C. R. McGill

The effects of salinity and osmotic stress on barley germination rate: sodium as an osmotic regulator

BACKGROUND AND AIMS Seed germination is negatively affected by salinity, which is thought to be due to both osmotic and ion-toxicity effects. We hypothesize that salt is absorbed by seeds, allowing them to generate additional osmotic potential, and to germinate in conditions under which they would otherwise not be able to germinate. METHODS Seeds of barley, Hordeum vulgare, were germinated in the presence of either pure water or one of five iso-osmotic solutions of polyethylene-glycol (PEG) or NaCl at 5, 12, 20 or 27 °C. Germination time courses were recorded and germination indices were calculated. Dry mass, water content and sodium concentration of germinating and non-germinating seeds in the NaCl treatments at 12 °C were measured. Fifty supplemental seeds were used to evaluate the changes in seed properties with time. KEY RESULTS Seeds incubated in saline conditions were able to germinate at lower osmotic potentials than those incubated in iso-osmotic PEG solutions and generally germinated faster. A positive correlation existed between external salinity and seed salt content in the saline-incubated seeds. Water content and sodium concentration increased with time for seeds incubated in NaCl. At higher temperatures, germination percentage and dry mass decreased whereas germination index and sodium concentration increased. CONCLUSIONS The results suggest that barley seeds can take up sodium, allowing them to generate additional osmotic potential, absorb more water and germinate more rapidly in environments of lower water potential. This may have ecological implications, allowing halophytic species and varieties to out-compete glycophytes in saline soils.

The effect of biochars on maize (Zea mays) germination

Abstract Biochar made from biosolids, corn stover, eucalyptus, fresh pine or willow pyrolysed at 550°C and incorporated into Manawatu fine sandy loam or Waitarere sand at rates from 0 to 10 t/ha did not significantly affect the germination or early growth (root and coleoptile length, and dry weight) of maize seeds. There were no interactions between type or rate of biochar with soil type. The results suggest that biochar incorporation prior to a maize crop should be investigated as a method of increasing stable soil carbon with the potential for mitigating carbon emissions.

Effects of a low-temperature pre-sowing treatment on the germination of tomato seed under temperature and osmotic stress

Abstract The germination responses of two seed lots of tomato seeds (Lycopersicon esculentum, cultivar ‘Moneymaker’) to a low-temperature pre-sowing treatment were investigated at 10, 20 and 30°C under ψms of 0, −0.25 and −0.5 MPa created by aqueous solutions of polyethylene glycol. After a low-temperature pre-sowing treatment, both lots of seeds germinated more rapidly than untreated controls under most combinations of temperature and polyethylene glycol concentration; the relative benefits increasing with the stress imposed. More significantly, seed treatment increased the final germination capacity of seeds under the severe stress conditions of high temperature and −0.5 MPa. Uniformity of radicle emergence differed greatly between the two seed lots. Pre-sowing treatment brought the uniformity of the poorer lot to levels comparable with, or better than, the other lot which responded less well to treatment in this respect. The physiological and horticultural implications of these results are discussed.

Seed dormancy and germination of a panel of New Zealand plants suitable for re‐vegetation

Abstract Seeds of Carex trifida, Coprosma robusta, Cyperus ustulatus, Hebe stricta, Lepto‐spermum scoparium, Muehlenbeckia australis, Myrsine australis, Phormium tenax, P. tenax ’Yellow Wave’, and Sophora prostrata were assessed for germination and dormancy. Seeds of Carex trifida, Cyperus ustulatus, and Myrsine australis showed no germination in light or dark at 20°C, but a high percentage of seed of Carex and Cyperus germinated after 12 weeks of low stratification. Despite high viability, germination of Myrsine australis seeds was low and occurred only after 12 weeks of low temperature stratification. Germination was low for Muehlenbeckia australis in the light, at 20°C, but increased after 4 weeks of low temperature stratification. After 12 months, 50% of Coprosma robusta seeds germinated but germination was greater after 4 weeks or more of stratification. No seeds of Coprosma robusta or Muehlenbeckia australis germinated in the dark. Rapid germination of Hebe stricta seeds was obtained, with 100% of the seed germinating in the light but 7% in the dark. Leptospermum scoparium had rapid germination, with 100% germinating in the light but 3% in the dark. A low percentage of Phormium tenax seeds germinated in both the light and dark in the first month with no further germination, but after 8 weeks or more of low temperature stratification there was almost complete germination. There was rapid germination of Sophora prostrata seeds with 100% of the seed germinating in the light and dark.

