, 300 e 600 mg dm -3 de P. Aos 180 dias após a germinação, as plantas foram colhidas e submetidas às análises químicas. Sob todas as doses de P, a paineira produziu maior biomassa da parte aérea e das raízes em comparação à aroeirinha e jambolão, além de apresentar maiores teores de P orgânico (Po) e maior participação relativa do P orgânico (Po) em relação ao P total solúvel em ácido em todos os níveis de fertilização fosfatada. O melhor crescimento da paineira foi atribuído a uma maior eficiência nos ajustes metabólicos desta espécie em relação à nutrição fosfatada, o que indica que esta espécie pode ser plantada em solos com diferentes níveis de P.Termos para indexação: Schinus terenbinthifolius, Chorisia speciosa, Syzygium jambolanum, plântulas, adubação fosfatada, relação planta-solo. INITIAL GROWING, PHOSPHORUS CRITICAL LEVELS AND PHOSPHATE FRACTIONS IN FOREST SPECIESABSTRACT -An experiment under greenhouse conditions was carried out at Universidade Federal de Lavras, Minas Gerais State, Brazil, to evaluate the growth of seedlings, phosphorus critical levels and phosphate fractions, in forest species growing under four phosphorus levels. Seedlings of Schinus terenbinthifolius Raddi (aroeirinha), Chorisia speciosa St. Hill. (paineira) and Syzygium jambolanum Lam. (jambolão) were cultivated on an Oxisol fertilized with 0, 150, 300 and 600 mg dm -3 of P. The seedlings were harvested 180 days after germination and analysed for P forms. In all levels of phosphorus addition, the paineira produced higher aerial and root biomass than aroeirinha and jambolão, and presented higher contents of organic phosphorus and higher relative participation of organic phosphorus in the total acid soluble phosphorus under all phosphorus levels. The improved growth rate of paineira was attributed to a greater efficiency in the metabolic adjustments of this species in relation to phosphorus nutrition, indicating that this species can grow in soils of variable P levels.

The interaction between ammonium and potassium during influx was examined in roots of dark-grown decapitated corn seedlings (Zea mays L., cv. Pioneer 3369A). Influx was measured during a 10-min exposure to either ((15)NH4)2SO4 ranging from 10 to 200 μM NH 4 (+) with and without 200 μM K((86)Rb)Cl or to K((86)Rb)Cl ranging from 10 to 200 μM K(+) with and without 200 μM NH 4 (+) as ((15)NH4)2SO4. The simple Michaelis-Menten model described the data well only for potassium influx in the presence of ambient ammonium. For the other three instances, the data were improved by assuming that a second influx mechanism became operative as the low-concentration phase approached saturation. Two distinct mechanisms are thus indicated for both ammonium and potassium influx within the range of 10 to 200 μM.The influx mechanism operating at low concentrations showed greater affinity for potassium than for ammonium, even though the capacity for ammonium transport was twice as large as that for potassium. It is suggested that this phase involved a common transport system for the two ions and that localized low acidity next to the internal surface, following H(+) extrusion, favored ammonium deprotonation and dissociation from the transport system-ammonium complex. Parallel decreases in V max and increases in Km of the low-concentration saturable phase occurred for ammonium influx when ambient potassium was present and for potassium influx when ambient ammonium was present. The data support a mixed-type inhibition in each case. Simultaneous measurement of potassium and ammonium influx showed that they were highly negatively correlated at the lower concentrations, indicating that the extent to which influx of the inhibited ion was restricted was associated with influx of the inhibitor ion. Presence of ambient ammonium eliminated the second phase of potassium influx. In contrast, the presence of ambient potassium decreased the concentration at which the second phase of ammonium influx was initiated but did not restrict the rate.

Com o objetivo de se estudar as exigências nutricionais no desenvolvimento inicial de mudas de Peltophorum dubium (Spreng.) Taub., foi realizado um experimento em casa de vegetação. Usou-se como substrato um latossolo vermelho-amarelo com baixa disponibilidade de nutrientes, utilizando-se 10 tratamentos, sob a técnica do elemento faltante. Foram aplicados um tratamento completo (com N, P, K, Ca, Mg, S, B e Zn), outros com omissão de um nutriente por vez (-N, -P, -K, -Ca, -Mg, -S, -B e -Zn) e uma testemunha (solo natural). Foram avaliadas as seguintes características: altura, diâmetro, peso de matéria seca da parte aérea e das raízes, e teor de nutrientes na matéria seca da parte aérea. Concluiu-se que as plantas do angico-amarelo apresentam elevada exigência nutricional; os nutrientes P, N, S e o Ca seguidos pelo Mg, K e pelo B são limitantes ao crescimento das plantas; as omissões de K, Ca e de Mg afetam a absorção de S pelas plantas.

RESUMO O experimento, realizado em casa de vegetação no Departamento de Ciência do Solo da

Potassium (86Rb) influx from 200 mmol m −3 KCl into dark grown, decapitated maize seedlings 6 d old) was stimulated by nitrate pretreatment. The stimulus was clearly evident by 6h exposure to nitrate and required 12–24 h for maximal expression. Decay of the nitrate‐stimulated potassium influx was more than 50% complete within 3 h after transfer to nitrogen‐free solutions. The stimulation of potassium influx was entirely accounted for by an increase in the influx component that was resistant to inhibition by presence of 200 mmol m−3 ambient ammonium. In contrast, the component of potassium influx that was sensitive to inhibition by ambient ammonium was unaffected by nitrate pretreatment. Exposure to the glutamine synthetase inhibitor L‐methionine‐dl‐sulphoximine (MSX) during nitrate pretreatment stimulated the resistant component but the sensitive component was nearly eliminated. Pretreatment with ammonium increased the resistant component of potassium influx within 3 h, i.e. before it was increased by nitrate pretreatment, but the sensitive component was concomitantly restricted. The latter recovered partially during extended pretreatment with ammonium. The data indicate that the resistant component responded positively to increases in tissue ammonium concentrations whereas the sensitive component was unaffected by tissue ammonium except at concentrations in excess of 10μmol g−1. Ammonium influx was also stimulated by nitrate pretreatment and to a greater extent than potassium influx. Presence of MSX with nitrate during pretreatment resulted in a further stimulation in ammonium influx. The parallel increases in root ammonium concentrations with the two pretreatments imply that part of the increase in ammonium influx was a consequnce of increased counter‐transport with endogenous ammonium.