The Vegetation of South Africa, Lesotho and Swaziland

Twenty-four articles by biologists, ecologists, and other scientists represent a year's progress in the field. Among the topics addressed: the effects of introduced species, paleobiogeography, genetics and geographic structure, marine fisheries management, time as an ecological resource, genetic var

Annual Review of Ecology, Evolution and Systematics

The predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity. Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that ...

Acclimation and adaptive responses of woody plants to environmental stresses

A suite of mechanisms or processes that together have been implicated in the acquisition and maintenance of desiccation tolerance in orthodox seeds is discussed in the context of the behaviour of desiccation-sensitive seeds, and where appropriate, parallels are drawn with the situation in vegetative plant tissues that tolerate dehydration. Factors included are: physical characteristics of cells and intracellular constituents; insoluble reserve accumulation; intracellular de-differentiation; metabolic ‘switching off’; presence, and efficient operation, of antioxidant systems; accumulation of putatively protective substances including LEAs, sucrose and other oligosaccharides, as well as amphipathic molecules; the presence and role of oleosins; and the presence and operation of repair systems during rehydration. The variable response to dehydration shown by desiccation-sensitive seeds is considered in terms of the absence or incomplete expression of this suite of mechanisms or processes. Three categories of damage are envisaged: (i) reduction in cell volume which can lead to mechanical damage; (ii) aqueous-based degradative processes, probably consequent upon deranged metabolism at intermediate water contents. This is termed ‘metabolism-induced damage’ and its extent will depend upon the metabolic rate and the rate of dehydration; and (iii) the removal of water intimately associated with macromolecular surfaces leading to denaturation: this is referred to as desiccation damage sensu stricto . The effects of drying rate and the maturity status of seeds are considered in relation to the responses to dehydration, leading to the conclusion that the concept of critical water contents on a species basis is inappropriate. Viewing seed postharvest physiology in terms of a continuum of behaviour is considered to be more realistic than attempting precise categorization. Rapid dehydration of excised embryonic axes (or other explants) from desiccation-sensitive seeds permits retention of viability (in the short term) to water contents approaching the level of non-freezable water. This opens up the possibility of long-term conservation, by cryopreservation techniques, of the genetic resources of species producing non-orthodox seeds.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0960258599000033

A review of recalcitrant seed physiology in relation to desiccation-tolerance mechanisms

The nucleation of crystals in liquids is one of nature’s most ubiquitous phenomena, playing an important role in areas such as climate change and the production of drugs. As the early stages of nucleation involve exceedingly small time and length scales, atomistic computer simulations can provide unique insights into the microscopic aspects of crystallization. In this review, we take stock of the numerous molecular dynamics simulations that, in the past few decades, have unraveled crucial aspects of crystal nucleation in liquids. We put into context the theoretical framework of classical nucleation theory and the state-of-the-art computational methods by reviewing simulations of such processes as ice nucleation and the crystallization of molecules in solutions. We shall see that molecular dynamics simulations have provided key insights into diverse nucleation scenarios, ranging from colloidal particles to natural gas hydrates, and that, as a result, the general applicability of classical nucleation theory...

Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations.

When intact recalcitrant (homoiohydrous) seeds of Landolphia kirkii were stored over silica gel, only 50% viability was retained after 15 days, axis water content having declined to around 120% (dmb) from the original average of 220%. After 20 days, only 7% of the sample (axis water content 49% [dmb]) remained viable. In contrast, when excised embryonic axes were «flash dried» to a water content of 16% within 60 minutes, essentially undiminished viability (95%) was retained (...)

Recalcitrant (homoiohydrous) seeds: the enigma of their desiccation-sensitivity.

Cryopreservation of Somatic Embryos of Four Species With and Without Cryoprotectant Pre-treatment

Slow growth-storage, for up to 10 months, has been achieved for Eucalyptus grandis shoot cultures by either the addition of 10 mg l−1 abscisic acid to the growth medium or by the halving of nutrient supply (half MS) and removal of exogenous plant growth regulators. Reduction of light intensity or the addition of mannitol to the media were less effective in reducing growth rate. Isolated in vitro axillary buds encapsulated in calcium alginate and stored under low temperature and low light intensities survived for up to 3 months without loss in viability. Storage of such encapsulated fresh axillary buds at higher temperature resulted in a loss in viability. These methods have immediate applications to forestry breeding and clonal programs.

In vitro storage of Eucalyptus grandis germplasm under minimal growth conditions.

South Africa has a rich flora which exhibits among the highest species density in the world, distributed across nine biomes that support an impressive diversity of animal life. However, a variety of human actions, invasion by alien species, natural disturbances and climate change collectively impact negatively on the great diversity of both plant and animal species. In situ conservation has long been practised, primarily in nature reserves, complemented by ex situ conservation in national botanic gardens, but in vitro plant conservation is not common. In the context of animal biodiversity conservation, the Wildlife Biological Resource Centre of the National Zoological Gardens utilises cryobanking as one of its major focuses and is now poised to expand as the repository for the cryoconservation of plant germplasm, particularly for indigenous recalcitrant-seeded and poor-seeding species. However, there are particular problems associated with successful germplasm cryostorage of such tropical and subtropical plants. As we see the science and application of cryobiology and cryoconservation as cross-cutting and transdisciplinary, we have entrained formal networking among scientists offering a range of specialisations aimed at a deeper understanding of common problems and practical outcomes to facilitate both plant and animal biobanking. The endeavours are aimed at elucidating the basis of both successes and failures in our efforts to attain optimal outcomes. With focus on best practices, standard operating procedures, validation and risk management for cryopreserved and cold-stored plant and animal material, our ultimate aim is to facilitate restoration by the safe reintroduction of indigenous species.

D.J. Mycock

Papers

Recalcitrant (homoiohydrous) seeds: the enigma of their desiccation-sensitivity.

Cryopreservation of Somatic Embryos of Four Species With and Without Cryoprotectant Pre-treatment

In vitro storage of Eucalyptus grandis germplasm under minimal growth conditions.

Cryoconservation of South African plant genetic diversity

Experimental parameters underlying failure or success in plant germplasm cryopreservation : a case study on zygotic axes of quercus robur l.