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Silvia Jiménez, Mónica Pérez, Blanca María Plaza, Roberto Salinas, and María Teresa Lao

correspond to an interval of 20 d. Estimation of phosphorus uptake and phosphorus uptake concentration. Net uptake was measured each 20 d as the difference between the initial and the final ion concentrations of the nutrient solution, as proposed by

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William B. Evans and Darryl D. Warncke

Six potato cultivars (Atlantic, Sebago, Onaway, Russet Burbank, Lemhi Russet,and Norland) were evaluated for phosphorus uptake efficiency in solution culture. Individual rooted cuttings of each cultivar were transferred from a standard 1/5 Hoagland's solution into solutions containing one of six P concentrations (0.05,0.1,0.22,0.5,1.1 and 2.3mg/l). After a 24h adjustment period P uptake was followed over a 6h period by collecting solution aliquots every two hours. All cultivars depleted the two lowest initial P concentrations to similar stable P concentration. The P uptake rate per unit length of root showed a sigmoidal relationship to the initial P solution concentration. The general nature of the P uptake relation to solution P concentration was similar among the cultivars, although the actual values varied. In general, P uptake rate increased from 5.0 × 10-4 at the lowest concentration to 7.0 × 10-2μg·cm-1·h-1 at the highest P solution concentration.

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Andrew G. Ristvey, John D. Lea-Cox, and David S. Ross

than 1.5% in the high P treatments of 25 mg/week. Phosphorus uptake efficiencies would have dropped to ≈20% for azaleas given the sufficient P rate of 5 mg/week compared with the calculated range of 40.1% to 49.2% ( Table 3 ). Interestingly, the uptake

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Matthew D. Taylor, Paul V. Nelson, and Jonathan M. Frantz

Sudden pH decline (SPD) describes the situation where crops growing at an appropriate pH rapidly (within 1–2 weeks) cause the substrate pH to shift downward one to two units. ‘Designer Dark Red’ geraniums (Pelargonium ×hortorum Bailey) were grown in three experiments to assess possible effects of light on SPD and phosphorous (P) uptake. The first experiment tested the effect of four light intensities (105, 210, 575, and 1020 ± 25 μmol·m−2·s−1) on substrate acidification. At 63 days, substrate pH declined from 6.0 to 4.8 as light intensity increased. Tissue P of plants grown at the highest two light levels was extremely low (0.10%–0.14% of dry weight). P stress has been reported to cause acidification. Because plants in the two lowest light treatments had adequate P, it was not possible to determine if the drop in substrate pH was a direct light effect or a combination of light and P. The second experiment used a factorial combination of the three highest light levels from Expt. 1 and five preplant P rates (0, 0.065, 0.13, 0.26, or 0.52 g·L−1 substrate) to assess this question. When tissue P concentrations were deficient, pH decreased by 0.6 to 1.0 pH units within 2 weeks and deficiency occurred more often with high light intensity. These data indicated that P deficiency caused substrate acidification and indicated the possibility that P uptake was suppressed by high light intensity. The third experiment was conducted in hydroponics to determine the direct effect of high light intensity on P uptake. In this experiment, cumulative P uptake per gram root and the rate of P uptake per gram root per day both decreased 20% when light intensity increased from 500 to 1100 μmol·m−2·s−1. It is clear from this study that P deficiency causes geraniums to acidify the substrate and that high light suppresses P uptake.

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John M. Smagula

Liquid phosphorus (23% phosphoric acid) was applied preemergence at 0, 22.4, 44.8, 67.2, or 89.6 kg·ha-1 to 9 fields: 3 commercial blueberry fields having plants with very low (<.111%), 3 low (.111-.125%), and 3 adequate (>.125%) leaf phosphorus concentrations. Years of application ('89,'89+'91,'89 + '91 + '93) were assigned in a split-block RCB design with 4 replications at each location. A linear increase in leaf phosphorus concentration with increasing rates of P application was found in both 1989 and 1991. Differences in response were found among locations. A second application in 1991 was effective in raising leaf P levels at most locations to higher levels than the application in 1989. Also, there were higher levels of leaf P in treatment plots that only received P fertilizer in 1989 compared to controls, indicating a carry over effect.

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Keitaro Tawaraya, Maman Turjaman, and Hanna Artuti Ekamawanti

The effect of arbuscular mycorrhizal (AM) colonization on nitrogen (N) and phosphorus (P) uptake and shoot growth of Aloe vera was investigated. Plants were inoculated with one of two AM fungi, Glomus clarum or Gigaspora decipiens. Control plants were not inoculated. Plants were grown under glasshouse conditions in a peat land soil without fertilizers for 12 months. Inoculated A. vera plants were colonized with AM fungi. Total length of leaves and number of leaves were higher in inoculated plants than uninoculated plants 12 months after inoculation. Shoot N and P concentrations were higher in inoculated plants than uninoculated plants. Shoot fresh weight was increased by AM colonization. This result suggests that AM colonization can increase the nutrient uptake and growth of A. vera.

