Coding and description of growth stages of precocious walnut in Xinjiang based on the extended BBCH scale

Xinjiang is one of the important origins of walnut (Juglans regia L.) and harbors abundant germplasm resources. However, a standardized phenological description system for precocious walnuts in this region is lacking. This study aimed to establish such a system based on the extended BBCH scale.

We conducted systematic phenological observations over two growing seasons (2023 -2024) on six nine-year-old precocious walnut materials (two main cultivars and four elite strains) planted in Alar, Xinjiang. The study covered eight principal growth stages: bud development (0), leaf development (1), shoot development (3), inflorescence emergence (5), flowering (6), fruit development (7), fruit maturity (8), and senescence and dormancy (9), further subdivided into 38 secondary stages, with coded morphological photographs provided for each stage.

Significant differences were observed among varieties and elite strains. BT-2 had the earliest budburst, the highest number of female flowers, and reached standard fruit size first; XW-185 had abundant male flowers and could serve as a suitable pollen donor; BT-1 exhibited the highest kernel percentage and the thinnest nutshell. Correlation analysis revealed that the trait correlation patterns of BT-2 and BT-3 were relatively complex, showing opposite trends. PCA and clustering heatmap indicated clear differentiation among the six materials, classifying them into four clusters.

The BBCH_based system provides a practical tool for pollinizer arrangement, irrigation scheduling, and breeding of precocious walnuts. Complementary traits among varieties support targeted utilization, and phenological stages can guide stage_specific irrigation. However, the two_year observation period limits the assessment of long_term stability, and future research should further elucidate the interaction network between early fruiting traits and quality traits.