Seed Recalcitrance:a Current Assessment

Seeds have been categorized as orthodox, recalcitrant and intermediate seeds according to their dehydration behaviors. Identification of desiccation-tolerance and -sensitivity of seeds is the basis making storage strategy of seeds and long-term conservation of species gene resources. In addition to the inherent characteristics of the species, developmental status of the seeds, dehydration rate, and the conditions under which they are dried and subsequently re-imbibed are very important factors influencing desiccation tolerance of seeds. Survival, electrolyte leakage rate, and germination/growth rate produced by survived seeds are a excellent synthetic parameter when discussing desiccation tolerance of seeds. Desiccation tolerance of seeds is a quantitative feature. The term 'critical water content' is incorrect and has caused some confusion in assessment of seed recalcitrance. A new working approach to quantify the degree of seed recalcitrance has been proposed in this paper.

Ecophysiology of Lophira lanceolata Seeds Germination and Conservation Perspectives

Lophira lanceolata is a multi-purpose woody plant species used by local populations in Benin. Its seeds are used for the manufacture of edible vegetable oil and also for medicinal care. However, reproduction by seed of this plant species is compromised not only by the multiple uses made of the seeds, but also by the difficulties of germination in the natural environment. Several ecological factors could explain this difficulty of germination. In this study, we investigated the species seeds ecophysiology in order to develop seedlings production techniques for its reintroduction. Beta regression was performed to test the effect of water, light and provenance on seeds germination rate. The germination speed was expressed as the median germination time. The results showed that the germination rate was better under light conditions (β = 0.77674 ± 0.14954, Z = 5.194, p < 0.0000). The germination rate decreased as the frequency of watering increased (β = -0.28222 ± 0.14809, Z = -1.906, p = 0.0567). The best provenance was the phytodistrict of Atacora chain. The cumulative germination rate reached 50% after 54 days for seeds collected from Atacora chain and after 57 days under light conditions. Regarding the level of watering, the germination rate remained below 50% regardless the treatment. The germination speed was higher for seeds collected from Atacora chain and that have germinated under light conditions and watered twice a week. For the sustainable management of L. lanceolata, domestication by establishment of plantations by rural communities is recommended based on seeds from Atacora chain.

Conservation and fruit biology of Sichou oak {Quercus sichourensis,Fagaceae)- A critically endangered species in China

Several conservation programs have been started for the critically endangered Sichou oak(Quercus sichourensis) since 2007.These programs include detailed field investigations,seedling cultivation and research on the fruit biology of the species.In this study,we first report on the five mature individual trees found in our 9-year field investigation.Thus far,a total of 10 mature individuals have been recorded.All Q.sichourensis trees are healthy and most produce healthy acorns.Acorns of Q.sichourensis are large with dry masses of 8.0-14.0 g.These acorns had high moisture contents at collection and died shortly after(7-28 d) when dried with silica gel.Characteristics of Q,sichourensis acorns varied between populations.Compared with the acorns from Funing,the acorns collected from Ceheng were bigger,more viable(germination percentage was up to 96%),less sensitive to desiccation,and germinated faster.Q.sichourensis occurs in regions with a distinct 5-6 month dry season.Habitat degradation is largely responsible for the rareness of Quercus sichorensis,but desiccation sensitivity of the acorns may also limit the regeneration of the species and potentially lead to its continued rareness.As a species with extremely small populations(PSESP),Q.sichourensis is facing high risk of extinction and should be defined as a Critically Endangered species in the global IUCN Red List.

Dormancy Breaking and Storage Behavior of Garcinia cowa Roxb. （Guttiferae） Seeds： Implications for Ecological Function and Germplasm Conservation

Abstract: The dormancy breaking and storage behavior of Garcinia cowa Roxb. seeds were investigated. The seeds of G. cowa had 8–11 months dormancy in their natural habitat. Seeds were matured and dispersed at the end of the rainy season (mid-late August to late September) and were scatter-hoarded by rodents as food for winter after the seeds had fallen to the ground. Seedlings often emerged in the forest during the rainy season (May to August) the following year. Intact seeds of G. cowa failed to germinate after being sown at 30 °C for 120 d and the mean germination time (MGT) of seeds cultured in a shade (50% sunlight) nursery was 252 d. The most effective method of breaking dormancy was to remove the seed coat totally, which reduced the MGT to 13 d at 30 °C. Germination was also promoted by partial removal of the seed coat (excising the hilum and exposing the radicle) and chemical scarification (immersion in 1% H2O2 for 1 d). Unscarified seeds take up water rapidly in the first 96 h, but water was absorbed by the outside seed coat, without penetrating through it. The moisture content (MC) of G. cowa seeds was high (50% in fresh weight) at shedding. The seeds could tolerate desiccation to some extent, until the MC reached approximately 40%; below that, the viability decreases rapidly and all seeds died at approximately 17% of MC. Seed viability decreased rapidly when seeds were chilled at 4 °C; germination was 2% after storage for 1 week. Even stored at 10 °C, seeds began to be damaged after 4 weeks. Seed storage for 1 yr revealed that in both dry (relative humidity (35 ± 5)%) and moist (wet sand) storage conditions, seed viability declined, but germination percentages for seeds stored under moist conditions are better than for seed stored under dry conditions. Because of their low tolerance to desiccation, marked chilling sensitivity and relatively short lifespan, G. cowa seeds should be classified into the tropical recalcitrant category. The ecological implications of dormant recalcitrant seeds and cues on storing recalcitrant seeds were discussed.

