Exploring low-chill genotypes of Persian walnut (Juglans regia L.) in west of Iran

Climate change may cause a significant decline in winter chill over the coming decades and is becoming an important issue for walnut (Juglans regia L.) breeding programs to address. The current study was conducted to identify superior walnut genotypes with low-chilling requirements in western Iran (Ilam province). In the first step, approximately 200 walnut trees of seed-origin were pre-selected based on a questionnaire. Based on this primary evaluation for phenotypic and horticultural traits, 55 of the 200 genotypes were selected for further study. Morphological and phenological traits of these 55 were evaluated for two consecutive years and 11 genotypes were selected as superior. These superior genotypes were moderate to late-leafing with high yield, lateral fruitfulness, thin shells, and light to extra light kernel color. Nut weight, kernel weight and kernel percentage of the 11 superior genotypes ranged between 12.6-15.9 g, 6.9-8.2 g, and 49.7-62.0%, respectively. These superior genotypes had high levels of protein (13.3-18.2%), oil (55.3-65.9%) and PUFAs content. Assessment of chilling requirement of these superior genotypes showed that eight (IrIlZg1, IrIlZg13, IrIlZg7, IrIlZg12, IrIlAa92, IrIlAa47, IrIlAa48 and IrIlAa27) had significantly lower chilling requirements than the cultivar 'Chandler'. Although having low-chill requirements, these genotypes were moderate to late-leafing. This may be due to their high heat requirement, since chilling requirement together with heat accumulation determines the leafing and flowering date. In conclusion, the selected low chilling requirement genotypes having moderate to late leafing could be used in walnut breeding programs to release commercial cultivars which are suitable for areas with spring frost concerns and expected future loss of winter chill due to climate change.