Early Imbibition Research Articles

Isolation and characterization of a cDNA encoding a sar-like monomeric GTP-binding protein in Avena fatua L.

Differential display of mRNAs from embryos of Avena fatua L. caryopses was used to isolate an mRNA more abundant in nondormant than dormant caryopses during early imbibition. The DNA sequence of the corresponding 579 bp cDNA, termed Af SAR1, is 92% identical to an Arabidopsis cDNA encoding the monomeric GTP-binding protein sar1p. Predicted amino acid sequences of the four conserved GTP binding and hydrolysis domains in Af SAR1 are 100% identical to sar1p. Af SAR1 mRNA levels increased 6-fold or more than 10-fold in nondormant embryos during the first 48 h of imbibition in water or GA 3, respectively. However, mRNA levels increased only slightly and transiently in dormant embryos imbibed in water. mRNA abundance was highest in meristematic and actively growing tissues of A. fatua seedlings. Af SAR1 belongs to a small (two to four members) gene family as judged by Southern hybridizations. Increased abundance of this mRNA during early germination and in actively growing tissues indicates that the respective protein is associated with rapid cell elongation, cell division and growth.

The dormancy-related peroxiredoxin anti-oxidant, PER1, is localized to the nucleus of barley embryo and aleurone cells.

Protection against desiccation-induced injury, including damage by reactive oxygen species (ROS), is a necessary component of the genetic programmes active during late seed development. Likewise, protection against ROS respiration by-products is required during seed imbibition and germination. Late embryogenesis abundant (LEA) proteins are proposed to protect seed tissues against desiccation-induced damage. Specifically, the atypical Lea gene Per1 in barley (Hordeum vulgare L.) has been proposed to play a protective role in embryo and aleurone cells against free-radical damage during late seed development and early imbibition. PER1 represents a subgroup of the peroxiredoxin family of thiol-requiring anti-oxidants with one conserved cysteine residue (1-Cys), and displays in vitro anti-oxidant activity. In this work, we use antiserum generated against PER1 to study protein accumulation patterns as well as localization at the tissue, cellular and subcellular level. While previous studies have shown the Per1 transcript to be dormancy-related, we show here that the protein level is maintained in imbibed dormant seeds, but not in non-dormant seeds. Our data identify the location of this seed-specific peroxiredoxin as the nucleus of immature embryos and aleurone layers. Highest levels of protein are detected in nucleoli. In contrast, in mature imbibed dormant seeds, cytosolic levels are comparable to that of the nucleus. A putative nuclear localization signal (NLS) of bipartite nature was identified in the C-terminal end of the PER1 sequence. Protective roles for PER1 in seeds are discussed.

Peroxiredoxin antioxidants in seed physiology

AbstractPeroxiredoxins are thiol–requiring antioxidants found in organisms ranging from bacteria to humans. They can be divided into two subgroups with either one or two conserved cysteine residues. In plants, 1–Cys peroxiredoxins have been identified in a number of grasses and cereals, and in the dicotyledonous speciesArabidopsis thaliana. In contrast to other antioxidants, the 1–Cys peroxiredoxin genes are expressed solely in seeds, and only in the parts of the seeds surviving desiccation, i.e. the embryo and the aleurone layer. The expression pattern is characteristic of late embryogenesis–abundant genes. The PER1 protein of barley is present in high concentrations in the nucleus at the onset of desiccation. 1–Cys genes are expressed in a dormancy–related manner in mature seeds, in that transcript levels are high in imbibed dormant seeds, but disappear upon germination of their non–dormant counterparts. 1–Cys transcript levels can be up–regulated by ABA and osmotic stresses and suppressed by gibberellic acid. Two hypotheses have been put forward on the function of 1–Cys peroxiredoxins in seed physiology. First, these proteins might protect macromolecules of embryo and aleurone cells against damaging reactive oxygen species during seed desiccation and early imbibition. And second, seed peroxiredoxins might play a role in the maintenance of dormancy. These hypotheses are discussed, taking into account present knowledge of the biochemistry and molecular biology of peroxiredoxins.

The expression of a peroxiredoxin antioxidant gene, AtPer1, in Arabidopsis thaliana is seed-specific and related to dormancy.

