SUMMARY
Loss of moisture in mature seeds of Quercus robur L. was associated with loss of viability, a rise in lipid peroxidation and build-up of free radicals. Radical-initiated damage was largely confined to the embryonic axes as their moisture contents declined to below 47 %. The accumulation of a stable free radical in axial tissue, detected by electron para-magnetic resonance (EPR), was indistinguishable from the EPR response previously shown in a moss on droughting and maize roots on desiccation. A minor higher-field component appeared to represent an intermediate stage in the sequence of free radical reactions associated with loss of water. Using seeds from freshly abscised fruits dried to different moisture contents, protective mechanisms against activated forms of oxygen were monitored in cotyledons and in embryonic axes. The two tissues exhibit distinctly different molecular defences against oxidative attack; that in the cotyledons being predominantly enzymatic, with relatively high and increasing activities of superoxide dismutase and glutathione reductase; that in the axes being largely through the anti-oxidants, ascorbic acid and alpha-tocopherol. We conclude that a decrease in enzymic protection against oxidative attack in the axes, associated with diminishing concentrations of alpha-tocopherol (and depletion of the precursor gamma-tocopherol) as moisture was lost, was directly linked with lipid peroxidation and free radical formation in the axes and that these events taken together may contribute to loss of viability in these recalcitrant seeds.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1992.tb04231.x

Free radical processes and loss of seed viability during desiccation in the recalcitrant species Quercus robur L.

Summary
• An analysis was made of seed traits along a north–south gradient spanning 19° of latitude in Europe using Aesculus hippocastanum, which originates from Greece and the Balkans and has been introduced throughout Europe.
• Because summer temperatures decrease with increasing latitude in Europe, we tested the hypothesis that the heat sum during seed development influences physical, physiological and biochemical seed traits.
• Seeds from Greece (within the natural range) had a fresh mass five times higher than those from Scotland (most northerly seed lot), and a lower axis moisture content and solute potential. In addition, Greek seeds germinated at cooler temperatures, and were more desiccation tolerant. Principal component analysis (PCA) showed that the observed patterns in seed characteristics were consistent and predictable: a single PCA axis explained 86.6% of the variation in the dataset. This axis correlated strongly with the heat sum accumulated by the seeds during development.
• The results support the notion that seeds from further north were shed less developed as a consequence of cooler temperatures during development, and provide a quantitative explanation for intraspecies variability in recalcitrant seed traits for this and, possibly, other species.

Developmental heat sum influences recalcitrant seed traits in Aesculus hippocastanum across Europe

Embryonic axes of tea (desiccation sensitive) and pea (desiccation tolerant) were dried at different rates or stored at different water contents to distinguish between damage associated with the immediate effects of water loss and the longer-term effects of a partially hydrated state. No loss of viability was observed if pea axes were dried sufficiently rapidly (from 1.8 to 0.1 g H2O g-1dry mass (g/g) within 5 d). However, viability was lost in tea axes dried below 0.5 g/g (approximately -15 MPa) even if axes were dried within 1 h. Death in tea axes dried to moisture contents less than 0.5 g/g probably resulted from the removal of water necessary for cellular structural integrity (i.e. desiccation damage sensu stricto). When axes of both species were dried at slower rates, viability losses were observed at water potentials between about -3 and -15 MPa. The timing for this type of damage was species dependent, occurring within 2 d for tea and after 5 d for pea, and may be explained by higher oxidative activity in tea compared to pea. Embryos of both species with water potentials below -3 MPa were lethally damaged if oxygen consumption exceeded 1000–5000 μmol O2 g-1dry mass. Recalcitrant seeds are different than orthodox seeds because the former do not survive drying below a critical water content, regardless of the drying rate. Rapid drying is required for accurate assessment of the critical water content. Slow drying leads to metabolic imbalance and artefactual assessment of the critical water content for desiccation damage. Both tea and pea seeds were susceptible to damage from metabolic imbalances, suggesting that the predominant stress from slow drying is ageing.