Assessment of germination and vigour status of 24‐year‐old seeds of a Sophora hybrid

Abstract Germination and vigour of 24‐year‐old seeds collected from a single individual of a hybrid Sophora and stored at room temperature are compared with seeds from the 2001 season from the same tree. Germination of machine‐cleaned seed showed the hard seed character was still evident after 24 yr. Hand‐chipped seeds of the 2001 harvest gave 93% germination; 1977 seeds were 8%. Of the 2001 harvest there was little difference in germination between machine‐cleaned seeds and seeds removed by hand from pods. The majority of 1977 embryos were dead within their hard seed coat. Hand‐chipped and machine‐scarified seeds of both harvests were subjected to two accelerated ageing treatments designed to impose physiological stress. Germination of 1977 seeds was suppressed to zero but 2001 seeds were minimally affected. Tetrazolium tests for viability showed that viability was lost from most 1977 seeds. The loss in germinability and viability of the 1977 seeds was accompanied by a loss in vigour of remaining viable seeds. Leakage of solutes from 1977 seeds was greater than from 2001 seeds, suggesting cell membrane and cytoplasmic damage.

Defining the pathways of symbiotic Epichloë colonization in grass embryos with confocal microscopy

ABSTRACT Asexual cool-season grass endophytes of the genus Epichloë (Ascomycota: Clavicipitaceae) are strictly vertically disseminated. The hosts of these mutualistic fungi express no symptoms during the fungal lifecycle that takes place entirely within the plant, while their hosts receive beneficial outcomes. These fungi are distributed in two major locations within the mature seeds of their hosts; namely, within the embryo (including the scutellum, coleoptile, plumule, radicle, and coleorhiza tissues) and between the aleurone and pericarp layers, with the latter hyphae playing no role in transmission of the fungus to the next plant generation. Conflicting evidence remains in the literature on the timing of embryo colonization. In a detailed investigation, utilizing confocal microscopy to observe the distribution of Epichloë coenophiala strain AR601 in tall fescue (Lolium arundinaceum), we tracked endophyte hyphal colonization in the ovary (pre-fertilization) through to the fully mature seed stage. Confocal microscopy images revealed that at the early and mature developmental stages of the embryo sac, before host grass fertilization, there were large quantities of endophyte mycelium present, especially around the antipodal cells, indicating that this endophyte enters the embryo sac before the fertilization stage. After host fertilization, fungal hyphae could be seen in the true embryo and early nonstarchy endosperm. Understanding the mechanisms of transmission to the seed is important for commercial seed producers and end users.

Biology, structure, and germination characteristics of seeds of puriri Vitex lucens (Verbenaceae)

Abstract Germination behaviour of puriri (Vitex lucens) is described. Seedlings emerging from fruits from six localities at the southern range of the species in North Island, New Zealand, largely followed a normal distribution. There were distinct locality differences in fecundity. Fruits that produced at least one seedling ranged from 5 to 43% (1.3–16.1% of potential seedlings). An average of 35% of those seeds within fruits that germinated produced multiple seedlings; 22% produced 2, 6% produced 3, and 1% produced 4. Light and electron microscopy of mature seeds revealed an embryo with well‐developed cotyledons (with cells containing storage lipids) and axis. A membranous endosperm and testa encloses the embryo, the latter containing fewer and thinner cells with maturity. Seed moisture contents (SMC) and percentage germination within 25 days were recorded from seeds extracted from green (32.6% SMC, 60% germination, 25% fresh ungerminated) and red fruits (27.2% SMC, 58% germination, 14% fresh ungerminated). In addition, further seeds were extracted from red fruits, dried in a glasshouse, and germinated (1.9% SMC, 40% germination, 30% fresh ungerminated) suggesting that seeds do not exhibit recalcitrant behaviour.