We have isolated a gene, AtPer1, from the dicotyledon Arabidopsis thaliana, which shows similarity to the 1-cysteine (1-Cys) peroxiredoxin family of antioxidants. In higher plants, members of this group of antioxidants have previously only been isolated from monocotyledons. It has been suggested that seed peroxiredoxins protect tissues from reactive oxygen species during desiccation and early imbibition and/or are involved in the maintenance of/protection during dormancy. AtPer1 expression is restricted to seeds. Despite differences in seed development between monocots and dicots, AtPer1 shows an expression pattern during seed development and germination similar to the dormancy-related transcript Per1 in barley. In situ hybridization identifies AtPer1 as the first aleurone-expressed transcript characterized in developing Arabidopsis seeds. The transcript is also expressed in the embryo. AtPer1 expression in seeds is unaltered in an ABA-deficient mutant (aba-1) during seed development, while expression in seeds of an ABA-insensitive mutant (abi3-1) is reduced. The transcript is not induced in vegetative tissue in response to stress by ABA or drought. AtPer1 transcript levels are correlated to germination frequencies of wildtype seeds, but AtPer1 transcript abundance is not sufficient for expression of dormancy in non-dormant mutants. Hypotheses on peroxiredoxin function are discussed in view of the results presented here.

Active oxygen and their scavengers in rice seeds (Oryza sativacv. IET 4094) aged under tropical environmental conditions

AbstractElectron paramagnetic resonance studies of high vigour (99% viable) and low vigour (38% viable) dry embryos of rice seeds (Oryza sativaL.) stored in a natural (warm and humid) environment were carried out. Loss in viability due to hot and humid conditions was found to be correlated with a decrease in free radical levels. The free radicals could be carbon-based and derived from quinones. Presence of the active oxygen scavenging enzyme, superoxide dismutase (EC 1.15.1.1) in embryos isolated from dry seeds showed a positive corrrelation with the state of vigour or viability. Anodic peroxidase (EC 1.11.1.7) activity in imbibed seeds also declined with the decline in vigour and viability. It is concluded that the deterioration of cells in the embryonic axis depends on the balance between free radical accumulation and the activity of active oxygen-scavenging enzymes which constitutes the active oxygen scavenging system (AOSS) during early imbibition. During prolonged storage under hot and humid conditions, cumulative effects of macromolecular damage due to oxidative chain products, compounded with the loss of enzyme activity, result in the final catastrophe, the death of the embryo.

Structural Changes in Lettuce Seed during Germination Altered by Genotype, Seed Maturation Temperature, and Priming

Thermotolerance in lettuce seed at high temperature was investigated using primed and nonprimed seed or seeds matured at 20/10°C and 30/20°C. During seed germination at 36°C, the structural changes of the seed coverings in front of the radicle tip were observed in an anatomical study. In all seeds during imbibition, regardless of seed maturation temperature or priming, a crack appeared on one side of the cap tissue and the endosperm separated from the integument in front of the radicle tip. Additional changes took place during imbibition: the protein bodies in the vacuoles enlarged and were gradually depleted, large empty vacuoles formed, the cytoplasm condensed, the endosperm shrank, the endosperm cell wall dissolved and ruptured, then the radicle elongated toward this ruptured area. The findings suggested that the papery endosperm layer presented mechanical resistance to lettuce seed germination and the weakening of this layer was a prerequisite to radicle protrusion at high temperature. Seeds of `Dark Green Boston', `Everglades', and PI 251245 matured at 30/20°C had greater thermotolerance than those matured at 20/10°C. Results of the anatomical study indicated that the endosperm cell walls in front of the radicle of seeds matured at 30/20°C were more easily disrupted and ruptured during early imbibition than seeds matured at 20/10°C, suggesting that these seeds could germinate quickly at supra-optimal temperatures. From anatomical studies conducted to identify and characterize thermotolerance in lettuce seed germination, it was observed that genotype thermotolerance had the ability to reduce physical resistance of the endosperm by weakening the cell wall and by depleting stored reserves.

