The effect of water stress on glutathione reductase and catalase activities was evaluated in leaf blades of field-grown winter wheat (Triticum aestirum L.). Wheat was sown at two seeding rates under both irrigated and dryland conditions. Flag leaves from dryland plants sown at 60 kilograms/hectare showed no change in either glutathione reductase or catalase activities per unit leaf area, while leaves from the basal portion of the canopy exhibited a 273% increase in glutathione reductase activity and a 60% increase in catalase activity. Glutathione reductase activity in dryland plants sown at 120 kilograms/hectare increased 25% in flag leaves and 225% in basal leaves. No change in catalase activity was observed in either flag or basal leaves from these same plants. The increase in glutathione reductase activity in response to water stress was observed when activity was expressed on either a per unit leaf area, protein, or chlorophyll basis. No change in catalase activity was detected when enzyme activity was expressed on a protein basis.The midday closure ofstomates (20) in response to water stress may optimize the water use efficiency of the plant on a daily basis (3), yet result in a reduction in CO2 assimilation at times when peak irradiances are commonly encountered. Because of stomatal closure, CO2 fixation is low while photosynthetic electron transport is operating at normal rates. Under these conditions, limited quantities of NADP are available to accept electrons, therefore oxygen can function as an alternative electron acceptor (6). Although this pseudocyclic pathway for electron transport provides additional ATP (15), it can result in the production of superoxide and H202.Superoxide and H202 are toxic oxygen molecules and upon reaction with chloroplast components can form even more reactive oxygen products such as the hydroxyl radical (7) and singlet oxygen (19). These toxic products produced within the chloroplast must be effectively removed to prevent lipid peroxidation, inhibition of C02-fixation, and the photooxidation of chloroplast pigments.Photosynthetic cells can tolerate elevated oxygen levels because of the presence of several endogenous protective mechanisms including glutathione, ascorbate, carotenoids, and enzymes which effectively scavenge and remove the toxic products before cellular damage occurs (13). Plant cells contain millimolar concentrations of reduced glutathione (GSH) which can prevent the inactivation of enzymes by oxidation of essential thiol groups.Because of the availability of GSH, it can be preferentially oxidized thereby protecting the enzymes from inactivation. GSH can also reduce an oxidized sulthydryl group thereby reactivating certain enzymes. When GSH reacts with oxygen, the glutathione is oxidized to form glutathione disulfide (GSSG). The subsequent re-reduction of GSSG to GSH is catalyzed by the enzyme glutathione reductase in an NADPH-dependent reaction. Glutathione reductase, therefore, plays an essential role in the protection of chloroplasts against oxid...

Treatment of dark-grown barley with 0.1 mM fluridone inhibited carotenoid accumulation but did not alter plastid biogenesis. Plastids isolated from dark-grown control and dark-grown fluridone-treated plants were similar in size and protein compositions. Dehydration of dark-grown control barley caused abscisic acid levels to increase 30-40-fold in 4 h, while plants treated with 0.1 mM fluridone accumulated very little abscisic acid in response to dehydration. These results suggest that fluridone-treated plants do not accumulate abscisic acid because of carotenoid deficiency rather than plastid dysfunction. Dark-grown barley plants treated with 0.31 pM fluridone accumulated low levels of carotenoids. Dehydration of these plants resulted in a 4-8-fold increase in abscisic acid and a decrease in antheraxanthin, violaxanthin and neoxanthin, but no change in j?-carotene or lutein plus zeaxanthin levels. This result is consistent with the suggestion that xanthophylls are precursors to abscisic acid in dehydrated plants.A striking response of plants to dehydration is the accumulation of the plant growth regulator, abscisic acid (ABA) [l]. The increase in ABA levels results from increased biosynthesis [4 -81 and correlates with decreased cell turgor [2] or a cell-wall/plasmalemma perturbation [3]. It has recently been shown that transcription [9] and cytoplasmic translation [9, 101 must be active in order for ABA levels to increase in dehydrated plants. However, it is not known which step(s) in the ABA biosynthetic pathway are altered to cause the increase.ABA is synthesized from mevalonic acid [ l l , 121 (Fig. 1). Two isoprenoid compounds have been proposed as intermediates in the ABA biosynthetic pathway (Fig. 1). The first, farnesyl pyrophosphate, has been shown to be a precursor of ABA in Cercospora [13, 141. However, in higher plants evidence for the involvement of this compound is less clear [14]. The second proposed intermediate, violaxanthin, was originally suggested because of its structural similarity to ABA [15]. It was also found that violaxanthin could be converted to xanthoxin by light [I61 or by lipoxygenase [17] and that plants could convert xanthoxin to ABA [18]. Additional support for xanthophylls as precursors to ABA has come from two types of experiment. First, exposure of dehydrated plants to "02 during synthesis of ABA implicated a preformed oxygenated precursor to ABA [19, 141. Second, it was found that carotenoid-deficient plants (Fig. 1 a link between chloroplast function and ABA biosynthesisIn this paper we have tried to separate the direct effect of carotenoid deficiency on ABA biosynthesis from the effect of plastid photo-oxidation, which occurs in carotenoid-deficient plants, on ABA biosynthesis. Our results show that carotenoid deficiency in the absence of chloroplast photo-oxidation inhibits dehydration-induced ABA biosynthesis.[lo, 201. MATERIALS AND METHODS Plant growth , jlur idone treatment and dehydration proceduresBarley (Hordeum vulgare L. var Morex) seeds were soaked for 24 h in...

