Understanding Poor Storage Potential of Recalcitrant Seeds

Trees producing recalcitrant seeds are dominant among Asian (89%) and neo-tropical species (44%), but are rare among African tree species (9%). Among the families, Dipterocarpaceae is the most populous with 98% of recalcitrant species, followed by Meliaceae (17%), Moraceae (14%), Malvaceae (8%), Rubiaceae (7%) and Bignoniaceae (3%). Since desiccationsensitive recalcitrant seeds are shed from the tree after maturation, with high moisture content, high metabolic activity and poor storage potential, it leads to significant challenge for ex situ conservation through seed-gene bank. A general understanding of the mechanisms underlying the poor viability potential of recalcitrant seeds is important to redefine our research programmes on these desiccation-intolerant seeds. There are three general categories of damage that may take place in recalcitrant seeds, eventually causing loss of viability: (i) mechanical damage, (ii) metabolism-induced damage, and (iii) macromolecular denaturation. This article discusses the occurrence of these three categories of damage, in the light of the various biochemical and structural changes reported in the different recalcitrant seeds, relative to (i) storage temperature, (ii) seed moisture content and (iii) rate of seed drying. Among these factors, maintenance of appropriate storage temperature is viewed as the most appropriate way to contain all the three types of damage. Storage of seeds in lowest suitable temperature is known to restrain the rate of moisture loss from seeds, thereby reducing mechanical damage and macromolecular denaturation; further, it can also minimize aberrant metabolism, eventually reducing metabolism- induced damage. Thus, the most practical way to extend the storage life of intact recalcitrant seeds is by storing them at lower temperatures, wherein the above damages can be reduced and at the same time chilling or freezing injury may not occur. Nevertheless, the 'lowest temperature' which is most suitable for seeds of a particular recalcitrant species should be standardized through dedicated research.