Respiration and Mitochondrial Characteristics of Imbibing Maize Embryos Damaged by High Temperatures during Desiccation

Hybrid maize (Zea mays L.) seed artificially dried to 120 g H2O kg−1 moisture in air temperatures of either 35 (LT) or 45°C (HT) was used to study the effect of desiccation temperature on mitochondrial function during early imbibition. In embryo culture experiments, embryo growth was delayed for embryos excised from HT treated seed compared with LT samples. After 1 h of imbibition, the rate of O2 uptake by LT axis tissue was greater than for HT dried samples. Uptake rates continued to increase for LT samples through the first 6 h of imbibition while rates for HT axes remained constant. Mitochondria isolated from LT axis tissue that had undergone imbibition for 4 h exhibited respiratory control with oxidative phosphorylation efficiency values of 0.79 and 0.77 for NADH and succinate assays, respectively. Mitochondria from imbibing HT axis tissue exhibited no respiratory control. Electron micrographs of cells from the radicle meristem region of both treatments showed that mitochondrial development during the initial 24 h of imbibition was impaired in HT samples. These results suggest that mitochondrial function and development, during the initial stages of embryonic tissue hydration, is impaired when maize seed are artificially dried with high air temperatures. We conclude that the development of mitochondrial function during this period is a key element in determining the rate of germination and subsequent seedling growth.

Characterization of cDNA clones for differentially expressed genes in embryos of dormant and nondormant Avena fatua L. caryopses.

The molecular regulation of seed dormancy was investigated using differential display to visualize and isolate cDNAs representing differentially expressed genes during early imbibition of dormant and nondormant Avena fatua L. embryos. Of about 3000 cDNA bands examined, 5 cDNAs hybridized with mRNAs exhibiting dormancy-associated expression patterns during the first 48 h of inhibition, while many more nondormancy-associated cDNAs were observed. Dormancy-associated clone AFD1 hybridized with a 1.5 kb mRNA barely detectable in dry dormant and nondormant embryos that became more abundant in dormant embryos after 24 h of imbibition. Clone AFD2 hybridized with two mRNAs, a 1.3 kb message constitutively expressed in dormant and nondormant embryos and a 0.9 kb message present at higher levels in dormant embryos after 3 h of imbibition. Nondormancy-associated clones AFN1, AFN2 and AFN3 hybridized with 1.5 kb, 1.7 kb and 1.1 kb mRNAs, respectively, that were more abundant in nondormant embryos during imbibition. Expression patterns of some mRNAs in dormant embryos induced to germinate by GA3 treatment were different than water controls, but were not identical to those observed in nondormant embryos. DNA sequence analysis revealed 76% sequence identity between clone AFN3 and a Citrus sinensis glutathione peroxidase-like cDNA, while significant sequence similarities with known genes were not found for other clones. Southern hybridization analyses showed that all clones represent low (1 to 4) copy number genes.

Accumulation of reactive oxygen species and oxidation of cytokinin in germinating soybean seeds.

Seed germination is an important developmental switch when quiescent seed cells initiate oxidative phosphorylation for further development and differentiation. During early imbibition of soybean seeds (Glycine max L. cv. Weber), a superoxide dismutase (SOD) activity peak was observed, in embryonic axes, after 6 h imbibition. Peroxidase activities, including catalase, were significantly increased after 12 h inhibition and during germination phase III. Catalase was the most efficient enzyme in catabolizing H2O2 in embryonic axes. When stored at 42 degrees C and 100% relative humidity, seeds were stressed and lost their viability in a time-dependent manner. A significant increase in the Cu, Zn-superoxide-dismutase activity, and to a lesser extent, Mn superoxide dismutase activity was observed during germination in low-viability (stressed) seeds as compared to high-viability (unstressed) seeds. Northern blot analysis confirmed that superoxide dismutase induction resulted from an accumulation of its transcripts in response to the production of O2-. The induction of catalase did not occur in low-viability seeds, resulting in dramatic accumulation of H2O2. Using capillary electrophoresis, HPLC and NMR we found that the endogenous cytokinin, zeatin riboside, was present in large quantities in the high-viability seeds, but it was oxidized into adenine in the low-viability seeds. In vitro superoxide anion could also oxidize the cytokinin. Zeatin riboside, but not adenine, was found to act as a scavenger of superoxide anions and may help to maintain seed viability by detoxifying reactive oxygen species. Germination of stressed seeds was partially restored by the addition of exogenous cytokinin (zeatin riboside). Protection against oxidative stress by cytokinin seemed to be a general phenomenon, as Escherichia coli cells were also protected against superoxide stress in the presence of cytokinin.

