Pecan Tree Growth and Precocity

Precocity of pecan (Carya illinoinensis (Wangenh.) C. Koch) seedlings (year of first fruit production) was studied in relation to original seed measurements (nut weight, buoyancy, volume, and density) and in relation to growth index (GI) measurements of seedling trees for 4 years. A total of 2,071 pecan seedlings, representing nine controlled-cross families, were studied. Original seed measurements were not related to precocity of resultant seedling trees; but seed weight, buoyancy, and volume were significantly correlated with seedling growth rates. Nut density was negatively related to growth of seedlings. These relationships show the importance of original seed measurements and seed parentage in determining seedling growth, and have direct relevance in pecan nursery operations to increase general rootstock seedling vigor. Seedling growth rate was significantly correlated to precocity levels, with measurements taken in the later years of the study showing the highest correlations with precocity. This strong growth-precocity relationship may have negative genetic implications since a common breeding objective is to produce more precocious cultivars that maintain smaller tree size in mature orchards. Limited success has been obtained in developing an artificial method to induce pistillate flowers on juvenile clones in the second year of growth (Thompson, 1986). A 2-year generation time using this technique is a possibility. Presently this system is not efficient on large numbers of clones and must await further refinement before it can be routinely used in the BBP. Also of concern is what level of precocity, vigor, and tree size are most desirable horticulturally. Romberg and Smith (1950) reported that hybrid vigor is desirable in pecan. Alben et al. (1953) showed that pecan cultivars differ for initial orchard growth. Persian walnut (Juglans regia L.), is a highly productive crop in California that offers a prototype for pecan genetics advancement. Current thinking in walnut is that maximum tree vigor (both rootstock and scion) is an absolute requirement for maximum production. Initial tree growth in walnut is rapid (faster than pecan), then scion growth is minimal during the mature tree phase, being limited by fruit production. This growth scenario seems ideal, since it contributes to early tree development, then stable production with reduced vegetative growth. These desirable genetic characteristics seem obtainable in pecan. It has been generally known for many years that precocity is positively related to growth in seedling trees (Zimmerman 1972), but little information has been published on this relationship in pecan breeding programs. Breeding efficiency over years is strongly affected by generation time, especially in pecan. These experiments utilized records from the pecan breeding program to investigate this relationship and to refine and improve the efficiency of the USDA- ARS pecan breeding program.