Beam Interactions with Materials B Atoms ELSEVIER Nuclear Instruments and Methods in Physics Research B 130 (1997) 381-387 Nuclear microprobe studies of elemental distributions in dormant seeds of Burkea africana E.T.F. Witkowski a* *, I.M. Weiersbye-Witkowski a, W.J. Przybylowicz b,‘, J. Mesjasz-Przybylowicz b a Botany Department, Uniuersity of the Witwatersrand, P.O. Wits, 2050, Johannesburg, Gauteng. South Africa h Van de Graaff Group, National Accelerator Centre. P.O. Box 72, Faure 7131, South Africa Abstract Seed nutrient stores are vital post-germination for the establishment of seedlings in harsh and unpredictable environ- ments. Plants of nutrient-poor environments allocate a substantial proportion of total acquired nutrients to reproduction (i.e. seeds). We propose that differential allocation of mineral resources to specific seed tissues is an indication of a species germination and establishment strategy. Burkea africana Hook is a leguminous tree typical of broad-leaved nutrient-poor savannas in southern Africa. Elemental distributions in dormant B. u$+cunu seed structures were obtained using the true elemental imaging system (Dynamic Analysis) of the NAC Van de Graaff nuclear microprobe. Raster scans of 3.0 MeV protons were complemented by simultaneous BS and PIXE point analyses. Mineral nutrient concentrations varied greatly between seed tissues. Elevated levels of metals known to play an important role as plant enzyme co-factors were found in the seed lens and embryonic axis. Distributions of most of these metals (Ca, Mn, Fe and Zn, but not K or Cu) were positively correlated with embryonic P distribution, and probably represent phytin deposits. The distribution of metals within seed structures is ‘patchy’ due to their complexation with P as electron-dense globoid phytin crystals, which constrains the interpretation of PIXE point analyses. 0 1997 Elsevier Science B.V. 1. Introduction Detailed quantitative studies integrating demogra- phy and ecophysiology of indigenous shrubs/trees are rare [l], particularly where the physiology of the seed itself is concerned. Burkea africana Hook (Leguminosae) is the dominant tree of the ‘ Eragros- tis pallens - Burkea africana’ savanna at the South African Ecosystem Programme study area. B. africana frequents deep (l-2 m) acidic sandy soils impoverished in most nutrients essential for plant growth [2], with a particularly low P status. B. africana is dioecious (separate male and female plants) and produces an annual cohort of seeds. In nutrient-poor ecosystems, seeds generally represent the largest investment a plant makes of scarce nutri- ent reserves [3]. Concentrations of nutrients in seeds are often over two orders of magnitude greater than those found in the soil and much higher than in other * Corresponding author. Fax: Johannesburg 403 1429; email: ed@gecko.biol.wits.ac.za ’ On leave from the Faculty of Physics and Nuclear Tech- niques, University of Mining and Metallurgy, Cracow, Poland. plant tissues. These differences are even greater when one uses levels of plant-available, rather than total nutrient concentrations in the soil [4]. Seeds of many tree/shrub species growing in nutrient-poor environ- 0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO168-583X(97)00231-0 VIII. BIOLOGY