Dormancy‐associated embryonic mRNAs and proteins in imbibing Avena fatua caryopses

The mechanisms controlling seed dormancy maintenance and release are not understood. To characterize the molecular events accompanying dormancy release, two‐dimensional gel electrophoresis was used to monitor changes in soluble proteins and in vitro translation products of embryonic mRNA populations during imbibition of dormant and nondormant (after‐ripened) Avena fatua L. caryopses. No differences were observed between in vitro translation products of mRNA extracted from dry dormant and nondormant embryos. However, the expression patterns of several imbibition‐ and germination‐associated mRNAs were temporally modulated during the first 24 h of imbibition. Two dormancy‐associated mRNAs, represented by polypeptides D1 and D2, were differentially overexpressed in dormant embryos after 3 h of imbibition. mRNA levels for D1 and D2 were about 8‐ and 3‐fold higher, respectively, in dormant embryos than in nondormant embryos after 3 h of imbibition. Overexpression of D1 continued through 12 h of imbibition, while expression of both mRNAs fell to low and equivalent amounts in dormant and nondormant embryos after 24 h. Similar dormancy‐associated changes in two soluble proteins were observed during imbibition. The results demonstrate that steady‐state levels of specific mRNAs and proteins change during early imbibition of dormant and nondormant A. fatua embryos and indicate that these changes may be associated with differential gene expression responsible for the maintenance of dormancy.

Respiratory pathways in germinating maize radicles correlated with desiccation tolerance and soluble sugars

We have assessed the activities of the cytochrome and alternative pathways in total respiration and their role in each stage of germination of Zea mays L. radicles. Throughout imbibition, the salicylhydroxamic acid (SHAM) concentration needed to inhibit the cyanide‐resistant pathway, without any side effects, decreased from 15 mM in quiescent embryos to 5 mM at 72 h after imbibition. Electrons predominantly flowed through the cytochrome pathway although the alternative pathway was already present at early imbibition. The capacity of the alternative path was about 70% of the control rate of respiration. Its engagement progressively increased from 18% after 10 min of imbibition to 70% at the radicle emergence and then decreased to 50% at 96 h after imbibition, concomitant with the onset of radicle growth. The alternative pathway was, however, not essential for germination. The observed activity of the alternative path correlated with the monosaccharide (glucose + fructose) content, suggesting that the alternative pathway could be acting according to the ‘energy overflow model’. On the other hand, up to 24 h after imbibition at 16°C, maize radicles tolerate a severe desiccation, becoming intolerant at 72 h. On reimbibition of tolerant radicles, respiration increased immediately and the alternative pathway was rapidly engaged. At 72 h, no respiration was measured, indicating a total loss of the respiratory systems. The possible correlation between carbohydrate content, loss of desiccation tolerance and activity of the two respiratory pathways is discussed.

Superoxide anion and hydrogen peroxide metabolism in soybean embryonic axes during germination

The total rate of mitochondrial O 2 − production in the presence of NADH as substrate increased from 200 to 1340 pmol/min per axis between 2 and 30 h of imbibition. The activities of the enzymes involved in hyperoxide metabolism, e.g., superoxide dismutase, catalase, peroxidase and glutathione and ascorbate peroxidases, markedly changed during the germination of soybean embryonic axes. Superoxide dismutase was the enzymatic activity affected the most during the initial stages of germination. Intracellular O 2 − steady-state concentration, calculated from the rate of O 2 − production and superoxide dismutase activity, showed a 2-fold increase from 2 · 10 −8 M to 4 · 10 −8 M in germination phase I, declined in phase II to 2 · 10 −8 M and remained constant over the rest of the incubation period. The reaction of H 2O 2 and luminol catalyzed by Co 2+ was utilized to measure H 2O 2 diffused out of the soybean axes after 5 to 10 min of incubation. The catalase-sensitive luminol emission of diffusates prepared from axes previously imbibed from 2 to 30 h corresponded to a H 2O 2 intracellular steady-state concentration in the range of 0.3 to 0.9 μM. The activity of metal-containing antioxidant enzymes was determined in the extracellular fluid. Cell wall peroxidase activity increased from 10 to 300 μmol/min per mg protein and appears as a potentially important pathway for H 2O 2 utilization. Hydrogen peroxide metabolism in soybean embryonic axes during early imbibition appears to have the following main features: (a) mitochondrial membranes are the most important source of cytosolic O 2 − and H 2O 2; (b) H 2O 2 is regulated at a steady-state concentration of 0.3–0.9 μM; (c) catalase is the main enzyme in terms of H 2O 2 utilization; (d) H 2O 2 exo-diffusion is quantitatively important destiny of intracellular H 2O 2; and (e) extracellular peroxidase located at the cell wall affords an enzymatic system able to use diffused H 2O 2.

