The role of nitric oxide (NO) in photosynthesis is poorly understood as indicated by a number of studies in this field with often conflicting results. As various NO donors may be the primary source of discrepancies, the aim of this study was to apply a set of NO donors and its scavengers, and examine the effect of exogenous NO on photosynthetic electron transport in vivo as determined by chlorophyll fluorescence of pea (Pisum sativum) leaves. Sodium nitroprusside-induced changes were shown to be mediated partly by cyanide, and S-nitroso-N-acetylpenicillinamine provided low yields of NO. However, the effects of S-nitrosoglutathione are inferred exclusively by NO, which made it an ideal choice for this study. Q A 2 reoxidation kinetics show that NO slows down electron transfer between Q A and Q B , and inhibits charge recombination reactions of Q A 2 with the S 2 state of the water-oxidizing complex in photosystem II. Consistent with these results, chlorophyll fluorescence induction suggests that NO also inhibits steady-state photochemical and nonphotochemical quenching processes. NO also appears to modulate reaction-center-associated nonphotochemical quenching.Plants, as well as animals, respond to ambient levels of nitric oxide (NO), and also generate NO themselves via various enzymatic and nonenzymatic pathways (Yamasaki, 2000;Neill et al., 2003;Río et al., 2004). Indeed, in the past years, a growing amount of research has provided evidence for the multiple physiological roles of this gaseous free radical in plants (for review, see Wendehenne et al., 2004;Delledonne, 2005). The turnover of NO depends on its concentration, the ambient redox status, and the concentration of target molecules. In biological systems, NO is capable of targeting thiol-and metal-containing proteins (Lamattina et al., 2003). Photosynthetic and mitochondrial electron transport chains are abundant in transition metalcontaining complexes, and NO and its derivative peroxynitrite are known to inhibit the mitochondrial electron transport chain (Millar and Day, 1996;Yamasaki et al., 2001). Yet, the effect of exogenous NO on photosynthetic activity in intact leaves has so far been poorly addressed, with often conflicting results.Previous research suggests that NO gas decreases net photosynthesis rates in oat (Avena sativa) and alfalfa (Medicago sativa) leaves (Hill and Bennett, 1970). Lum et al. (2005) have identified a number of intracellular targets of NO signalization including mitochondria, peroxisomes, and chloroplasts. They found that the NO donor sodium nitroprusside (SNP) decreases the amount of Rubisco activase and the b-subunit of the Rubisco subunit-binding protein in mung bean (Phaseolus aureus).NO is also able to influence the photosynthetic electron transport chain directly. An important action site of NO is PSII. Electron paramagnetic resonance and chlorophyll fluorescence measurements using NO gas treatment of isolated thylakoid membrane complexes have clearly demonstrated that NO can reversibly bind to several sites in PSII and...

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We compared the enzymatic antioxidative defence mechanisms of some regional subspecies of Allium (A. cepa L., A. ascalonicum auct. hort., A. sativum L.) cultivated mainly in the western regions of Romania, and two modern Hungarian climate resistant F 1 hybrids. The variability in the activities of antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione reductase (GR) and glutathione S-transferase (GST) and their changes under soil moisture stress were investigated. 1-week-long water stress revealed that among three Allium species, relative water content decreased only in A. ascalonicum leaves (up to 16%). Unlike root enzymes, the activities of the shoot enzymes, especially POD, GR and GST showed a stronger correlation with the water content of the leaves after one week of water withdrawal; regression coefficients (R 2 ) were 0.359, 0.518 and 0.279, respectively. The ancient populations with elevated (or highly inducible) antioxidant enzyme activities may be interesting for further research and for breeding of new Allium varieties.

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Structural analysis of K+ channel pores suggests that the selectivity filter of the pore is an inherent sensor for extracellular K+ (Ko+); channels seem to be inactivated at low Ko+ because of a destabilization of the conducting state and a collapse of the pore. In the present study, the effect of depleting Ko+ on the activity of a plant K+ channel, KAT1, from Arabidopsis thaliana was investigated. This channel is thought to be insensitive to Ko+. The channel was therefore expressed in mammalian HEK293 cells and measured with patch clamp technology in the whole cell configuration. The effect of Ko+ depletion on channel activity was monitored from the tail currents before, during, and after washing Ko+ from the medium. The data show that a depletion of Ko+ results in a decrease in channel conductance, irrespective of whether K+ is simply removed or replaced by either Na+ or Li+. Quantitative analysis suggests that the channel has two binding sites for K+ with the dissociation constant in the order of 20 microM. This high sensitivity of the channel to Ko+ could serve as a safety mechanism, which inactivates the channel at low Ko+ and, in this way, prevents leakage of K+ from the cells via this type of channel.

Nitric oxide (NO) is one of the key elements in the complex signalling pathway leading to stomatal closure by inducing reversible protein phosphorylation and Ca(2+) release from intracellular stores. As photosynthesis in guard cells also contributes to stomatal function, the aim of this study was to explore the potential role of NO as a photosynthetic regulator. This work provides the first description of the reversible inhibition of the effect of NO on guard cell photosynthetic electron transport. Pulse amplitude modulation (PAM) chlorophyll fluorescence measurements on individual stomata of peeled abaxial epidermal strips indicated that exogenously applied 450nM NO rapidly increases the relative fluorescence yield, followed by a slow and constant decline. It was found that NO instantly decreases photochemical fluorescence quenching coefficients (qP and qL), the operating quantum efficiency of photosystem II (ΦPSII), and non-photochemical quenching (NPQ) to close to zero with different kinetics. NO caused a decrease in NPQ, which is followed by a slow and continuous rise. The removal of NO from the medium surrounding the epidermal strips using a rapid liquid perfusion system showed that the effect of NO on qP and ΦPSII, and thus on the linear electron transport rate through PSII (ETR), is reversible, and the constant rise in NPQ disappears, resulting in a near steady-state value. The reversible inhibition by NO of the ETR could be restored by bicarbonate, a compound known to compete with NO for one of the two coordination sites of the non-haem iron (II) in the QAFe(2+)QB complex.

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