Active oxygen species (AOS) are involved in various aspects of seed physiology. Their generation, which occurs during seed desiccation, germination and ageing, may lead to oxidative stress and cellular damage, resulting in seed deterioration. However, cells are endowed with detoxifying enzymes and antioxidant compounds that scavenge AOS and participate in seed survival. The detoxifying mechanisms play a key role in acquisition of desiccation tolerance of developing seeds, completion of seed germination and seed storability. However, AOS must also be regarded as molecules intervening in cellular signalling. They are involved in growth processes occurring at early embryogenesis during seed development, and participate in the mechanisms underlying radicle protrusion during seed germination. AOS might also have a regulatory function in the changes in gene expression during seed development, dormancy and germination. Their interplay with other molecules, particularly with hormones such as abscisic acid, suggests that they should be considered as key components of an integrated signalling network involved in many aspects of seed physiology.

Mineral nutrient stores in cereal grains are mainly phytate, a salt of the phosphorus-rich compound phytic acid. Quantitative measures of total phosphorus, phytic acid-phosphorus, potassium, magnesium, calcium, iron, manganese and zinc were obtained for whole grains, embryos and rest-of-grain portions of cv. Kaybonnet rice (wild type) (Oryza sativa L.) and a low phytic acid (lpa1–1) mutant strain with a 45% reduction in phytic acid. P, K and Mg were present in higher amounts than Ca, Mn, Fe and Zn in both grain types. Whole-grain amounts of total P, Ca, Mn and phytic acid-phosphorus were lower in whole lpa1–1 grains than in wild-type grains; K, Mg and Fe amounts were similar, and Zn was higher. Embryos, which comprise 3.5% or less of grain dry weight, were comparatively rich in all measured elements. The lpa1–1 mutation influenced the phytic acid content of the embryo more than that of the aleurone layer. Aleurone-layer cells of wild-type grains had many phosphorus-rich globoids 2μm or larger in diameter, whereas lpa1–1 grains contained more of the smaller globoids. The reduction in globoid size was consistent with the reduction in phytate. Energy-dispersive X-ray analysis of both aleurone-layer cells and sections of globoids in aleurone-layer cells revealed that P, K and Mg were the main mineral nutrient elements present in both grain types; traces of Ca, Mn, Fe or Zn were present. Starchy endosperm cells contained virtually no P, K or Mg, whereas scutellum cells were rich in these elements.

The impact of rehydration on a multicellular organism was studied in lettuce (Lactuca sativa L.) embryos, using cryo-scanning electron microscopy (cryo-SEM). Naked embryos were sensitive to imbibitional stress, whereas embryos with an intact, thick-walled endosperm were not. Imbibitional injury to naked embryos was mainly confined to the outer 2–3 cell layers of the axis. These cells failed to swell and appeared poorly hydrated. Deeper layers were not affected even after extended periods of cold rehydration. The proportion of damaged cells (6–7% of total) roughly corresponded with the additional K+ that gradually leached from the embryos. Damaged embryos were able to survive the loss of their surface layers and form adventitious roots. The swelling of inner tissues caused the dead surface layers to rupture into patches. Plasma membranes in dried embryos showed normal bilayer structure with a homogeneous distribution of intra-membrane particles (IMPs), also after non-injurious rehydration. Imbibitionally damaged plasma membranes showed many irregularities, such as globular insertions, that probably resulted from malfusions in the ruptured membrane, but the IMPs were still randomly distributed.

Senna multijuga and Plathymenia reticulata are tropical tree species native to the Brazilian Atlantic Forest and the Brazilian Cerrado, respectively. Seed-coat dormancy variation was evaluated within and among natural populations of these two species. Scarified and non-scarified seeds from different plants within populations were germinated at 28°C, and the percentage of germinated seeds was estimated for both species. Mean germination percentages of non-scarified seeds tended to be higher for P. reticulata populations (40 and 62%) than for S. multijuga populations (9 and 35%). After scarification, germination percentages increased significantly in both species, with all populations showing mean values above 84%. The level of seed dormancy, evaluated through the experiment with non-scarified seeds, differed significantly within and among populations of both species (P < 0.05). The values of the coefficient of genotypic determination were high for populations of both species (b = 0.85). Although this coefficient is an overestimation, since it includes non-genetic maternal effects, its high values suggest that a considerable part of the phenotypic variation in seed dormancy in S. multijuga and P. reticulata is of genetic origin. Variation in seed dormancy can be an important factor for increasing genetic diversity in populations of these species, making them able to respond to environmental changes.

