogen assimilatory enzyme activities due to the Mo shortage.