72 Arboricultural Abstracts NITROGEN AVAILABILITY, LOCAL LIGHT REGIME AND LEAF RANK EFFECTS ON THE AMOUNT AND SOURCES OF N ALLOCATED WITHIN THE FOLIAGE OF YOUNG WALNUT (JUGLANS NIGRA × REGIA) TREES Ela Frak, Xavier Le Roux, Peter Millard, Sabine Guillaumie, and Renate Wendler Early season leaf growth depends largely on nitrogen (N) provided by remobilization from storage, and many studies have tested the effect of N availability to roots on the amount of N provided for new leaf development by remobilization. Although it is well known that the light regime experienced by a leaf influences the amount of N per unit leaf area (LA), the effect of the local light regime on the amount of N derived either directly from root uptake or from remobilization for early season leaf growth has never been tested at an intra- canopy scale. The objective of this study was to quantify the relative importance of (1) N availability to roots, (2) local light regime experienced by the foliage (at the shoot scale) and (3) leaf rank along the shoot, on the total amount of N allocated to leaves and on the proportions of N provided by remobilization and root uptake. To quantify the importance of N uptake and remobilization as sources of leaf N, potted hybrid walnut trees (Juglans nigra L. × regia L.) were grown outdoors in sand and fed with a labeled (15N) nutrient solu- tion. By removing the apical bud, the trees were manipulated to produce only two shoots. The experimental design had two factors: (1) high (HN; 8 mol N m–3) and low (LN; 2 mol N m–3) N availability; and (2) high (HL; 90% of incident pho- tosynthetically active photon flux (PPF)) and low (LL; 10% of incident PPF) light. Total leaf N per tree was unaffected by either N availability or irradiance. The HN treatment in- creased the amount of leaf N derived from root uptake at the whole-tree scale (typically around 8 and 2% in the HN and LN treatments, respectively). Nitrogen allocation within fo- liage of individual trees was controlled by the local light regime, which strongly affected individual leaf characteristics as leaf mass per unit LA and area- based amount of leaf (Na). Decreasing the light availability to a branch decreased the amount of N allocated to it, benefiting the less shaded branches. In contrast, shading of the lower branch did not affect the fraction of total leaf N remobilized for either the lower, shaded branch or the upper, unshaded branch. The relevance of these findings for tree growth modeling is dis- cussed. (Tree Physiology 2006. 26(1):43–49) PRINCIPLES OF PLANT HEALTH MANAGEMENT FOR ORNAMENTAL PLANTS Margery L. Daughtrey and D. Michael Benson Economic, environmental, and technological influences com- plicate the task of achieving disease-free products in the or- namentals industry. Integrated pest management (IPM) is a cornerstone of floriculture and nursery crop production: strat- egies include sanitation, clean stock, host resistance, and con- trol through biological, cultural, environmental, chemical, and regulatory means. Sanitation measures and cultural con- trols must keep pace with new production technologies. Clean stock programs are used for many crops that are prop- agated vegetatively. Breeding, selection, and biotechnology provide crops resistant to pathogens. Offshore production for economic competitiveness can introduce pathogens that make regulatory programs necessary. New biocontrol and chemical products continue to improve control while meeting the re- quirement for minimal environmental impact. Continual in- troduction of new crops and new production technologies creates new opportunities for pathogens to exploit, such that new disease management tactics must be discovered and old ones rediscovered to achieve optimum health management for ornamentals. (Annual Review of Phytopathology 2005. 43:141–169) PHYTOPHTHORA RAMORUM: INTEGRATIVE RESEARCH AND MANAGEMENT OF AN EMERGING PATHOGEN IN CALIFORNIA AND OREGON FORESTS David M. Rizzo, Matteo Garbelotto, and Everett M. Hansen Phytophthora ramorum, causal agent of sudden oak death, is an emerging plant pathogen first observed in North America associated with mortality of tanoak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia) in coastal forests of California during the mid-1990s. The pathogen is now known to occur in North America and Eu- rope and have a host range of over 40 plant genera. Sudden oak death has become an example of unintended linkages between the horticultural industry and potential impacts on forest ecosystems. This paper examines the biology and ecol- ogy of P. ramorum in California and Oregon forests as well discussing research on the pathogen in a broader management context. (Annual Review of Phytopathology 2005. 43:309– 335) ©2007 International Society of Arboriculture
January 2007
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