Peer review

Hybrid necrosis is the premature gradual death of leaves and leaf sheaths in certain wheat hybrids. It is based on two complementary genes Ne 1 and Ne 2. Hitherto 89 Ne 1-carriers and 207 Ne 2-carriers from many countries are known (table 1 and 2). This corresponds with more than 18,000 different necrotic cross combinations. The degree of necrosis in the F1's varies greatly, due to multiple allelism of Ne 1 and Ne 2. Nine necrosis grades are described in table 3. Grades 6–8 are called severe necrosis (no seed is produced), 3–6 moderate necrosis (premature seed), 0–3 weak necrosis (normal seed). Most severely necrotic F1's could be brought to seed production by raising them under extra favourable conditions in a small growth-chamber. Growing moderately necrotic F1's in pots outside, at a high fertility level of the soil, resulted in a striking increase of the seed production. On the basis of the above mentioned data and results some practical advice is given to the breeders about the handling of necrotic crosses. In addition some ways to preclude hybrid necrosis are indicated, though the wheat breeder is advised against a systematic avoiding of necrosis because many combinations of excellent varieties then remain untried.

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