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      Implications of the lack of desiccation tolerance in recalcitrant seeds

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          Abstract

          A suite of interacting processes and mechanisms enables tolerance of desiccation and storage (conservation) of orthodox seeds in the dry state. While this is a long-term option under optimized conditions, dry orthodox seeds are not immortal, with life spans having been characterized as short, intermediate and long. Factors facilitating desiccation tolerance are metabolic “switch-off” and intracellular dedifferentiation. Recalcitrant seeds lack these mechanisms, contributing significantly to their desiccation sensitivity. Consequently, recalcitrant seeds, which are shed at high water contents, can be stored only in the short-term, under conditions not allowing dehydration. The periods of such hydrated storage are constrained by germination that occurs without the need for extraneous water, and the proliferation of seed-associated fungi. Cryopreservation is viewed as the only option for long-term conservation of the germplasm of recalcitrant-seeded species. This is not easily achieved, as each of the necessary procedures imposes oxidative damage. Intact recalcitrant seeds cannot be cryopreserved, the common practice being to use excised embryos or embryonic axes as explants. Dehydration is a necessary procedure prior to exposure to cryogenic temperatures, but this is associated with metabolism-linked injury mediated by uncontrolled reactive oxygen species generation and failing anti-oxidant systems. While the extent to which this occurs can be curtailed by maximizing drying rate (flash drying) it cannot be completely obviated. Explant cooling for, and rewarming after, cryostorage must necessarily be rapid, to avoid ice crystallization. The ramifications of desiccation sensitivity are discussed, as are problems involved in cryostorage, particularly those accompanying dehydration and damage consequent upon ice crystallization. While desiccation sensitivity is a “fact” of seed recalcitrance, resolutions of the difficulties involved germplasm conservation are possible as discussed.

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          Reactive species and antioxidants. Redox biology is a fundamental theme of aerobic life.

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            Active oxygen species and antioxidants in seed biology

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              Cryopreservation of nucellar cells of navel orange (Citrus sinensis Osb. var. brasiliensis Tanaka) by vitrification.

              The nucellar cells of navel orange(Citrus sinensis Osb. var. brasiliensis Tanaka) were successfully cryopreserved by vitrification. In this method, cells were sufficiently dehydrated with highly concentrated cryoprotective solution(PVS2) prior to direct plunge in liquid nitrogen. The PVS2 contains(w/v) 30% glycerol, 15% ethylene glycol and 15% DMSO in Murashige-Tucker medium(MT) containing 0.15 M sucrose. Cells were treated with 60% PVS2 at 25°C for 5 min and then chilled PVS2 at 0°C for 3 min. The cell suspension of about 0.1 ml was loaded in a 0.5 ml transparent plastic straw and directly plunged in liquid nitrogen for 30 min. After rapid warming, the cell suspension was expelled in 2 ml of MT medium containing 1.2 M sucrose. The average rate of survival was about 80%. The vitrified cells regenerated plantlets. This method is very simple and the time required for cryopreservation is only about 10 min.
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                Author and article information

                Journal
                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                1664-462X
                22 November 2013
                2013
                : 4
                : 478
                Affiliations
                Plant Germplasm Conservation Research, School of Life Sciences, University of KwaZulu-Natal (Westville Campus) Durban, South Africa
                Author notes

                Edited by: Jill Margaret Farrant, University of Cape Town, South Africa

                Reviewed by: Nabil I. Elsheery, Tanta Univeristy, Egypt; Julia Buitink, Institut National de Recherche en Agronomie, France

                *Correspondence: Patricia Berjak, Plant Germplasm Conservation Research, School of Life Sciences, University of KwaZulu-Natal (Westville Campus), South Ring Road, Durban 4001, South Africa e-mail: berjak@ 123456ukzn.ac.za

                This article was submitted to Plant Physiology, a section of the journal Frontiers in Plant Science.

                Article
                10.3389/fpls.2013.00478
                3837223
                24319450
                d4df3174-07f4-4073-8f55-e40710e2775b
                Copyright © 2013 Berjak and Pammenter.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 30 September 2013
                : 04 November 2013
                Page count
                Figures: 0, Tables: 0, Equations: 1, References: 85, Pages: 9, Words: 0
                Categories
                Plant Science
                Review Article

                Plant science & Botany
                desiccation sensitivity,desiccation damage,recalcitrant seeds,dehydration,ice crystallization,vitrification,cryopreservation,ros

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