Synthesis of D2, a Photosystem II reaction center protein encoded by psbD, is differentially maintained during light‐induced chloroplast maturation. The continued synthesis of D2 is paralleled by selective light‐induced accumulation of two psbD‐psbC transcripts which share a common 5′ terminus. In the present study, we examine the nature of the photoreceptor and the fluence requirement for psbD‐psbC transcript induction. The light‐induced change in psbD‐psbC RNA population can be detected between 1 and 2 h after 4.5 day old dark‐grown barley seedlings are transferred to the light. Light‐induced transcript accumulation occurs normally in the chlorophyll‐deficient barley mutant, xan‐f10, indicating that light‐activated chlorophyll formation and photosynthesis are not required for RNA induction. High fluence blue light fully induces psbD‐psbC transcript accumulation; low or high fluence red or far‐red light do not. However, psbD‐psbC transcript accumulation elicited by blue light pulses can be partially attenuated if far‐red light is given immediately following the blue light treatment. Thus, although blue light is needed to initiate transcript accumulation, phytochrome modulates the amplitude of the response. Pretreatment of dark‐grown plants with cycloheximide blocks light‐induced psbD‐psbC transcript accumulation. This could implicate a blue‐light responsive nuclear gene in the light‐induced accumulation of the two psbD‐psbC transcripts.

Four plastid genes, psaA, psaB, psbD and psbC, were localized on the barley plastid genome. PsaA was adjacent to psaB in one transcription unit and psbD was adjacent to psbC in a second transcription unit. The transcription units containing psaA-psaB and psbD-psbC are separated by approximately 25 kbp on the barley plastid genome and are transcribed convergently. Transcripts hybridizing to each transcription unit were characterized by northern blot analysis, S1 protection experiments and primer extension analysis. Two 5.3 kb transcripts hybridize to psaA-psaB. The two transcripts have a common 5' end but differ at their 3' ends by about 26 nucleotides. The transcription unit which contains psbD-psbC also includes trnS(UGA), trnG(GCC), and an open reading frame which codes for a 62 amino acid protein. Six large transcripts ranging from 5.7 kb to 1.7 kb hybridize to the psbD-psbC transcription unit as well as several RNAs of tRNA size. The large transcripts arise from three 5' ends and two clusters of 3' ends. The 3' ends map near trnG(GCC) and trnS(UGA) and could be generated by RNA processing or termination of transcription. Two of the six transcripts hybridize to psbC but not psbD suggesting that translation of psbD and psbC could occur on separate RNAs.

The accumulation of radiolabeled plastid-encoded chlorophyll a-apoproteins is light dependent and is controlled at a posttranscriptional level.