Water Content, Raffinose, and Dehydrins in the Induction of Desiccation Tolerance in Immature Wheat Embryos

Desiccation tolerance is initiated in wheat (Triticum aestivum L.) embryos in planta at 22 to 24 d after anthesis, at the time that the embryo water content has decreased from about 73% fresh weight (2.7 g water/g dry weight) to about 65% fresh weight (1.8 g water/g dry weight). To determine if desiccation tolerance is fully induced by the loss of a relatively small amount of water, detached wheat grains were treated to reduce the embryo water content by just a small amount to approximately 69% (2.2 g water/g dry weight). After 24 h of such incipient water loss, subsequently excised embryos were able to withstand severe desiccation, whereas those embryos that had not previously lost water could not. Therefore, a relatively small decrease in water content for only 24 h acts as the signal for the development of desiccation tolerance. Embryos that were induced into tolerance by a 24-h water loss had no detectable raffinose. The oligosaccharide accumulated at later times even in embryos of detached grains that had not become desiccation tolerant, although tolerant embryos (i.e. those that previously had lost some water) contained larger amounts of the carbohydrate. It is concluded that desiccation tolerance and the occurrence of raffinose are not correlated. Immunodetected dehydrins accumulated in embryos in planta as desiccation tolerance developed. Detachment of grains induced the appearance of dehydrins at an earlier age, even in embryos that had not been made desiccation tolerant by incipient drying. It is concluded that a small reduction in water content induces desiccation tolerance by initiating changes in which dehydrins might participate but not by their interaction with raffinose.     

Acquisition of desiccation tolerance in developing wheat embryos correlates with appearance of a fluid phase in membranes

Membrane behaviour in developing wheat (Triticum aestivum cv Priokskaya) embryos was studied in relation to the acquisition of desiccation tolerance, using spin probe techniques. Fresh embryos were able to develop into seedlings at day 15 after anthesis, but it took 18 d before fast‐dried, isolated embryos could germinate. On the basis of membrane integrity measurements it was estimated that between 14 and 18 d after anthesis the proportion of embryonic cells surviving fast drying increased and the critical moisture content, to which embryonic cells could be dehydrated, decreased. Apparently, embryonic cells do not acquire the same level of desiccation tolerance simultaneously. Only when all cells had become desiccation tolerant was germination of air‐dried embryos possible. Using 5‐doxylstearic acid as the probe molecule, an approximately similar lipid–water interface ordering of membranes was observed in all hydrated embryos, irrespective of age. Dehydration had a dual effect on the lipid interface: further ordering of the major part of the interface and the appearance of additional, disturbed regions. The proportion of these regions correlated with the proportion of desiccation‐tolerant cells. We propose that the membrane surface disturbance be caused by endogenous amphiphiles that partition from the cytoplasm into membranes during drying. The absence of such disturbed regions in dried, desiccation‐sensitive embryos might reflect a lack of sufficient amphiphiles. The relevance of membrane surface disturbance for desiccation tolerance is discussed.     

成熟脱水对种子发育和萌发的作用

成熟脱水是正常性种子发育的末端事件。种子在成熟时胚的脱水耐性增加;当种子萌发时胚变得不耐脱水。当种子获得脱水耐性时,糖、蛋白质和抗氧化防御系统等保护性物质积累;当脱水耐性丧失时,这些物质被降解。成熟脱水是种子从发育过程向萌发过程转变的“开关”,它降低发育的蛋白质和ｍＲＮＡ的合成,终止发育事件和促进萌发事件。顽拗性种子不经历成熟脱水的发育阶段,对脱水高度敏感。     

Developmental and environmental induction of Lea and LeaA mRNAs and the postabscission program during embryo culture.

