TheeffectofthenaturalcompoundS-methylmethionine(SMM)onthefunctioningofthephotosyntheticapparatus,theefficiencyof photosynthesis and the synthesis of stress-induced phenoloids and anthocyanins involved in defence was investigated in young maize plantsexposedtomoderateandseverechillingstress.DamagetoPSIIwasobservedasareductioninthevalueofvariablefluorescence (Fv/Fm) which could be detected even after few hours of mild chilling stress. At temperatures below 10°C, the reduction in Fv/Fm was more pronounced. Changes in the value of net photosynthesis exhibited a similar tendency. SMM has a moderating effect on this reduction and itsprotectiveeffectwasmorepronouncedunderlong-lastingchillingstressandatthelowesttemperatures.Monitoringoffluorescence intensitiesandratioscorrelatedwiththelevelsofstressdefencecompounds.Thefluorescenceintensitieswerefoundtoincreaseover the course of chilling stress in response to SMM, with the highest values being recorded in plants exposed to the longest period of stress. Asimilartendencywasobservedforthequantityofanthocyanins.TheresultsconfirmthecomplexroleofSMM,whichismanifestedbothin preserving the ability of the photosynthetic apparatus to function and in stimulating the synthesis of metabolites involved in stress defence.

S-methylmethionine (SMM), a naturally occurring, biologically active compound, is a free amino acid derivative, which is increasingly recognised as playing an important part in the plant metabolism. SMM, which is synthesised from methionine, is involved in crucial processes in the S metabolism, such as the regulation of methionine and S-adenosyl methionine levels, the methylation processes taking place in cells, and the transport and storage of sulphur in certain phases of development. It is of great importance in the development of resistance to abiotic and biotic stress factors, as it is a direct precursor in the biosynthesis of the osmoprotectants and other S-containing compounds involved in defence mechanisms, while also influencing the biosynthesis of major plant hormones such as polyamines and ethylene. The present paper discusses our increasing understanding of the role played by SMM in the plant metabolism and its possible role in the improvement of traits that enable plants to overcome stress.

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S-methylmethionine (SMM) is an important intermediary compound in the sulphur metabolism and has been shown to play a possible role in moderating the damaging effects of low temperature stress. The present work investigated the extent to which SMM is capable of influencing the activity of antioxidant enzymes when the subtropical species maize is exposed to chilling temperatures during the early developmental phase. SMM was found to contribute to the protection of maize seedlings against low (<14°C) temperature stress by enhancing the activity of certain antioxidant enzymes to varying extents, and thus helping to neutralise the reactive oxygen species (ROS) formed at this temperature. Results obtained in a gradient plant growth chamber revealed that, with the exception of catalase, SMM increased the activity of all the antioxidants studied (glutathione reductase, glutathione-S-transferase, guaiacol peroxidase, ascorbate peroxidase), particularly in the lower ranges of the temperature gradient (6–14°C).

The level of UV-B radiation reaching the surface of the earth is increasing due to the thinning of the ozone layer in the stratosphere over recent decades. This has numerous negative effects on living organisms.Some of the Hungarian inbred maize lines examined under the climatic conditions in Chile exhibited an unusually high proportion of pollen mortality, flowering asynchrony and barrenness. The evidence suggests that this can be attributed to the approx. 30% greater UV-B radiation in Chile.The investigation of this problem within the framework of abiotic stress breeding programmes is extremely important in the light of the global rise in UV-B radiation, which may make it necessary to elaborate a selection programme to develop inbred lines with better tolerance of this type of radiation.In the course of the experiment the same ten inbred lines, having different maturity dates and genetic backgrounds, were tested for five years in Chile and Hungary. The tests focussed on anthocyanin, a flavonoid derivative involved in the absorption of damaging UV-B radiation.Averaged over years and varieties, the total anthocyanin content in the leaf samples was significantly higher in Chile than in Hungary. This was presumably a response at the metabolic level to the negative stress represented by higher UV-B radiation.In the five early-maturing flint lines the anthocyanin contents were more than 45% greater than those recorded in Hungary. This suggests that these genotypes, originating from northern regions, were not sufficiently adapted to the higher radiation level. In these samples higher UV-B caused a sharp rise in the quantity of anthocyanin, which absorbs the dangerous radiation. In late-maturing genotypes the initial content of the protective compound anthocyanin was higher at both locations, so in these types the high radiation level was not a problem and did not cause any substantial change.Similar conclusions were drawn from the results of fluorescence imaging. The F440/F690 ratio indicative of the stress level was higher in late lines with a high anthocyanin content, good tolerance and good adaptability.Key words: Zea mays L., inbred line, ultraviolet-B radiation, flavone, anthocyanin, stress acclimation, fluorescence imaging J. PINTÉR et al. Hungarica, 55, 2007 8 Acta Agronomica IntroductionOne negative consequence of global climate change is the thinning of the ozone layer in the stratosphere, which functions as a shield against dangerous radiation. This has been caused in part by human activities, especially by the ozone-antagonistic gases emitted by heavy industry (Anderson et al., 1991;McFarland and Kaye, 1992;McKenzie et al., 1999;Reddy and Hodges, 2000).A decrease of 1% in the ozone level may result in a 1.3-1.8% rise in the UV-B radiation (280-320 nm), the most biologically active form of ultraviolet radiation, with the most damaging effect on living organisms (Madronich, 1993). Naturally, the level of dangerous radiation depends on many factors, such as the latitude and altitude of the given location, ...