Correlations of Crop Load and Return Bloom with Root and Shoot Concentrations of Potassium, Nitrogen, and Nonstructural Carbohydrates in Pecan

Abstract

The current theory of pecan [Carya illinoinensis (Wangenh.) C. Koch] alternate bearing is the “growth regulator–carbohydrate theory” in which flowering is first controlled by growth regulators produced by fruit and leaves, and then by the size of the carbohydrate pool near budbreak. Lack of nitrogen (N) reserves has also been proposed to be limiting after large crops, thus reducing return bloom. Annual production was determined for 12 individual trees for 3 years. Return bloom was monitored on four previous-season shoot types: 1) vegetative shoots, 2) bearing terminal shoots without a second growth flush, 3) bearing lateral shoots without a second growth flush, and 4) bearing shoots that were primarily in the terminal position with a second growth flush. Nonstructural carbohydrates, organically bound N, and potassium (K) concentrations were determined in roots and shoots. Regression analysis was used to determine the effect of yield on subsequent nonstructural carbohydrates, N, and K in the roots and shoots, and their postyield concentrations on subsequent flowering. Alternate bearing was evident because there were reductions of 18%, 16%, and 18% in the percentage of current season shoots flowering for every 10 kg/tree production increase in the previous season's yield in 2002, 2003, and 2004 respectively. Flower production in 2002 decreased by 2.6 flowers/1-year-old branch and 1.6 flowers/1-year-old branch in 2003 for each 10 kg/tree increase in production. The third year of the study, neither previous season shoot type nor yield affected subsequent flower production. The previous year's shoot type did not affect the percentage of current season shoots flowering; however, the previous year's shoots that had a second growth flush produced more flowers the following year than the other shoot types. Results suggested that crop load was not related to nonstructural carbohydrates, N, or K in the roots and shoots during January in these well-managed trees. Stored nonstructural carbohydrates, N, and K were also not related to return bloom. These data suggest that the current “growth regulator–carbohydrate theory” may not be valid in these well-managed trees. Nonstructural carbohydrates, K, and organically bound N do not appear to be critical factors regulating flowering.