To evaluate the response of three tropical forage species to varying rates of nitrogen (N) fertilization [0, 39, 78, 118, 157 kg of N/(ha x cutting)] and five summer harvests, forage DM mass and nutritive value were evaluated in a randomized complete block design with a split-split plot arrangement of treatments. Plots (n = 60) were established in 1996, and five harvests were conducted every 28 d from June through September in 1997 and 1998, with fertilizer applications occuring after each harvest. Fertilization with 78 kg of N/(ha x cutting) increased forage mass in these grasses by 129% (P < 0.01) compared with no N fertilization. Additional N did not result in further increases of forage mass. Bermudagrass (Cynodon dactylon) produced more forage DM [P < 0.01; 1,536 +/- 43 kg/(ha x cutting)] than stargrass [Cynodon nlemfuensis; 1,403 +/- 43 kg/(ha x cutting)] or bahiagrass [Paspalum notatum; 1,297 +/- 43 kg/(ha x cutting)]. Peak forage mass for all species occurred in late June and July. In vitro organic matter digestibility (IVOMD) of stargrass increased (P < 0.01) linearly with fertilization. A quadratic response to N fertilization (P < 0.01) was noted in IVOMD of bermudagrass, whereas bahiagrass was not affected. Bermudagrass was more (P < 0.01) digestible (57.5 +/- 0.4) than stargrass (54.6 +/- 0.4) and bahiagrass (51.9 +/- 0.4%). As fertilization level increased, NDF decreased linearly (P < 0.01) in all three forages. Total N concentration increased (P < 0.01) linearly as N fertilization increased in all forages. Total N concentration was highest (P < 0.01) in stargrass (2.4%, DM basis) compared with bermudagrass (2.2%) and bahiagrass (2.0%). Total N concentration was depressed in all forages for late June and July harvests (P < 0.01). Fertilization increased (P < 0.05) the concentration (% of DM) of all protein fractions. In July and August, nonprotein N was reduced 11.8% (P < 0.01), whereas ADIN increased in July (P < 0.01). Bahiagrass had less N in cell contents than did bermudagrass and stargrass but had a greater concentration of N associated with the cell wall. Managerial factors, including rates of N fertilization and harvest dates, can have profound effects on the nutritional value of forage. An increased understanding of these effects is imperative to improve supplementation programs for ruminants.

Translocation of '4C-photosynthates to mycorrhizal (+ +), half mycorrhizal (0+), and nonmycorrhizal (00) split-root systems was compared to P accumulation in leaves of the host plant. Carrizo citrange seedlings (Poncirus trifoliata [L.] Raf. x Citrus sinensis [L.] Osbeck) were inoculated with the vesicular-arbuscular mycorrhizal fungus Glomus intraradices Schenck and Smith. Plants were exposed to 14 CO2 for 10 minutes and ambient air for 2 hours. Three to 4% of recently labeled photosynthate was allocated to metabolism of the mycorrhiza in each inoculated root half independent of shoot P concentration, growth response, and whether one or both root halves were colonized. Nonmycorrhizal roots respired more of the label translocated to them than did mycorrhizal roots. Label recovered in the potting medium due to exudation or transport into extraradical hyphae was 5 to 6 times greater for (+ +) versus (00) plants. In low nutrient media, roots of (0+ ) and ( + + ) plants transported more P to leaves per root weight than roots of (00) plants. However, when C translocated to roots utilized for respiration, exudation, etc., as well as growth is considered, (00) plant roots were at least as efficient at P uptake (benefit) per C utilized (cost) as (0+) and (+ +) plants. Root systems of ( + + ) plants did not supply more P to leaves than (0+ ) plants in higher nutrient media, yet they still allocated twice the 14C-photosynthate to the mycorrhiza as did (0+) root systems. This indicates there is an optimal level of mycorrhizal colonization above which the plant receives no enhanced P uptake yet continues to partition photosynthates to metabolism of the mycorrhiza.ranged from 4 to 17% of fixed C (5, 12,13,17,22 (12). One can describe differences in root physiology without compounding effects of different shoots by inoculating one half-root system with a mycorrhizal fungus. In such a system, mycorrhizal root halves of sour orange and Carrizo citrange seedlings accumulated 3 to 5% more of total '4C-photosynthate than the paired nonmycorrhizal half-root system (12).The following experiments were conducted to detail C partitioning in below ground fractions of split-root Carrizo citrange and to quantify C cost relative to net P acquisition in leaves. The first portion of this objective was accomplished through the development of a method of wet acid digestion and oxidation of soil organic matter. Below ground respiration and partitioning within roots also were measured. Thus, all components of below ground partitioning of carbon have been quantified in the present study.Vesicular-arbuscular mycorrhizal (VAM)4 fungi have been shown to increase growth and P uptake in a wide variety of forest and fruit trees (14,19,20) as well as agronomic crops (9,10,16). These fungi are assumed to be obligate symbionts because they have not been cultured axenically. As obligate symbionts they require organic compounds from their hosts, and indeed, the transfer of 14C-labeled compounds from host plant to VAM fungus has been noted (6,13

Photosynthate partitioning was examined in seedings of sour orange (Citrus aurantium L.) and Carrizo citrange (Poncirus trifoliata jL.j Raf.x C. sinensis [L.] Osbeck) grown with split root systems inoculated on one side with vesicular-arbuscular mycorrhizal fungus (Glomus intraradices Schenck and Smith). Source-sink relations were studied without mitipting differences in mineral content or physiological age that can occur in separate plant comparisons, because phosphorus was evenly distributed between leaves on opposite sides of the seedlings. Aboveground portions of each plant were exposed to '4CO2 for 8.5 minutes and ambient air for 2 hours, followed by extraction and identification of labeled assimilates. Mycorrhizal halves of root systems accumulated 66 and 68% of the '4C-labeled photosynthates translocated to roots of sour orange and 'Carrizo' citrange, respectively, as well as an average of 77% greater disintegrations per minute per gram fresh weight. Distribution of '4C-labeled assimilates was independent of phosphorus effects on photosynthate partitioning in leaves and did not reflect fresh or dry weights of roots or degree of mycorrhizal dependency of the species. Differences in radioactivity between mycorrhizal and nonmycorrhizal root halves after 2 hours indicated at least 3 to 5% of the whole plant '4C-labeled pbotosynthates were allocated to mycorrhizae-related events on one side and that twice this amount, or 6 to 10%, might be expected if the entire root system was infected.

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