Effect of heat stress on subsequent germination and viability of ischaemum afrum (j.f.gemel.) dandy seeds

Seeds of Ischaemum afrum were collected from fields in the Rahad Scheme; Central Sudan and were stored for six months at 20 _ 30 deg C. These were placed in Petri dishes, lined with filter paper, moistened with deionized water and incubated at various temperatures for varying periods. Treatments were replicated four times and germination recorded. Ungerminated seeds were transferred to 30/20 deg C (12/12 hrs light/dark) for 7 days and seed germination recorded. The viability of ungerminated seeds was determined by 1% Triphenyltetrazolium chloride (TTC). The results indicated that exposing unimbibed Ischaemum afrum seeds to 60 deg C for 24 hrs had no adverse effect on their germination and viability. However, both germination and viability were reduced under moist conditions in the incubator or in the water bath at 45-60 deg C. At these temperatures, it was observed that the higher the temperature and/or the longer the exposure period, the greater were the reductions in seed viability. Viability of seeds without glumes followed the same trend but was apparently more sensitive to heat stress compared to seeds with glumes. Indisciminately, 7 days of heat stress at 50 deg C under moist conditions killed all the seeds. At 55 and 60 deg C, exposure period required to reduce viability of glumed seeds by 90% was 24 hrs and 6 hrs, respectively. Seeds that were previously imbibed at 30 deg C were more susceptible to a 60 min exposure at 60 deg C in a water bath than previously unimbibed ones. A 60 min exposure at 60 deg C in a water bath had no adverse effect on seed viability. In contrast, the reduction in viability amounted to 70% following 20 min exposure periods made daily for 3 consecutive days. The first indication of seed damage by heat stress was a delay in germination. The results are briefly discussed in relation to solarization