Desiccation damage, accelerated ageing and respiration in desiccation tolerant and sensitive seeds

Prediction of Desiccation Sensitivity in Seeds of Woody Species: A Probabilistic Model Based on Two Seed Traits and 104 Species

Summary
1.The ability of seeds to tolerate desiccation plays an important role in plant regeneration ecology. Globally, the majority of species produce desiccation-tolerant (orthodox) seeds, whilst comparatively few produce desiccation-sensitive (recalcitrant) seeds that are unable to survive dehydration. The trait has important implications for species conservation, as desiccation-sensitive species cannot be conserved using traditional seed banking techniques. Additionally, these species may be less resilient to the increases in droughts predicted for some regions under climate change scenarios.

2.The best available resource on seed desiccation tolerance is the Royal Botanic Gardens, Kew's Seed Information Database. This database contains seed desiccation sensitivity data for over 18 000 taxa, approximately 3% of which have desiccation-sensitive seeds. However, this database is likely biased towards desiccation-tolerant species. Previous attempts to estimate the proportion of seed plants with desiccation-sensitive seeds have ranged from 7% to 50%. Here, we aimed to overcome sampling bias to derive a best estimate for the proportion of seed plants with desiccation-sensitive seeds, based on current data.

3.We used a recently developed method, based on taxonomic relatedness, to account for sampling bias and estimate the proportion of seed plants with desiccation-sensitive seeds. As a comparison, given that seed desiccation-sensitivity is strongly related to habitat, we repeated our analyses using habitat as a basis.

4.The predictions from our taxonomy-based models ranged between estimates of 7.5 and 19.6% of the world's seed plant species with desiccation-sensitive seeds, depending on model type, whilst the habitat-based models suggested a value of approximately 8%. Our evidence suggests that, based on current data, the best estimate of the proportion of species with desiccation-sensitive seeds is likely to be approximately 8%. Tropical and subtropical moist broadleaf forests had the highest incidence of seed desiccation-sensitivity, where an estimated 18.5% of the seed plant flora possessed the trait.

5.Synthesis. Alongside our estimation of the numbers of species with desiccation-sensitive seeds, we provide data on taxa and habitats where this trait may be most prevalent. These findings can be used to support conservation planning, particularly with respect to providing decision support for in and ex situ conservation techniques.

This article is protected by copyright. All rights reserved.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2745.12725

Predicting the global incidence of seed desiccation sensitivity

Mature seeds of Norway maple (Acer platanoides L.) are tolerant of desiccation, at least to moisture contents of about 7% (fresh weight basis), but those of sycamore (Acer pseudoplatanus) are killed by drying below about 45% moisture content. Sycamore seeds are thus recalcitrant; while the classification of those of Norway maple as orthodox is confirmed by the fact that between 19% and 7.5% moisture content their longevity is increased in a predictable way by reduction of seed moisturecontent. However, the period of useful storage of the latter in seed banks may be much less than for many crop species. The rates of water loss to a dry environment of both fruits and seeds of sycamore are much less than those of Norway maple, suggesting a degree of desiccationavoidance in the desiccation-intolerant species. Seed physiological maturity (maximum dry weight) occurred 2–3 weeks earlier in Norway maple than insycamore, but in both species this occurred about 150–160 days after peak flowering. Tetrazolium staining is a good indicator of embryo viability in both species, correlating well with germination test results. In Norway maple both methods of viability testing indicated that whole-seed desiccation tolerance coincided with the attainment of maximum dry weight. Tetrazolium staining indicated the development of desiccation tolerance in the radicles/hypocotyls of both species approximately 2–4 weeks before physiological maturity. Possible correlation between changes in the level of embryo dormancy during development and the acquisition of desiccation tolerance are discussed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258500000829

S. V. A. Morris

Papers

The effects of desiccation on seed survival in Acer platanoides L. and Acer pseudoplatanus L.