Seed storage and seed storage reserves in Chatham Island forget‐me‐not (Myosotidium hortensia, Boraginaceae)

Abstract Chatham Island forget‐me‐not is endemic to the Chatham Islands, New Zealand, where it is mainly confined to the outer islands. The conditions required to maintain seed viability during storage are not well understood and there are conflicting reports as to whether the seed will retain viability in storage. Seed of M. hortensia stored at 7.5% seed moisture content showed no decline in viability after 21 months, but seed stored at 9.5% seed moisture content showed a significant (P < 0.05) loss of viability after 9 months storage. The composition of the seed storage reserves of M. hortensia was assessed using transmission electron microscopy. The storage reserves were found to be predominately lipid (24%) and protein with no starch reserves detected. Gas chromatographic analysis of fatty acid methyl esters derived from the lipid storage reserves identified a high percentage (9%) of the commercially important γ‐linolenic (cis, cis, cis‐6, 9, 12‐octadecatrienoic) acid. The decline in seed viability at relatively low seed moisture content is typical of oil storage seeds and consistent with orthodox seed behaviour.

The mechanism of seed coat-imposed dormancy revealed by oxygen uptake in Chatham Island forget-me-not Myosotidium hortensia (Decne.) Baill.

ABSTRACT Chatham Island forget-me-not Myosotidium hortensia (Boraginaceae) is a monotypic genus endemic to the Chatham Islands of New Zealand. Myosotidium hortensia is threatened in its natural environment by introduced grazing animals and weeds. Further to this, the conservation and reintroduction efforts of M. hortensia are hindered by their dormant seeds. The dormancy is imposed by the seed coat but the underlying dormancy mechanisms are not well understood. Understanding the dormancy mechanisms in this species will help in revegetation efforts. This work aims to elucidate mechanisms by which the seed coat imposes dormancy in M. hortensia by determining the germination after manipulation of the seed coat and measuring changes in oxygen uptake in response to this manipulation. The structure of the seed coat was assessed using scanning electron microscopy and the presence or absence of lignin in the seed coat determined. As with other Boraginaceae, the seed coat of M. hortensia does not restrict water penetration into the seed. Oxygen uptake was 4.5 fold higher in decoated than in intact seed. There was no significant increase in oxygen uptake for the seeds when their seed coat had been pierced or pierced and resealed with vaseline. However, the germination rate, as measured by radicle emergence, was faster in these seeds. These findings, in addition to the structural analysis of the seed coat which revealed that the seeds have secondary thickening but no lignin, confirmed that mechanical constraint of embryo growth was preventing germination.

Saving rainforests in the South Pacific: challenges in ex situ conservation

Rainforests in the South Pacific hold a considerable amount of plant diversity, with rates of species endemism >80% in some countries. This diversity is rapidly disappearing under pressure from logging, clearing for agriculture or mining, introduced pests and diseases and other anthropogenic sources. Ex situ conservation techniques offer a means to limit the loss of plant diversity. Seed banking is considered the most efficient and cost effective of these techniques but is applicable only to seed capable of tolerating desiccation and cold storage. Data on the degree of tolerance of these conditions was lacking for more than half of the 1503 South Pacific rainforest genera examined for this review. Of the 710 genera for which data were available, the storage behaviour of 324 was based on an assessment of only one or two species, although 76% of those genera contained at least 10 species. Many of the unstudied or poorly studied genera are shared across several South Pacific nations, providing an excellent opportunity for collaboration on future ex situ research and conservation. Of the 386 genera for which three or more species have been studied, 343 have a very high proportion of species (>95% of those tested) that are suitable for seed banking. Seed banking could therefore provide a suitable means for preserving a large proportion of the rainforest flora before it becomes extinct in the wild. Alternatives for preserving species that are not suitable for seed banking are also discussed.