Possible Involvement of Reactive Oxygen Species Scavenging Enzymes in Desiccation Sensitivity of Antiaris toxicaria Seeds and Axes

The relationships among desiccation sensitivities of Antiaris toxicaria seeds and axes, changes in activities of superoxide dismutase （SOD）, ascorbate peroxidase （APX）, catalase （CAT）, glutathione reductase （GR） and dehydroascorbate reductase, （DHAR）, production rate of superoxide radical （.O2^-）, and the contents of hydrogen peroxide （H2O2） and thiobarbituric acid （TBA）-reactive substance were studied. Desiccation tolerance of seeds and axes decreased with dehydration. Desiccation tolerance of axes was higher than that of seeds, and that of epicotyls was higher than radicles. Activities of SOD, CAT and DHAR of seeds increased during the initial phase of dehydration, and then decreased with further dehydration, whereas activities of APX and GR decreased with dehydration. These five enzyme activities of axes, however, increased during the initial phase of dehydration, and then decreased with further dehydration. The rate of superoxide radical production, and the contents of H2O2 and TBA-reacUve products of seeds and axes gradually increased with dehydration. These results show that the A. toxicaria seed is a typical recalcitrant seed. Loss of desiccation tolerance in seeds and axes was correlated with the increase in .O2- production rate, content of H2O2 and TBA-reactive products, and the decline of antioxidant enzyme activities of seeds and axes.

Induction of desiccation tolerance in desiccation sensitive Citrus limon seeds

Many economically important perennial species bear recalcitrant seeds, including tea, coffee, cocoa, mango,citrus, rubber, oil palm and coconut. Orthodox seeds can be dried almost completely without losing viability, but so-called recalcitrant seeds have a very limited storage life and die upon drying below a higher critical moisture content than orthodox seeds. As a result, the development of longterm storage methods for recalcitrant seeds is compromised.Lowering this critical moisture content would be very valuable since dry seed storage is the safest, most convenient and cheapest method for conserving plant genetic resources.Therefore, we have attempted to induce desiccation tolerance(DT) in the desiccation sensitive seeds of Citrus limon. We show that DT can be induced by paclobutrazol(an inhibitor of gibberellin biosynthesis) and we studied its associated transcriptome to delineate the molecular mechanisms underlying this induction of DT. Paclobutrazol not only interfered with gibberellin related gene expression but also caused extensive changes in expression of genes involved in the biosynthesis and signaling of other hormones. Paclobutrazol induced a transcriptomic switch encompassing suppression of biotic-and induction of abiotic responses.We hypothesize that this is the main driver of the induction of DT by paclobutrazol in C. limon seeds.

Dehydration Kinetics of Embryonic Axes from Desiccation-sensitive Seeds:An Assessment of Descriptive Models

The response of desiccation-sensitive plant tissues to dehydration is significantly affected by dehydration conditions, particularly the rate of drying. Consequently it is important to be able to quantify drying rate. The aim of the study was to assess two models that have been proposed to describe drying kinetics, and thus to provide a quantification of non-linear drying rates, of embryonic axes excised from recalcitrant seeds. These models are an exponential drying time course, and a modified inverse relationship, respectively. For the six species investigated here the inverse function was generally found to fit drying data better than the exponential function under both rapid and slow drying conditions, and so is recommended. The rate of drying, under the conditions used here, was determined by axis size and possibly the nature of the axis outer coverings, rather than the water activity difference between the tissue and surrounding air.

Response of Chinese Wampee Axes and Maize Embryos to Dehydration at Different Rates

Survival of wampee （Clausena lansium Skeels） axes and maize （Zea mays L.） embryos decreased with rapid and slow dehydration. Damage of wampee axes by rapid dehydration was much less than by slow dehydration, and that was contrary to maize embryos. The malondialdehyde contents of wampee axes and maize embryos rapidly increased with dehydration, those of wampee axes were lower during rapid dehydration than during slow dehydration, and those of maize embryos were higher during rapid dehydration than during slow dehydration. Activities of superoxide dismutase （SOD）, ascorbate peroxidase （APX） and catalase （CAT） of wampee axes markedly increased during the early phase of dehydration, and then rapidly decreased, and those of rapidly dehydrated axes were higher than those of slow dehydrated axes when they were dehydrated to low water contents. Activities of SOD and APX of maize embryos notable decreased with dehydration. There were higher SOD activities and lower APX activities of slowly dehydrated maize embryos compared with rapidly dehydrated maize embryos. CAT activities of maize embryos markedly increased during the early phase of dehydration, and then decreased, and those of slowly dehydrated embryos were higher than those of rapidly dehydrated embryos during the late phase of dehydration.