The aim of this study was to establish associations of kernel weight and colour with ecogeographic factors and molecular markers, based on ten wild barley [Hordeum spontaneum (C. Koch) Thell.] populations sampled in Israel across a southward transect of increasing aridity. Kernel weight and colour category were scored using barley kernels (naked caryopsis). Small kernel sizes (0.011 g kernel–1) and dark kernels were found in xeric populations. A higher variation of kernel weight was observed in xeric populations. A higher proportion of variation occurred within, rather than among, populations. Water, temperature and soil factors were associated with kernel size variation. Among 18 simple sequence repeats (SSRs) investigated, HVM14, HVM36, HVM43, BMS64 and BMS90 were associated with kernel weight, and HVM68 with kernel colour. The results indicated that high phenotypic variation and genetic diversity are related to ecological stress, and that the association of phenotypic traits with molecular markers, based on natural plant populations, should be interpreted cautiously due to the high chance of spurious associations between traits and molecular markers.

Epizoochory is widely recognized as an effective long-distance seed dispersal mechanism. Nevertheless, few studies have focused on the investigation of its influencing factors. One of the key aspects of epizoochory is the adhesive interaction between seeds and furs. We describe a new method to quantify experimentally and standardize the adhesivity of seeds to animal fur, as a measure of epizoochorous dispersal potential. The method excludes the impact of animal behaviour and environmental factors, and allows the ranking of species according to their adhesivity score. We measured adhesivity scores for 66 species on the furs of seven mammals. Deep furs with long, rough, undulated hairs implanted at a large angle were most suited for seed adhesion, while seeds adhered less well to shallow furs with short, smooth, straight hairs implanted at small angles. Seeds with specialized adhesive appendages had higher adhesivity scores than seeds with unspecialized appendages and seeds without appendages. However, an interaction effect between certain seed and fur types exists. Although seed morphology is a good predictor for seed adhesivity on fur, less well-adhering seed types often still have relatively high adhesivity scores. Therefore, it is likely that nearly all species are, to some extent, able to disperse epizoochorously.

The desiccation sensitivity of seeds of Inga vera Willd. subsp. affinis, a recalcitrant-seeded tree from Brazil, was analysed, focusing on water relations and cell-cycle aspects, including DNA content and the microtubular cytoskeleton. Seeds were collected at four developmental stages, dried to different moisture contents (MCs), assessed regarding water activity and set to germinate. Samples of fresh (non-dried) developing and mature seeds were used for assessment of DNA content by flow cytometry. Immunohistochemical detection of microtubules (MTs) was done in mature seeds at different MCs. Slight desiccation of immature seeds increased germination, but further drying resulted in a quick decline of germinability. During seed development the desiccation sensitivity decreased slightly, but DNA content of the embryonic axis cells remained constant, suggesting no relation between those two parameters. Embryonic axis cells of mature seeds showed abundant cortical microtubule arrays, which were not affected by mild desiccation, but broken down by further drying. It appeared that, upon rehydration, damaged cells were not able to reconstitute the microtubular cytoskeleton. The failure of germination of Inga vera seeds after drying could not be attributed to cellular damage to DNA synthesis and mitosis, since the radicle protruded by means of cell elongation, without a need for cell division. However, the breakdown of MTs during desiccation, and their subsequent inability to reassemble upon rehydration, may be related to the decreased germination, since MTs are required for cell elongation.

The concept of ‘imbibitional damage’ arose when it was observed that considerable leakage of cell contents could occur when dry seed or pollen tissues are plunged directly into water. It is now common practice to imbibe dehydrated tissue slowly, to permit the re-establishment of functional membranes, prior to placing the tissue into liquid water. However, this argument may not hold if the tissue of interest is inherently desiccation-sensitive. Slow drying of desiccation-sensitive (recalcitrant) seeds or excised embryonic axes results in damage at high water contents, because it permits time for aqueous-based deleterious processes to occur. The same argument may apply if partially dried material is re-imbibed slowly, as this technique will also expose the tissue to intermediate water contents for protracted periods. This hypothesis was tested using embryos or axes from seeds of three recalcitrant species (Artocarpus heterophyllus, Podocarpus henkelii and Ekebergia capensis). Excised material was rapidly dried to water contents within the range over which viability is lost during drying, and re-imbibed either rapidly, by plunging directly into water, or slowly, by placing the material on damp filter paper or exposing it to a saturated atmosphere for several hours. Although details of the response differed among species and developmental stage, in all cases direct re-imbibition in water resulted in higher (or similar, but never lower) survival than either of the slow rehydration techniques.

