SummaryStress ethylene emission is positively correlated with ozone sensitivity in various plant species, indicating that ethylene may be involved in the control of ozone damage. This study shows that ozone exposure of tomato plants for 5 h at 85 nl 1-1 and above leads to leaf injury within 24 h. 1-aminocyclopropane-l-carboxylic acid (ACC) content and ACC synthase activity were accordingly elevated within 1-2 h. Pre-treatment of leaves with inhibitors of ACC synthase and ACC oxidase significantly inhibited the evolution of ethylene and reduced ozone-induced visible damage. Transcript levels for only one out of three SadenosyI-L-methionine (SAM) synthetase genes (SAM3), and one out of four ACC synthase genes (LE-ACS2) were induced by ozone (maximum at 2 h). Treatment with protein kinase (K-252a) and phosphatase inhibitors (calyculin A) revealed that ACC synthase activity was additionally regulated by protein phosphorylation/dephosphorylation. Transcripts of ACC oxidase (pTOM13 cDNA probe) displayed the fastest response of the parameters tested (maximum at 30 min), suggesting a regulatory role for ACC oxidase in ethylene formation of ozone-exposed plants. The results demonstrate a highly selective ozone response by ethylene biosynthetic genes which resembles that of plant-pathogen interactions.
Ammonia was necessary for the origin of life. However, NH3 would not have been a significant component of the neutral or mildly reducing (N2, CO2, H2O) atmosphere which characterized the Hadean earth (>3·8 Gyr ago), especially in view of the u.v. lability of NH3, the greater u.v. output of the young sun than pertains today, and the absence of an atmospheric u.v. screen. Heterogeneous phase reactions, following atmospheric chemistry, have been proposed as a prebiotic NH3 source. Lightning, or bolides (meteorites and comets), generated NOx., and Fe2+ in the sea could have reduced NO2− resulting from the NOx. to NH4+; this disequilibrium NH4+ level could have supported the production of the nitrogenous organic building blocks of life. NOx. and NHy thus could both have had important roles in the origin and evolution of life. Burgeoning biota could soon have depleted abiotically generated NH4+, and biological N2 fixation could have evolved in its present Fe‐demanding, O2‐sensitive forms in the Archaean O2‐free, Fe2+‐rich environment. Organic matter could have driven biological denitrification reactions based on NO2− and NO3− generated abiologically using some redox components which evolved in earlier chemolithotrophs and photolithotrophs, regenerating atmospheric NOx. and producing N2O. Any atmospheric NH3 leaking from oceanic biology would have been subject to u.v. breakdown and rain‐out. Although O2‐evolving photosynthesis probably began in the Archean some 3·5 Gyr ago, any O2 accumulation was local until 2·0 Gyr ago (Proterozoic) due to consumption by oxidation of Fe2+ and S2−. However, localized O2 accumulation before 2·0 Gyr ago could account for the observed early evolution of cytochrome oxidase and the possibility of O2‐consuming chemolithotrophic and chemoorganotrophic nitrification, with further possibilities of NOx. production. Oxygen accumulation globally from 2·0 Gyr onwards coincided approximately with the evolution of eukaryotes, which contributed phagotrophy to the reactions of the N cycle as well as the nutrification‐like aerobic production of NO. by nitric oxide synthetase, while lightning and bolides could now generate NO. from N2 and O2. Evidence for terrestrial ecosystems is found from 1·0 Gyr onwards; NOx. and NH3 generated by terrestrial biota stands a greater chance of escaping to the atmosphere than do these compounds generated in the sea where recycling within the water body is likely. As CO2 levels fell and O2 levels rose, NH3 cycling in the photorespiratory carbon oxidation cycle might have been evident as early as 1 Gyr ago, although this does not seem to be a major contributor to atmospheric NH3 today. Embryophyte evolution on land 450 Myr ago, together with symbionts and biophages, increased primary productivity and N cycling on land, with greater quantitative possibilities for NOx. and NH3 escape to the atmosphere. The evolution of lignin (and related phenylpropanoids) at least 400 Myr ago, with associated NH3 recycling in vascular land plant, does not seem (on present evidence) to incre...
The impacts of various nitrogen sources, i.e. NO À 3 , NH 4 or NH 4 NO 3 in combination with gaseous NH 3 , on nitrogen-, carbon-and water-use eciency and 13 C discrimination (d 13 C) by plants of the C 3 species Triticum aestivum L. (wheat) and the C 4 species Zea mays L. (maize) were studied. Triticum aestivum and Z. mays were hydroponically grown with 2 mol á m A3 of N supplied as NO À 3 , NH 4 or NH 4 NO 3 for 21 and 18 d, respectively, and thereafter exposed to gaseous NH 3 at 320 lg á m A3 or to ambient air for 7 d. In T. aestivum and Z. mays over a 7-d growth period, nitrogen-use eciency (NUE) values were in¯uenced by N-sources in the decreasing order NH 4 NO 3 -N > NO À 3 -N > NH 4 -N and NO À 3 -N > NH 4 NO 3 -N > NH 4 -N, respectively. Fumigation with NH 3 decreased the NUE values of plants grown with any of the N-forms. During 28-and 7-d growth periods, N-sources aected water-use eciency (WUE) values in the decreasing order of NH 4 -N > NO À 3 -N % NH 4 NO 3 -N in non-fumigated T. aestivum, while fumigation with NH 3 increased the WUE of NO À 3 -grown plants. There were insigni®cant eects of N-sources on WUE values of Z. mays over 25-and 7-d growth periods. Furthermore, d 13 C values in plant tissues (leaves, stubble and roots) were higher (less negative) in NH 4 -grown plants of T. aestivum and Z. mays than in those supplied with NH 4 NO 3 or NO À 3 . Regardless of the N-form supplied to the roots of the plant species, exposure to NH 3 caused more-positive d 13 C values in the plant tissues. These results indicate that the variations in N-source were associated with small but signi®cant variations in d 13 C values in plants of T. aestivum and Z. mays. These dierences in d 13 C values are in the direction expected from dierences in WUE values over long or short growth periods and with dierences in the extent of non-Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase, EC 4.1.1.39) carboxylate contribution to net C acquisition, as a function of N-source.
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