The major programs of gene expression during late embryogenesis are the muturation or reserve accumulation program and, after ovule abscission, the postabscission program that is composed largely of Lea and LeaA mRNAs that probably encode desiccation protectants. There are diverse opinions about the developmental regulators of these programs. Several candidates are evaluated here by measuring, in cultured embryos, the accumulation kinetics of cloned mRNAs specifically expressed in the normal maturation, postabscission, or germination programs of cotton. Maturation-stage embryos both terminate the maturation program and induce the postabscission program after excision and culture, just as they do later in the plant after ovule abscission. However, they also induce simultaneously the germination program and are thus different from any normal stage of embryo development or germination. The developmental induction of the postabscission program in culture does not require exogenous abscisic acid, but its expression is enhanced by precocious desiccation or culture on abscisic acid or high osmoticum, probably by an environmentally responsive mechanism that normally operates during germination. Normal desiccation does not control any of these programs because the embryo acquires all of the characteristics of a mature embryo before it desiccates. These and other results suggest regulation of normal embryogenesis by a maternal maturation factor, a postabscission factor, and the postabscission program.     

成熟脱水对种子发育和萌发的作用

成熟脱水是正常性种子发育的末端事件。种子在成熟时胚的脱水耐性增加;当种子萌发时胚变得不耐脱水。当种子获得脱水耐性时,糖、蛋白质和抗氧化防御系统等保护性物质积累;当脱水耐性丧失时,这些物质被降解。成熟脱水是种子从发育过程向萌发过程转变的“开关”,它降低发育的蛋白质和mRNA的合成,终止发育事件和促进萌发事件。顽拗性种子不经历成熟脱水的发育阶段,对脱水高度敏感。     

Requirements for zygotic gene activity during gastrulation in Drosophila melanogaster

Mutations at the folded gastrulation (fog) and twisted gastrulation (tsg) loci interfere with early morphogenetic movements in Drosophila melanogaster. fog embryos do not form a normal posterior midgut and although their germbands do elongate, they do not extend dorsally. As a result, when normal embryos have fully extended germbands, the germbands in mutant embryos are folded into the interior on the ventral side of the embryo. tsg embryos have abnormally deep dorsal folds during early gastrulation, associated with the failure of dorsal cells to slip laterally to make way for the expanding germband. Both fog and tsg embryos continue to develop, but form disorganized first instar larvae. fog and tsg are zygotically active genes expressed at least by 10 and 20 min after the onset of gastrulation. Both mutations are viable in homozygous germ cells and the wild-type genes need not be expressed during oogenesis for survival of heterozygous progeny. Elimination of fog+ gene product from maternal germ cells does, however, affect the extent of folding observed during gastrulation in viable heterozygotes. Analysis of fog adult and larval gynandromorphs indicates that normal folded gastrulation gene function is only required at the posterior region of the embryo, most probably in the cells giving rise to the posterior midgut or proctodeum. The relative survival of fog mosaics suggests that embryos with mosaic "lethal foci" also die during embryogenesis, although the typical fog phenotype is only produced when the entire focus is mutant. In contrast to the fog focus, no particular cell must be wild type in tsg mosaics for survival. Wild-type cells on the dorsal side of the embryo, however, are most effective in rescuing the embryo. This indicates that normal tsg gene product may be required only on the dorsal side of the embryo, potentially in the region which gives rise to the amnion serosa.     

Mechanisms associated with cellular desiccation tolerance in the animal extremophile artemia

Using differential scanning calorimetry, we demonstrated the presence of biological glasses and measured the transition temperatures in dry encysted embryos (cysts) of the brine shrimp, Artemia franciscana. Cysts from the following three geographic locations were studied: San Francisco Bay (SFB); the Great Salt Lake, Utah (GSL); and the Mekong Delta, Vietnam (VN; these cysts were produced from previous sequential inoculations of SFB cysts into growth ponds). Values for the glass transition temperature, T(g), were highest in VN cysts. This study indicates that the composition and properties of these biological glasses can be altered by natural selection and thermal adaptation. To our knowledge, T(g) values for all three kinds of cysts were significantly higher than those for any other desiccation-tolerant animal system. To gain insight into the significance of T(g), we examined the thermal stability of these dry cysts at 80 °C. GSL cysts were the least tolerant, by far, with VN cysts being extremely tolerant and SFB cysts not far behind. Those results correlated with the thermal transition values. Also measured were alcohol-soluble carbohydrates, ~90% of which is the disaccharide trehalose, a known component of biological glasses. Amounts in the GSL cysts were significantly less than those in the other two kinds of cysts. Several stress proteins were measured in the three groups of cysts, with all of them being in lesser amounts in GSL cysts compared with the SFB and VN cysts. We interpret the data in terms of mechanisms involved with desiccation tolerance and thermal conditions at the sites of cyst collection.     