For recalcitrant seeds, mortality curves of germination versus water content typically imply a wide range of desiccation sensitivities within a seed population. However, seed to seed differences in water content, during desiccation, may confound our interpretation of these mortality plots. Here, we illustrate this problem for two batches of Vitellaria paradoxa (Sapotaceae) seeds collected in 1996 and 2002. Whole seeds were desiccated to various target water contents (TWCs) using silica gel. During desiccation, smaller seeds in the population dried most rapidly. Consequently, there was a significant linear relationship between whole-seed water content and seed mass during the drying process. In addition, following desiccation to low TWCs, only the largest seeds in the population retained viability. Taken together, this suggests that the larger seeds survived, not as a consequence of great relative desiccation tolerance, but as a result of taking longer to desiccate. Subsequently, the critical water content (CWC) for viability loss was calculated, based on the assumptions that in the seed population whole-seed water content during desiccation was normally distributed and the smallest, and hence driest, seeds were killed first. Using this approach, the driest seeds in the population that were killed, at each TWC, were always below a single CWC (c. 20% and 26% in 1996 and 2002, respectively). In subsequent experiments the effect of seed size variation on the response to desiccation was confirmed by conducting desiccation screens on seeds sorted into two discrete size classes, i.e. the seed-lot heterogeneity in mass was reduced. Using this approach, the mortality curves had a steeper slope. Furthermore, data for 24 tropical tree species from the Database of Tropical Tree Seed Research (DABATTS) revealed that seed lots with less variability in mass had steeper mortality curves. Thus, taken together, the data suggest that, at least for whole seeds, the wide range of desiccation sensitivities typically inferred is an artefact of seed to seed variation in mass, and hence water contents, during drying.

Despite the importance of the palm family, Arecaceae, little has been systematically documented about the seed behaviour of the many species. The post-harvest seed behaviour of Phoenix reclinata, the highly utilized wild date palm species distributed along the eastern seaboard of Africa, is investigated in the present study. While both embryo and endosperm water concentration declined as the seeds of Phoenix reclinata matured, they remained relatively high: this is a characteristic of (but not confined to) non-orthodox seeds. The ultrastructure of embryo cells, and the finding that negligible water uptake was required for the initiation of germination, were in keeping with the possible non-orthodox nature of the seeds. A developmental study revealed that between the acquisition of full germinability and complete pre-shedding maturity, germination performance appeared to be constrained, suggesting the presence of an inhibitor. Pre-treatment by soaking, mechanical or acid scarification had no significant promotory effect on either rate or totality of germination of mature P. reclinata seeds, while use of water transiently at 100°C was highly deleterious. However, germination of partially dehydrated seeds was initiated sooner if they had been soaked or scarified. Mature P. reclinata seeds tolerated dehydration to a mean embryo water concentration of 0.40 g g–1 (dry mass basis; dmb), but at 0.14 g g–1, both rate and totality of germination were adversely affected. However, viability of seeds dehydrated to the mean embryo water concentration 0.40 g g–1 declined during storage for 16 weeks. It is concluded that P. reclinata seeds are non-orthodox, and are best categorized as showing intermediate post-harvest behaviour.

Imbibitional stress was imposed on neem (Azadirachta indica) seeds by letting them soak for 1 h in water at unfavourable, low temperatures before further incubation at 30°C. Sensitivity to low imbibition temperatures increased with a decrease in seed moisture content (MC). To investigate a possible involvement of seed age in the extent of imbibitional damage, initially high-quality seed lots that differed in storage history (10 weeks versus 10 months) were examined at 4 and 7% MC (fresh weight basis). After 10 months of storage, the 7% MC seeds had become sensitive to imbibitional stress. Further drying (1 week) to 4% MC affected aged seeds more than non-aged seeds. Barrier properties of cellular membranes in axes excised after 1 d of rehydration were estimated using a spin-probe technique. The proportion of cells with intact membranes increased with increasing imbibition temperature. For each temperature tested, there were more cells with leaky membranes after 10 months than after 10 weeks of dry storage. Localization of embryo cells displaying loss of turgor and abnormal cellular structure was accomplished using cryo-planing, followed by cryo-scanning electron microscopy. Inspection of the cryo-planed surfaces confirmed that imbibitional damage was temperature dependent, occurring at the periphery. Ageing increased the number of imbibitionally damaged, peripheral cell layers. Germination was estimated to fail when less than 70% of axis cells were alive. We conclude that ageing increases the sensitivity to imbibitional stress. Both the fast ageing and the sensitivity to imbibitional stress might explain the apparent controversies about neem seed desiccation tolerance and storage behaviour.

