A multifactorial study of conditions influencing longevity of neem (Azadirachta indica) seeds

The longevity of neem, Azadirachta indica, seeds from African Sahelian (Burkina Faso) and Asian (Sri Lanka) provenances was studied over two years of storage under different conditions of moisture and temperature. After drying to equilibrium moisture content (MC) at different relative humidities at 20 degrees C, seeds were placed in open storage at 20 degrees C or hermetically sealed in packets at temperatures ranging from -20 to +20 degrees C. There was hardly any difference in storage behaviour between seed batches / lots, whatever their provenance. Seeds originating from mature yellow fruits lived longer than seeds from green or brown fruits. In all storage experiments with seeds having MCs greater than or equal to 10%, viability was preserved best at 10-15 degrees C, indicating that neem seed is chilling (and freezing) sensitive. There was no survival longer than 2 years under these conditions. At MCs of 4-8%, seeds were considerably more tolerant of low temperature storage and had 40-60% viability after 2 years at all temperatures tested (-20 to +20 degrees C). However, the seeds were sensitive to imbibitional stress, which could be alleviated by imbibition at temperatures of 25-30 degrees C or above. The difficult storage behaviour of neem seed seems to stem from: (1) the sensitivity to low temperatures at MCs greater than or equal to 10%; (2) the extreme sensitivity to imbibitional stress after storage at less than or equal to 8% MC; (3) underestimation of the water activity due to the high oil content of the neem seeds, causing unexpected metabolic stress in the higher MC and temperature range.