Zygotic embryo cell wall responses to drying in three gymnosperm species differing in seed desiccation sensitivity

Plant cell walls (CWs) are dynamic in that they can change conformation during ontogeny and in response to various stresses. Though seeds are the main propagatory units of higher plants, little is known of the conformational responses of zygotic embryo CWs to drying. This study employed cryo-scanning electron microscopy to compare the effects of desiccation on zygotic embryo CW morphology across three gymnosperm species that were shown here to differ in seed desiccation sensitivity: Podocarpus henkelii (highly desiccation-sensitive), Podocarpus falcatus (moderately desiccation-sensitive), and Pinus elliottii (desiccation-tolerant). Fresh/imbibed (i.e. fresh Podocarpus at shedding and imbibed Pi. elliottii) embryos showed polyhedral cells with regular walls, typical of turgid cells with an intact plasmalemma. Upon desiccation to c. 0.05 g g^?1 (dry mass basis), CWs assumed an undulating conformation, the severity of which appeared to depend on the amount and type of dry matter accumulated. After desiccation, intercellular spaces between cortical cells in all species were comparably enlarged relative to those of fresh/imbibed embryos. After rehydration, meristematic and cotyledonary CWs of P. henkelii and meristematic CWs of P. falcatus remained slightly undulated, suggestive of plasmalemma and/or CW damage, while those of Pi. elliottii returned to their original conformation. Cell areas in dried-rehydrated P. henkelii root meristem and cotyledon were also significantly lower than those from fresh embryos, suggesting incomplete recovery, even though embryo water contents were comparable between the two states. Electrolyte leakage measurements suggest that the two desiccation-sensitive species incurred significant plasmalemma damage relative to the tolerant species upon desiccation, in agreement with the CW abnormalities observed in these species after rehydration. Immunocytochemistry studies revealed that of the four CW epitopes common to embryos of all three species, an increase in arabinan (LM6) upon desiccation and rehydration in desiccation-tolerant Pi. elliottii was the only difference, although this was not statistically significant. Seed desiccation sensitivity in species like P. henkelii and P. falcatus may therefore be partly based on the inability of the plasmalemma and consequently CWs of dried embryos to regain their original conformation following rehydration.     

Glass formation in plant anhydrobiotes: survival in the dry state

Anhydrobiotes can resist complete dehydration and survive the dry state for extended periods of time. During drying, cytoplasmic viscosity increases dramatically and in the dry state, the cytoplasm transforms into a glassy state. Plant anhydrobiotes possess large amounts of soluble non-reducing sugars and their state diagrams resemble those of simple sugar mixtures. However, more detailed in vivo measurements using techniques such as Fourier transform infrared spectroscopy and electron paramagnetic resonance spectroscopy reveal that these intracellular glasses are complex systems with properties quite different from those of simple sugar glasses. Intracellular glasses exhibit a high molecular packing and slow molecular mobility, resembling glasses made of mixtures of proteins and sugars, which potentially interact with additional cytoplasmic components such as salts, organic acids, and amino acids. Above the glass transition temperature, the cytoplasm of biological systems still exhibits a high stability and low molecular mobility, which could serve as an ecological advantage. All desiccation-tolerant organisms form glasses upon drying, but desiccation-sensitive organisms generally lose their viability during drying at water contents at which the glassy state has not yet been formed, suggesting that other factors are necessary for desiccation tolerance. Nevertheless, the formation of intracellular glasses is indispensable to survive the dry state. Storage stability of seeds and pollens is related to the molecular mobility and packing density of the intracellular glass, suggesting that the characteristic properties of intracellular glasses provide stability for long-term survival.     

