STOCK QUALITY OF BLACK WALNUT (JUGLANS NIGRA) SEEDLINGS AS AFFECTED BY HALF-SIB SEED SOURCE AND NURSERY SOWING DENSITY

Abstract

Morphological quality of black walnut (Juglans nigra L.) seedlings largely dictates their capacity to establish well in plantations and their performance potential for use as rootstocks in grafting. Larger seedlings and rootstocks, attained from propagation at relatively low nursery sowing densities, have comparatively more expansive root systems for exploitation of soil resources and greater nutrient/carbohydrate reserves to support aboveground growth. The relative role of genetic selection in dictating morphological quality of nursery-grown black walnut seedlings, however, has not been thoroughly documented. We collected seed from three half-sib families (A, B, C) of black walnut in Indiana, USA which were sown as sprouted seeds into bareroot nursery beds at three sowing densities (L, 11.2 m -2 ; M, 24.2 m -2 ; H, 29.4 m -2 ). Sowing density significantly affected mean shoot height (H > M > L) and root volume (L > M > H), but not stem diameter. Shoot height was increased by 14.4 cm (43%) in H vs. L and root volume was increased by 32 cm 3 (39%) in L vs. H. Seedling response to family selection was significant for height, root volume, and stem diameter (C > B > A). The magnitude of the difference in response was greater for family selection compared to nursery sowing density, as height and root volume were increased by 15.2 cm (46%) and 44 cm 3 (60%) in C vs. A, respectively. Further, family C had greater mean stem diameter and root volume in H than did family A in L; mean height for these two families were about equal between H and L. Our results suggest that identification of superior genetic sources may be relatively more important than sowing density in nursery propagation of high quality black walnut seedlings.