Storage behaviour and antioxidant activities of Mimusops elengi seeds subjected to different drying rates

The tolerance of seed to desiccation depends on the species, developmental stage and drying conditions, especially the drying rate. Mimusops elengi seeds are classed as "intermediate" in terms of performance during storage. The object of this study was to test the effect of drying rate on viability, storage behaviour and antioxidant enzymes in M. elengi seeds. Rapid desiccation (over silica gel) for six days, where the water content (WC) of the seed was reduced from 39.7 to 7.5%, decreased the germination from 100 to 70%. Viability was fully retained after rapidly drying to 19.8% WC, whilst 95% of the seeds germinated following slow drying (equilibrium drying at 85% relative humidity). In contrast to this relative insensitivity to desiccation, M. elengi seeds were sensitive to low temperatures. All fresh seeds germinated, but only 18% of seeds stored at 4 degrees C for six months germinated and none stored for 12 months germinated. Storage at 15 degrees C was found to be optimal for M. elengi seeds. Drying rate had no obvious effect on seed germination although desiccation altered the activities of antioxidant enzymes and different drying rates led to different effects. Catalase (CAT) activity decreased with desiccation. Superoxide dismutase (SOD) activity increased with desiccation, with rapid drying increasing activity more than slow drying. Ascorbate peroxidase (APX) activity increased after initial desiccation, and then decreased with further desiccation. Rapid drying led to higher APX activity than slow drying. Activity of dehydroascorbate reductase (DHAR) and glutathione reductase (GR) had no clear trends with desiccation, but both enzymes had higher activity after slow drying than after rapid drying. The data show that rapid drying of M. elengi seeds leads to greater mobilisation of SOD, APX and CAT after drying to WC < 19.8%.