Proteomics of desiccation tolerance during development and germination of maize embryos

Maize seeds were used to identify the key embryo proteins involved in desiccation tolerance during development and germination. Immature maize embryos (28N) during development and mature embryos imbibed for 72 h (72HN) are desiccation sensitive. Mature maize embryos (52N) during development are desiccation tolerant. Thiobarbituric acid reactive substance and hydrogen peroxide contents decreased and increased with acquisition and loss of desiccation tolerance, respectively. A total of 111 protein spots changed significantly (1.5 fold increase/decrease) in desiccation-tolerant and -sensitive embryos before (28N, 52N and 72HN) and after (28D, 52D and 72HD) dehydration. Nine pre-dominantly proteins, 17.4 kDa Class I heat shock protein 3, late embryogenesis abundant protein EMB564, outer membrane protein, globulin 2, TPA:putative cystatin, NBS-LRR resistance-like protein RGC456, stress responsive protein, major allergen Bet v 1.01C and proteasome subunit alpha type 1, accumulated during embryo maturation, decreased during germination and increased in desiccation-tolerant embryos during desiccation. Two proteins, Rhd6-like 2 and low-molecular-weight heat shock protein precursor, showed the inverse pattern. We infer that these eleven proteins are involved in seed desiccation tolerance. We conclude that desiccation-tolerant embryos make more economical use of their resources to accumulate protective molecules and antioxidant systems to deal with maturation drying and desiccation treatment.     

Onset of desiccation tolerance during development of the barley embryo

We have investigated events which take place in the developing barley (Hordeum vulgare L.) embryo during its acquisition of desiccation tolerance. Excised embryos are capable of precocious germination as early as 8 d after pollination (DAP). At this age, however, they are not capable of resisting a desiccation treatment which induces a loss of 96–98% of their initial water content. At 16 DAP the embryos germinate despite the drastic drying treatment. The pattern of in-vivo and in-vitro proteins synthesized by the developing embryos from 12 DAP (desiccation-intolerant) and 16 DAP (desiccation-tolerant) were compared. A set of 25–30 proteins was identified which is denovo synthesized or enhanced during the developmental period leading to desiccation tolerance. Abscisic acid (ABA; 100 M) applied in vitro for 5 d to 12-DAP embryos induces desiccation tolerance and represses a subset of polypeptides preferentially associated with 16-DAP embryos. During in vitro culture of barley embryos ABA stimulates the appearance of a set of proteins and prevents the precocious germination allowing embryogenesis to continue in vitro. It also suppresses a set of germination-related proteins which appear 4 h after the incubation of the dissected embryo on a germination medium without ABA. Almost all mRNAs remain functional for translation when isolated embryos are dried at the desiccation-intolerant and tolerant stages of embryo development.Abbreviations ABA abscisic acid - DAP days after pollination - GM germination medium - poly(A)RNA polyadenylated RNA - SDS sodium dodecyl sulfate     

Developmental Changes in Relation to Desiccation Tolerance of Archontophoenix alexandrae (Palmae) Seeds

Germination of Archontophoenix alexandrae seeds and embryos were studied under gradient water content treatments throughout the seed development phases of maturation in 2005 to investigate seed desiccation tolerance and storage characteristics. During the maturation process, seed water content decreased gradually from55 DAF (days after flowering) to 70 DAF, and seeds reached the maximum dry-weight at 90 DAF. Seed germinability appeared after 60 DAF. Seeds germinated with a temperature range from15℃- 40℃ under alternating photoperiod (14 h light, 10 h dark, 12μmol m^{- 2}s ^{- 1} ), while the best germination percentage was obtained between 30℃- 35℃. A maximum germination capacity reached at 70 DAF. However, seed germination was greatly inhibited by light. Desiccation tolerance of seeds and embryos increasedgradually from 55 DAF to 90 DAF and reached the maximum at 90 DAF with a semilethal water content of 0.18 g/g ( seed) and 0.3 g/g ( embryo) respectively. Rapid dehydration maintained higher seed germination percentage than thatof slow dehydration when drying to the same water content. Seeds with without water content treatments failed to germinate after 1 month storage under - 18℃, whereas appropriate desiccation treatment prolonged seed longevity under 4℃, 10℃ and 15℃ storage temperatures. It revealed obviously the recalcitrant characteristics of Archontophoenix alexandrae seeds torage behaviour which are tolerant toward neither deep desiccation nor low temperatures.     