Heat Shock Proteins and Their mRNAs in Dry and Early Imbibing Embryos of Wheat

Two-dimensional gels of in vitro translation products of mRNAs isolated from quiescent wheat (Triticum aestivum) embryos demonstrate the presence of mRNAs encoding heat shock proteins (hsps). There were no detectable differences in the mRNAs found in mature embryos from field grown, from 25 degrees C growth chamber cultivated, or from plants given 38 degrees C heat stresses at different stages of seed development. The mRNAs encoding several developmentally dependent (dd) hsps were among those found in the dry embryos. Stained two-dimensional gels of proteins extracted from 25 degrees C growth chamber cultivated wheat embryos demonstrated the presence of hsps, including dd hsps. A study of the relationship of preexisting hsp mRNAs and the heat shock response during early imbibition was undertaken. Heat shocks (42 degrees C, 90 minutes) were administered following 1.5, 16, and 24 hours of 25 degrees C imbibition. While the mRNAs encoding the low molecular weight hsps decayed rapidly upon imbibition, the mRNAs for dd hsps persisted longer and were still detectable following 16 hours of imbibition. After 1.5 hours of imbibition, the mRNAs for the dd hsps did not accumulate in response to heat shock, even though the synthesis of the proteins was enhanced. Thus, an applied heat shock appeared to lead to the preferential translation of preexisting dd hsp mRNAs. The mRNAs for the other hsps, except hsp 70, were newly transcribed at all of the imbibition times examined. The behavior of the hsp 70 group of proteins during early imbibition was examined by RNA gel blot analysis. The mRNAs for the hsp 70 group were detectable at moderate levels in the quiescent embryo. The relative level of hsp 70 mRNA increased after the onset of imbibition at 25 degrees C and remained high through 25.5 hours of prior imbibition. The maximal levels of these mRNAs at 25 degrees C was reached at 17.5 hours of imbibition. Heat shock caused modest additional accumulation of hsp70 mRNA at later imbibition times.

Thermotolerance Is Developmentally Dependent in Germinating Wheat Seed

During the initial 9 to 12 hours of imbibition, the imbibing wheat (Triticum aestivum L.) seed was found to exhibit substantial tolerance to high temperature relative to later times of imbibition. Tolerance was assessed by seed viability and seedling growth. This initial high temperature tolerance gradually declines with increasing time of seed imbibition. A range of 2 hour heat pretreatments (38-42 degrees C) prior to imposition of a 2 hour heat shock (51-53 degrees C) during this same 9 to 12 hour interval was unable to increase survival or seedling growth over that of seed that did not receive a pretreatment. However, after 9 to 12 hours of imbibition the pretreatment provided both increased survival and increased seedling growth, measured 120 hours later, i.e., classical thermotolerance could be acquired. This response is called a ;thermotolerance transition.' Isolated embryos responded in a similar manner using a 2,3,5-triphenyltetrazolium chloride assay for viability determination following heat treatments. The high temperature tolerance during early imbibition indicates that the thermotolerance transition involves the loss of an existing thermotolerance coincident with acquiring the ability to become thermotolerant following heat pretreatment. Despite the inability to acquire thermotolerance, heat shock protein synthesis was induced by heat shock immediately upon imbibition of wheat seed or isolated embryos. Developmentally regulated heat shock proteins of 58 to 60, 46, 40, and 14 kilodaltons were detected at 1.5 hours of imbibition following heat shock, but were absent or greatly reduced by 12 hours. Constitutive synthesis of 70 and 90 kilodalton hsp groups appeared to be greater at 1.5 hours of imbibition than at 12 hours of imbibition.

