Nutrient partitioning and seedling development in the genus Leucaena

Abstract

Slow establishment of the genus Leucaena from seed has been attributed to low seedling vigor and late nodulation. Observation of early seedling growth indicated that partitioning of a large proportion of resources to the root of young Leucaena seedlings could account, in part, for the slow initial shoot growth observed in this genus. Therefore, a series of experiments were conducted to examine the partitioning of stored seed reserves, photosynthate, and nitrogen in developing Leucaena seedlings. The effects of nodulation and nitrogen fertilization on partitioning of nutrients in the seedling were also examined. Seed reserves were initially used for radicle growth in dark grown seedlings; however, partitioning soon shifted to the hypocotyl. By four days after imbibition, hypocotyl weight exceeded radicle weight in both species tested (L. leucocephala and L. retusa), at all temperatures above 20°C. Two experiments were conducted examining the carbon partitioning of L. leucocephala cultivar K-8 using ¹⁴CO₂ pulse labeling techniques. Initially, label was allocated largely to the water soluble fraction of the labeled leaf. A smaller amount of label was allocated to the starch and residual fractions of the labeled leaf (18 and 16% of the assimilated label, respectively). During the first light period following labeling, export of labeled photosynthate from the water soluble fraction was rapid. No label was lost from the starch fraction during the light period, but during the first dark period following labeling, expert was rapid. By 22 hours after labeling, most of the label in the labeled leaf was either exported or respired from the water soluble and starch fractions. Throughout the entire study a larger proportion of labeled photosynthate was partitioned to the shoot than to the root. The initiation of rapid nodule growth and nitrogen fixation appeared to greatly influence the partitioning of labeled photosynthate to the upper root, although only low levels of label were found in the nodules themselves. In an experiment examining the effect of nitrogen fertilization and inoculation treatments on nitrogen fixation and seedling growth, nodulation and dinitrogen fixation of L. retusa was reduced by all levels of nitrogen fertilization. However, low levels of nitrogen fertilization (2mM N in nutrient solution) actually increased nodulation and nitrogen fixation in L. leucocephala. Increasing nitrogen fertilization decreased the root/shoot ratio of both species in the study.