Effect of desiccation to low moisture content on germination,synchronization of root emergence,and plantlet regeneration of black spruce somatic embryos

A desiccation protocol was developed to evaluate the effect of different levels of desiccation on germination and plantlet regeneration of black spruce somatic embryos. Large desiccation chambers (80 l) with four liters of saturated salt solutions provided constant relative humidities (RH) of 63, 79, 88, and 97% (± 2%). Under these conditions, an embryo mass of 10 mg always dried fast even at 97% RH. In contrast, an embryo mass of 80 mg generated different kinetics of water loss, from fast drying at 63% RH to slow drying at 97% RH. Drying rates similar to those obtained with 80 mg embryos were also generated by combining 40 mg embryos with 40 mg water. The effects of drying rate and embryo MC on germination rate, root elongation, and plantlet regeneration were examined. A fast drying rate to 4–5% embryo MC, obtained under 63% RH, was detrimental to germination and plantlet development. However slower drying rates, obtained under 79–97% RH and generating 7–19% MC in the embryos, gave developmental responses similar to the control. Synchronization of root emergence was improved only for embryos desiccated to approx. 16% MC under 97% RH. The optimal desiccation protocol using large desiccation chamber at 97% RH and a constant embryo mass of 40 mg embryos plus 40 mg water was applied to five genotypes of black spruce. For all genotypes, desiccated embryos gave plantlet regeneration rates similar to the control undesiccated embryos. This revised version was published online in June 2006 with corrections to the Cover Date.     

Supercooling ability in apple seeds as affected by temperature-induced modifications in water relations in the seed parts

Prolonged storage of apple fruits ( Pyrus malus L. cv. Golden Delicious) at different temperatures (0, 12 and 35°C) decreased the water content in seed coats and endosperms, higher temperatures being much more effective than the lower (0°C) one. No effect of the temperature on the embryo hydration was found. However, a pronounced decrease in water potential in the embryos was observed during the first 9 weeks. The decrease was much faster and the water potential reached lower levels in embryos isolated from seeds pretreated with higher temperatures (12 or 35°C) than from cold-pretreated (0°C) material. Higher temperatures of fruit storage also resulted in a decreased permeability of the embryo membranes to electrolytes and sugars. At the same time, membrane permeability to water was not modified. It is proposed that the previously observed occurrence of the discontinuous type of freezing in apple seeds (Nguyen and Kacperska, Physiologia Plantarum (X): 000-000, 1989) is associated with the temperature-induced dehydration of seed coat and endosperm, whereas a higher super cooling ability of the high-temperature-pretreated embryos is due to a decrease in the free energy of water in the system, and to the effective protection of embryo cells against heterogenous ice nucleation. The changes in water potential showed a high negative correlation with the embryo phospholipid content determined in the other work (Nguyen et al. Plant Physiol. Bio chem. 25: 697–703, 1987). Therefore, it is proposed that changes in matrix potential play an important role in the regulation of the water potential in the embryo cells.     

Induction of tolerance to fast desiccation in black spruce (Picea mariana) somatic embryos: relationship between partial water loss,sugars, and dehydrins

Events associated with the induction of tolerance to fast desiccation in black spruce ( Picea mariana ) somatic embryos were investigated. An experimental approach using an initial period of partial water loss was developed to induce either no, partial, or complete tolerance to fast desiccation. Tolerance to subsequent fast desiccation was not promoted by decreasing embryo water content from 1.5 to 1.1 g H₂O g⁻¹ DW (g g⁻¹) throughout the first 24 h of slow desiccation. However, tolerance increased from 10 to 95% germination during the second 24-h period of slow desiccation after partial water loss from 1 to 0.55 g g⁻¹. Emphasis was also placed on the relationship between observed tolerance, and sugar and dehydrin contents. Compared to controls, sucrose content in embryos doubled after 24 h of slow desiccation and more than tripled after 48 h. Conversely, starch content was decreased by one half after 24 h and by three quarters after 48 h. Sucrose abundance and raffinose occurrence after 48 h of slow desiccation were congruent with complete tolerance to fast desiccation. The period of slow desiccation between 24 and 48 h also increased the content of a 24-kDa dehydrin and the appearance of a 42-kDa dehydrin. The relationship between partial water loss, sugars and dehydrins is discussed with respect to tolerance to fast desiccation in black spruce somatic embryos.     