Assessment of the rehydration procedure has been consistently overlooked in evaluation of factors contributing to successful cryostorage of partially dehydrated, embryonic axes excised from recalcitrant seeds. Conventional rehydration of Trichilia dregeana (Sond.) axes in distilled water resulted in the lack of root pole gravitropism after culture on medium in vitro. In comparison, a strong gravitropic response was observed in axes that had not been dehydrated, and by those rehydrated in a solution containing 1 M CaCl2 and 1 mM MgCl2. However, no marked loss of either cation from the tissue could be detected, whether axes were rehydrated in distilled water or the Ca2+/Mg2+ solution. Starch-packed statoliths differentiated rapidly in both non-dehydrated axes and those rehydrated in the divalent cation solution, but these organelles failed to develop or accumulate much starch following axis rehydration in water, as was the case for the amyloplasts of root cells generally. After rehydration in the Ca2+/Mg2+ solution and 48 h in culture, axis root-cap columella cells accumulated Ca2+, whereas axes rehydrated in water or solutions containing either Ca2+ or Mg2+ alone did not take up Ca2+. Rehydration with the Ca2+/Mg2+ solution also altered distribution of the actin component of the cytoskeleton. When rehydrated in the divalent cation solution, actin was associated with the nucleus and with the statoliths, which were located distally in statocytes of axes. In contrast, actin was largely confined to the perinuclear area in root-cap columella cells of the agravitropic, water-rehydrated axes. The present results indicate a definitive primary role for Ca2+ with Mg2+ in graviperception, via starch metabolism, and in the determination of statolith morphology, which appears to be linked with maintenance of the actin component of the cytoskeleton in root-cap statocytes.

Leymus Racemosus, The Mammoth Wild Rye, Is A Rhizomatous Perennial Grass, Mainly Distributed In The Moving Or Semi-Stabilized Sand Dunes In Deserts Of The Junggar Basin In Xinjiang, China. The Revival Ability Of The Young Seedling After Periods Of Desiccation Can Be Influenced By Several Factors: (1) The Stage Of Seedling Development – The Later The Stage At Dehydration, The Longer The Root Length And The Lower Is The Percentage Of Seedlings That Survive; (2) The Length Of The Period Of Desiccation – The Longer The Period That The Seedlings Are Under Desiccation, The Lower Is The Percentage Of Seedlings That Survive; (3) Endosperm Size – The Smaller The Proportion Of Endosperm That Remains In The Caryopses, The Lower Is The Percentage Of Seedlings That Revive, Determined By (A) The Stage Of Seedling Development, And (B) The Proportion Of The Endosperm That Is Removed By Cutting; And (4) Caryopsis Size – The Larger The Polymorphic Caryopses, The Higher Is The Percentage Of Young Seedlings That Revive From Periods Of Desiccation. The Physiological And Ecological Implications Of L. Racemosus Seedling Desiccation Tolerance Are That Under Extreme Desert And Unpredictable Environmental Conditions, The Chances Of Seedling Establishment Are Increased.

As a consequence of previous lack of success in cryostoring axes excised from newly shed seeds of Trichilia dregeana, the effects of the mode of axis excision on seedling production were investigated. Although vigorous root production occurred, no shoots were produced when the cotyledons were severed as closely as possible to the axis surface (explant-type 0). In contrast, shoot production was increasingly facilitated when small to larger segments of cotyledonary tissue were left attached to the axes (explant-types 1, 2 and 4), which did not compromise axis drying rate. However, root growth of explant-types 1, 2 and 4 was negatively affected, probably by leakage into the medium of an inhibitory or toxic substance(s) from the cut surfaces of the cotyledonary tissue. Microscopical examination revealed that the cotyledons were sessile, and their insertions were contiguous with the shoot apex in axes from newly shed seeds, leading to the suggestion that failure of shoot production by type 0 explants in vitro was the direct consequence of the proximity of the wound sites to the apical meristem. When seeds were stored hydrated for 6 months, the shoot apex had elongated, positioning the apical meristem some distance from the top of the cotyledonary insertions. In contrast to axes excised as type 0 explants from newly shed seeds, the equivalent explants from the stored seeds rapidly formed shoots and leaves in vitro. This indicates that the developmental status of axes, when excised, dictates failure or success in their further development in vitro, and that this aspect needs to be resolved before any further manipulations for cryostorage are attempted.

Cherry tomato (Lycopersicon esculentum Mill.) seeds harvested from fruits at four stages of development [2, 4, 6 and 8 weeks after flowering (WAF)] were washed (1% HCl) and germinated after 72 h of desiccation at 20°C, and 76%, 49% or 12% RH. Seed α-amylase activity was determined at each stage of development and correlated with seed germination. Desiccation at 76% and 49% RH had no significant effect on the germination of seeds at 4, 6 and 8 WAF, whereas it improved germination of seeds at 2 WAF. Low RH (12%) significantly reduced seed germination at all stages of development. There was a positive correlation (r = 0.68, P ≤ 0.05) between germination and α-amylase activity during seed development. Scanning electron microscopy also revealed a correlation between starch grain occurrence in the endosperm and α-amylase activity. Placenta water potential decreased with fruit and seed maturation, and correlated negatively with improved seed germination and α-amylase activity. Promotion of seed germination by desiccation in developing cherry tomato seeds was lost 4 weeks prior to mass maturity (80 ± 3% seed water content and –2.3 ± 0.1 MPa placenta water potential). It is proposed that the water potential differential between the placenta and the seeds influences α-amylase activity and germination behaviour during development.