Mechanism of seed dormancy and its relationship to bud dormancy in Persian walnut

Stratification, chilling, and heat requirements for both seeds and buds of five Persian walnut genotypes were compared. In experiments carried out between November 2006 and March 2008, 1-year-old twigs were collected after leaf-fall, placed in plastic bags, and kept at 4 ± 1 °C to provide chilling periods ranging from 400 to 1500 h. After chilling, twigs were transferred to a greenhouse for determination of chilling requirement and the number of growing degree hours (°C) needed for bud break. For germination experiments, mature seeds were stratified at 4 ± 1 °C for treatment periods ranging from 0 to 8 weeks and then grown in a greenhouse under natural conditions. Germination rate and time to radicle emergence were recorded weekly and seedling heights were recorded after 2 months growth. Results indicated genotypes could be classified into three dormancy groups based on their chilling and heat requirements: low (‘Ronde de Montignac’ and ‘Serr’), medium (‘Z53’), and high (‘Lara’ and ‘Z63’) dormancy. Stratification for 6–8 weeks was most appropriate to overcome walnut seed dormancy, to obtain the best germination percentage and germination rate, and to prevent physiological dwarfing. A relationship between the chilling and heat requirements for bud-break and the stratification requirement for germination of seeds was observed and data showed that the nutshell is a mechanical barrier to germination in walnut seeds with intermediate physiological dormancy.

► Chill requirements for walnut bud-break and seed germination are correlated.
► Chill requirement can be used to select genotypes for areas of spring frost.
► Walnut shell is a mechanical barrier to seed stratification and germination.