Physiological aspects of Taxus brevifolia seeds in relation to seed storage characteristics

Water relations, desiccation tolerance and longevity of Taxus brevifolia (Nutt.) seeds were studied to determine the optimal stage of development and storage conditions for seeds of this species. Seeds equilibrated to a range of relative humidities (RHs) had unusually low water contents which can be accounted for by the high lipid content of gametophyte tissues (71% of the dry mass). Water relations of embryonic tissue were more typical of those reported for other seed species. The water content below which freezing transitions were not observable in the embryo was ca 0.24 g H₂O (g dry weight)⁻¹ (g g⁻¹) for all maturity classes studied. Embryos did not achieve significant levels of desiccation tolerance (survival to water contents less than 0.5 g g⁻¹) until the latter stages of development when dry matter was maximal. Mature embryos could be dried to 0.025 g g⁻¹ (seed water content of 0.010 g g⁻¹) with no loss of viability. Thus, at the latter stages of development, embryo water content could be optimized to avoid both desiccation and freezing damage. Survival of mature seeds declined over a 2-year period when seeds were stored at temperatures between 5 and 35°C and RHs between 14 and 75%, corresponding to seed water contents between 0.015 and 0.07 g g⁻¹. The deterioration rate was slowest for seeds stored at the lowest RH and temperature. Our data indicate that seeds of Taxus brevifolia show orthodox rather than recalcitrant storage characteristics, but that the optimum water content for storage was extremely low. The results suggest that even if stored at optimal water contents and low temperatures, T. brevifolia seeds will be relatively short lived. The high quantity of lipids or reducing sugars may be contributing factors in the poor storage characteristics.     

Premature Drying, Fluridone-Treatment, and Embryo Isolation During Development of Maize Kernels (Zea mays L.) Induce Germination, but the Protein Synthetic Responses are Different. Potential Regulation of Germination and Protein Synthesis by Abscisic Acid

Freshly harvested, developing kernels of maize (Zea mays L.)do not germinate up to 77 d after pollination, but can be inducedto do so by fluridone, premature desiccation, and isolationof the developing embryo. The pattern of protein synthesis indeveloping maize embryos is distinct from that during germinationand subsequent seedling growth. Premature desiccation at 35DAP elicits a pattern of protein synthesis upon rehydrationwhich is similar to that in germinated embryos from mature drykernels. Fluridone-induced viviparous germination is accompaniedby changes in the synthesis of some proteins to a post-germinativepattern, but some developmental proteins continue to be synthesized.Embryos isolated from developing kernels at 35 DAP germinatewhen incubated on water; they also produce some developmentalproteins during germination. Kernels from developing cobs at35 DAP which are detached from the mother plant and maintainedin an atmosphere of high relative humidity (moist controls)do not germinate, but neither do they continue a clearly definedpattern of either developmental or germinative protein synthesis.Drying is thus critical to effect a clear transition of proteinsynthesis from a developmental to a germinative mode in maizeembryos. Abscisic acid within the developing embryos is reduced by fluridone,but to a lesser extent by premature drying or maturation drying.Changes in sensitivity to abscisic acid by the developing embryomay be as, or more, important in permitting germination, andthe attendant synthesis of proteins, than changes in abscisicacid content. Key words: Maize (Zea mays L.), germination, vivipary, desiccation, abscisic acid     

Mixed trehalose/sucrose glasses used for protein incorporation as studied by infrared and optical spectroscopy