The Role of Maturation Drying in the Transition from Seed Development to Germination. VI. Desiccation-Induced Changes in Messenger RNA Populations within the Endosperm ofRicinus communisL. Seeds

From 30 to 50 d after pollination (DAP) the total RNA content declines in the developing endosperm of castor bean and only a small amount is present in the mature dry (60 DAP) endosperm. During and following germination of the mature seed the RNA content of the endosperm increases up to 72 h after imbibition (HAI). A similar decline in RNA in the endosperm occurs when the seed is prematurely dried at 35 DAP and there is an increase again upon subsequent rehydration, although not to the level achieved in the endosperm of germinated mature seed. Quantitative and qualitative changes in the mRNA complement of the endosperm occur during development. In particular, messages coding for high mol. wt. proteins (putative storage protein precursors) decline substantially during maturation drying (50-60 DAP). A similar decline occurs as a consequence of premature desiccation at 35 DAP. Following rehydration of these dried seeds the pattern of proteins synthesized in vitro by the mRNAs extracted from the endosperm is very similar to that catalysed by messages from the mature seed endosperm during germination and growth. Some mRNAs associated with development, which persist in the endosperm following natural and imposed desiccation, decline during early imbibition. During germination and growth of mature and prematurely dried seeds a similar complement of mRNAs arise in the endosperm, in an identical temporal sequence. The mRNA for ricin D, a protein synthesized predominantly during development at 30-40 DAP, declines to a low level in the mature dry endosperm (60 DAP) and in that of the prematurely dried seed and does not increase substantially during germination or growth. Thus the changes in protein synthesis effected by drying in the endosperm of castor bean are mediated primarily through changes in their mRNAs, as a result of transcriptional control. Drying, whether occurring naturally or imposed prematurely, results in the permanent suppression of transcription of most developmentally-related messages and, upon subsequent rehydration, the induction of those associated with germination and growth. Some residual mRNAs for developmental proteins which are present in the dry seed are selectively degraded during germination. Thus there are some post transcriptional changes to the mRNA population following drying, in addition to the transcriptional modifications.

Early response to gibberellic acid of monophenolase (tyrosinase) activity in de-embryonated half-seeds of wheat

A significant enhancement (7–8 fold) of monophenolase activity by GA 3 was observed in de-embryonated half-seeds of wheat after 2 hr of imbibition. This early response to GA 3 (2–12 hr) was apparent only after lowering the endogenous levels of phytohormone in wheat half-seeds by leaching. Cycloheximide (CHI, 20 μg/ml) failed to inhibit GA 3-stimulated (GA 3, 10 μM) monophenolase activity. This ruled out the requirement of de novo protein synthesis for the hormone-regulated activity of monophenolase. Abscisic acid (100 μM) effectively stopped the GA 3-mediated activation of monophenolase. Curiously, the early stimulatory response to GA 3 of monophenolase activity was completely mimicked by the administration of phosphate ions (Pi, 75 mM). Simultaneous addition of GA 3 and Pi to wheat half-seeds, however, showed no cumulative effect on the activation of monophenolase. Both GA 3 and Pi activated monophenolase exhibited altered molecular properties, in terms of shift in the pH optimum of enzyme activity towards alkalinity (pH 9.0) and relatively increased thermostability of the enzyme at 50°. Molecular sieving of the enzyme fraction on Sephacryl S-200 revealed a single molecular form of monophenolase ( M r 45 000) both in control and GA 3-treated wheat half-seeds during the first 12 hr of imbibition. This indicated that the activation of monophenolase by GA 3 is due to some structural modification of the enzyme without its oligomerization. We envisage that the modulation of monophenolase activity by GA 3 during early imbibition of wheat half-seeds is a true primary response to the phytohormone.

Seed dormancy and germination of Echinochloa oryzicola Vasing.: Dormancy-breaking mechanism of potassium cyanide.