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S. Kuo

Acidifying soil to prevent annual bluegrass (Poa annua L.) from infesting creeping bentgrass (Agrostis palustris Hud.) reduces soil P and Ca availability. This study examined Ca and P effects on the growth of these two grasses in four moderately acidic soils using CaSO4 as a Ca source. Each soil received four P rates (0, 10, 40, or 80 mg·kg-1) and three Ca (as CaSO4) rates (0, 400, or 800 mg·kg-1). Neither Ca nor P treatments substantially changed pH or exchangeable soil Al. Clipping yields, tissue P concentration, and P uptake of both grasses were affected by soil NaHCO3-P levels. Compared to bentgrass, annual bluegrass had higher clipping yields and P uptake at high P rates or high NaHCO3-P levels; this result indicates that annual bluegrass was as acid-tolerant as the bentgrass, provided that available P in the soil is adequate. Adding CaSO4 to the Papac soil, which contained the least amount of exchangeable Ca among the four soils, markedly enhanced the clipping tissue P concentration and P uptake of creeping bentgrass but not those of annual bluegrass; this result indicates that a differential response to Ca existed between the two grasses. Maintaining an adequate soil Ca availability was necessary to improve bentgrass growth, particularly for the acid soil containing low available Ca initially.

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Conny W. Hansen and Jonathan Lynch

Whole-plant biomass accumulation, P dynamics, and root-shoot interactions during transition from vegetative to reproductive growth of `Coral Charm' chrysanthemum (Dendranthema ×grandiflorum Ramat.) (Zander, 1993) were investigated over a range of P concentrations considered to be deficient (1 μm), adequate (100 μm), and high (5 mm). In nondeficient plants, transition from vegetative to reproductive growth resulted in reduced relative growth rate and root and shoot biomass accumulation. Reproductive plants showed a higher commitment of the whole plant to the production of developing flowers than to leaves and roots, whereas, in vegetative plants, the highest component production rate was in leaves. This indicates changes in the source-sink relationships during transition from vegetative growth making developing flowers stronger sinks for photoassimilates than roots. Phosphorus allocated to developing flowers was predominantly lost from leaves. Phosphorus-deficient plants showed characteristic P-deficiency symptoms and favored root growth over shoot growth regardless of growth stage. Phosphorus availability in nondeficient plants affected root growth more than shoot growth. No substantial differences in shoot biomass production, relative growth rate, and CO2 assimilation rates were observed in adequate-P and high-P plants. However, the root component production rate, root to shoot ratio, root length ratio, specific root length, specific root area, root mass to leaf area ratio, and root respiration increased in adequate-P plants compared with high-P plants, which indicates that high root activity was maintained without affecting shoot biomass in buffered P conditions. Our results suggest that the high P concentrations used in many horticultural systems may have no benefit in terms of shoot growth and may actually be detrimental to root growth.

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R.G. Linderman and E.A. Davis

Formation of arbuscular mycorrhizae (AM) has been inhibited in soilless potting mixes that usually contain some proportion of peat moss. The cause of the inhibition has been thought to be high fertilizer P content in the media that suppresses spread of the fungal symbiont in the root tissue. However, there has also been some suggestion that the peats themselves may contribute to the inhibition. That possibility was explored in this study. A sandy-loam soil, in which mycorrhizae consistently enhance plant growth under P-limiting conditions, was amended with six different peats. Onions (Allium cepa 'White Lisbon'), as an indicator host, were grown in the mixes under P-limiting conditions, and were inoculated or not with the AM fungi Glomus deserticola or Gigaspora rosea. Plant growth response to inoculation with AM fungi (AMF) varied with the type of peat and AMF isolate. Inoculated plants generally had the highest root biomass when grown in soil amended with peat. Root colonization by the two fungal symbionts was also affected differently by different peat amendments. Root colonization by Glomus deserticola and Gigaspora rosea was inhibited by at least half of the peat types. However, the types of peat inhibitory to Gigaspora rosea colonization were not the same as those inhibitory to Glomus deserticola colonization. These results indicate that different peat amendments can suppress or enhance mycorrhiza formation on onion roots and resultant growth benefit under P-limiting conditions, depending on the mycorrhizal fungus used.