Evaporation of water from a 1/1 mixture of trehalose and sucrose gives rise to optically clear glasses that are transparent in the UV and visible ranges and do not crystallize when they are prepared at ambient temperatures. Two proteins, liver alcohol dehydrogenase and parvalbumin, and the tryptophan derivative N-acetyl-tryptophanamide were incorporated into the glasses. Infrared spectroscopy of the amide I band reveals that the proteins retain secondary structure in the glass over a temperature range of 20-300K. The amide II band of the protein and the HOH bending band of residual water in the glass shift with temperature changes, consistent with increased H-bonding strength as temperature is lowered. Phosphorescence of tryptophan can be seen from the proteins at room temperature, which shows the immobilization of the protein by the glass and the curbing of oxygen diffusion. It is suggested that using mixed sugars to form glasses is a way to immobilize proteins over a wide temperature range without distortions from solvent crystals.     

Storage oil breakdown during embryo development of Brassica napus (L.)

In this study it is shown that at least 10% of the major storage product of developing embryos of Brassica napus (L.), triacylglycerol, is lost during the desiccation phase of seed development. The metabolism of this lipid was studied by measurements of the fate of label from [1-(14)C]decanoate supplied to isolated embryos, and by measurements of the activities of enzymes of fatty acid catabolism. Measurements on desiccating embryos have been compared with those made on embryos during lipid accumulation and on germinating seedlings. Enzymes of beta-oxidation and the glyoxylate cycle, and phosphoenolpyruvate carboxykinase were present in embryos during oil accumulation, and increased in activity and abundance as the seeds matured and became desiccated. Although the activities were less than those measured during germination, they were at least comparable to the in vivo rate of fatty acid synthesis in the embryo during development. The pattern of labelling, following metabolism of decanoate by isolated embryos, indicated a much greater involvement of the glyoxylate cycle during desiccation than earlier in oil accumulation, and showed that much of the (14)C-label from decanoate was released as CO(2) at both stages. Sucrose was not a product of decanoate metabolism during embryo development, and therefore lipid degradation was not associated with net gluconeogenic activity. These observations are discussed in the context of seed development, oil yield, and the synthesis of novel fatty acids in plants.     

The Role of Maturation Drying in the Transition from Seed Development to Germination: V. RESPONSES OF THE IMMATURE CASTOR BEAN EMBRYO TO ISOLATION FROM THE WHOLE SEED: A COMPARISON WITH PREMATURE DESICCATION

Kennode, A. R, and Bewley, J. D. 1988. The role of maturationdrying in the transition from seed development to germination.V. Responses of the immature castor bean embryo to isolationfrom the whole seed; a comparison with premature desiccation.—J.exp. Bot. 39: 487–497. Desiccation is an absolute requirement for germination and post-germinativegrowth of whole seeds of the castor bean, whether desiccationis imposed prematurely during development, at 35 d after pollination(DAP) or occurs naturally during late maturation (50–60DAP). Desiccation also plays a direct role in the inductionof post-germinative enzyme synthesis in the cotyledons of embryosin the intact seed; this event is not simply due to the presenceof a growing axis. Isolation of embryos from the developingcastor bean seed at 35 DAP results in both germination and growth,despite the absence of a desiccation event. We have comparedthe metabolic consequences of premature drying of whole seeds(35 DAP) and isolation of the developing 35 DAP embryos. Inboth cases, hydrolytic events involved in the mobilization ofstored protein reserves proceed in a similar manner and mirrorthose events occurring within germinated mature seeds. Thereare differences, however, for post-germinative enzyme (LeuNAaseand isocitrate lyase) production occurs to a lesser extent innon-dried isolated embryos than in those from prematurely dried(35 DAP) whole seeds, or from mature dry (whole) seeds. Desiccationof the 35 DAP whole seed does not alter the subsequent responseof the embryo upon isolation. Thus, while drying does not affectthe metabolism of isolated embryos, it has a profound effecton that of embryos within the intact seed. Tissues surroundingthe embryo in the developing intact seed (viz. the endosperm)maintain its metabolism in a developmental mode and inhibitgermination. This effect of the surrounding tissues can onlybe overcome by drying or by their removal. Key words: Metabolism, isolation, desiccation, embryo, endosperm, castor bean, development, germination