The fundamental deteriorative processes that lead to loss of seed viability contrastingly vary between desiccation insensitive (orthodox) and desiccation sensitive seeds (recalcitrant). Orthodox seeds which undergo maturation drying are bestowed with protective mechanisms which guard the seeds against deterioration. They include the accumulation of antioxidants, non-reducing sugars, protective proteins such as late embryogenesis abundant proteins, heat-shock proteins, lipocalins, hormones and chemical protectants (raffinose family oligosaccharides, flavonoids, lignins, vitamin E). The nuclear DNA is packed denser and chlorophyll is degraded. Besides, the cytoplasm is capable of transitioning between liquid and glassy state depending on the moisture content of the seeds aiding in the maintenance of seed viability potential. In the dry seeds, the glassy state of the cytoplasm ensures the stabilization of cellular components by arresting cell metabolism. However, even with low moisture content and a glassy state of cytoplasm, reactive oxygen species generated due to the presence of oxygen in the storage atmosphere may cause the ageing of seed. As the seed moisture content increases, mitochondrial respiration gets activated, also leading to increased production of reactive oxygen species, owing to inefficient mitochondrial activity. The reactive oxygen species lead to the oxidation of essential molecules such as DNA, RNA, proteins and lipids. Further, mitochondrial membranes also get oxidized, leading to reduced aerobic respiration potential. When the damage is not substantial, orthodox seeds are capable of repairing the molecular damages that accumulate during storage, enabling the seeds to partially overcome the damages and extend their longevity. This includes activation of repair of cell membranes, DNA, RNA, proteins and mitochondria as the seeds imbibe water.Unlike the orthodox seeds, the recalcitrant seeds are largely devoid of protective mechanisms which guard the seeds against rapid deterioration. The recalcitrant seeds are shed from the mother tree at high moisture content while they are metabolically active. After dispersal, the seeds undergo deteriorative changes during drying due to the damage to the cytoskeleton (physical damage), besides reactive oxygen species-induced damage due to lack of antioxidant activity (metabolism-induced damage). Even when maintained under high moisture content, seeds exhibit dysfunction of the cell organelles and extensive vacuolization predisposing the seeds to deterioration. Thus, recalcitrant seeds are prone to deterioration either under low or high moisture content.
