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

SUMMARY
Recalcitrant seeds of Shorea robusta (sal) exhibit 100% viability up to 4 d after maturity. The rapid loss of viability after 4 d is associated with the reduction in moisture content below the lowest safe moisture content (37°o). Seed becomes non-viable on 8 d. Increased leakage of electrolytes in seeds and lipid peroxidation in embryonic axes was discernible immediately from 0 d. In embryonic axes, very low levels of superoxide (O2−) were maintained up to 4 d and a sharp increase was registered up to 7 d. It is suggested that loss of moisture content in sal seeds below 37% (after 4 d) induces substantial leakage loss probably due to increased lipid peroxidation and O2−) radical formation which are responsible for severe membrane perturbations leading to rapid loss of viability. In embryonic axes SOD activity was recorded only in 100% viable seeds.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1994.tb02957.x

Role of superoxide, lipid peroxidation and superoxide dismutase in membrane perturbation during loss of viability in seeds of Shorea robusta Gaertn.f

In most species, arrest of growth and a decrease in water content occur in seeds and pollen before they are dispersed. However, in a few cases, pollen and seeds may continue to develop (germinate). Examples are cleistogamy and vivipary. In all other cases, seeds and pollen are dispersed with a variable water content (2–70%), and consequently they respond differently to environmental relative humidity that affects dispersal and maintenance of viability in time. Seeds with low moisture content shed by the parent plant after maturation drying can generally desiccate further to moisture contents in the range of 1–5% without damage and have been termed ‘orthodox’. Pollen that can withstand dehydration also was recently termed orthodox. Seeds and pollen that do not undergo maturation drying and are shed at relatively high moisture contents (30–70%) are termed ‘recalcitrant’. Since recalcitrant seeds and pollen are highly susceptible to desiccation damage, they cannot be stored under conditions suitable for orthodox seeds and pollen. Hence, there are four types of plants with regard to tolerance of pollen and seeds to desiccation. Orthodoxy allows for dispersal over greater distances, longer survival, and greater resistance to low relative humidity. The advantage of recalcitrance is fast germination. Orthodoxy and recalcitrance are often related to environment rather than to systematics. It has been postulated that certain types of genes are involved during presentation and dispersal of pollen and seeds, since molecules (sucrose, polyalcohols, late embryogenic abundant proteins, antioxidants, etc.) that protect different cell compartments during biologically programmed drying have been detected in both.

Pollen and seed desiccation tolerance in relation to degree of developmental arrest, dispersal, and survival

http://planet.uwc.ac.za/nisl/Gwen's%20Files/Conservation%20Biology/Chapters/Info%20to%20Use/Chapter7/gapresearchmanageLee.pdf

P. B Tompsett

Papers

A review of the literature on storage of dipterocarp seeds