Dormancy-breaking mechanism of KCN in Echinochloa oryzicola seeds was studied by determining activities of respiration and redox enzymes in the seeds treated for 48hrs at the optimum concentration for breaking dormancy. KCN remarkably promoted germination of the seeds with primary dormancy. Any visible signs of germination were not observed during treatment, but started soon after seeds were transfered to the subsequent incubation without cyanide, indicating that the germination-induction was present during the 48hr treatment period. The KCN concentration causing the maximum germination at 100mM greatly inhibited both O2 consumption and cytochrome c oxidase activity. ADH activity of treated seeds initially decreased for 24hr, but thereafter increased. The RQ value increased to 3.6 during the germination-induction period, whereas fluctuations in neither ADH activity nor RQ value were observed in nontreated seeds. There was little difference in G6PDH activity between treated and nontreated seeds during treatment, but the treated seeds increased in activity as germination began. During the germination-induction period, catalase activity was remarkably inhibited by KCN, while, on the contrary, peroxidase was stimulated to activity two-folds higher than that of nontreated seeds. The amount of NAD (P) H in treated seeds significantly decreased soon after the subsequent incubation, suggesting the increased activities of NAD (P) H oxidases. The alternations observed in enzyme activities pointed out that the scheme HENDRICKS and TAYLORSON (1975) proposed for the mechanism of germination promotion for thiourea, nitrite and hydroxylamine was also applicable for KCN action in the seeds of E. oryzicola. The shift of aerobic respiration to alcohol fermentation by KCN was worthy of note since the latter is, as we previously reported, the main energy-yielding pathway of the rice paddy weed seed at early imbibition.

Desiccation during Germination of Pea Axes and its Effects on the Messenger RNA Population

Lalonde, L. and Bewley, J. D. 1986. Desiccation during germination of pea axes and its effects on the messenger RNA population.—J. exp. Bot. 37: 754-764. The messenger RNA population present in the axes of seeds of Pisum sativum changes during germination; particularly noticeable is the progressive decline in the population of messages present in the mature dry axes. Desiccation during germination does not affect the loss of many of these messages from the axes; their decline during germination is stimulated in response to desiccation, however. Another set of messages normally increases with time of germination; an interrupting desiccation period does not permanently suppress this increase, for it proceeds as normal upon subsequent rehydration. Desiccation of isolated pea axes at different rates reveals a differential stability of the message population, with slow drying resulting in a greater loss of translatable mRNAs. At least one message is synthesized in response to desiccation imposed during germination which is not present in the mature dry seed, or during very early imbibition. The level of control exerted by desiccation on protein synthesis appears to be at the level of transcription.

Soybean Seed Respiration During Simulated Preharvest Deterioration

Amable, R. A. and Obendorf, R. L. 1986. Soybean seed respiration during simulated preharvest deterioration.—J. exp. Bot. 37: 1364-1375. Rapid decline of seed quality limits soybean production in the lowland tropics. Seed deterioration is promoted by delayed harvest under high ambient temperature and relative humidity. To evaluate changes in respiration during deterioration, soybean seed were subjected to four controlled, constant or fluctuating, water content regimes at 25 °C and 32 °C to simulate the postmaturation, preharvest environment. Seed deteriorated rapidly under high water content regimes at 32 °C and were non-viable by 20 d. Low rates of oxygen uptake during early imbibition correlated with deterioration and were attributed to a decline in the integrity of the cyanide-sensitive cytochrome pathway. However, tissues subjected to high water content fluctuation at 32 °C for 20 d had high rates of oxygen uptake due mainly to 'residual' respiration. Only small increases in alternative respiration and lipoxygenase activity occurred during deterioration. A marked increase in ethanol content during imbibition indicated a shift from aerobic to anaerobic metabolism induced by the decline in the cytochrome pathway. A reduction in tetrazolium staining was not accompanied by decreased activity of alcohol dehydrogenase or malate dehydrogenase. Reduced pyruvate metabolism through the tricarboxylic acid cycle and a concomitant decline in NADH levels were proposed to limit dehydrogenase activity in vivo causing the decreased staining of tissues. The mitochondria appear to be the site of a primary lesion during seed deterioration under combinations of high water content and high temperature. Key words—Cytochrome, alternative-respiration, germination. Correspondence to: Department of Agronomy, Cornell University, Ithaca, N.Y. 14853-0144, U.S.A.

Changes in protein synthesis during the development of Agrostemma githago seeds

The transition from the growth to the maturation phase in developing seeds of Agrostemma githago L. was found to coincide with major changes in the rate of protein synthesis, the kinds of proteins synthesized, and the composition of the non‐proteinbound amino acid pool. Coincident changes were observed in viability and the ability to withstand desiccation. Desiccated mature Agrostemma seeds are dormant and need at least three months of after‐ripening. In imbibed dormant and after‐ripened seeds no synthesis of storage proteins was observed with the exception of one particular set of storage proteins. Dormant and after‐ripened seeds synthesized the same kinds of proteins during early imbibition, indicating an almost identical metabolic state which differs considerably from that of developing seeds.