02767naa a2200277 a 450000100080000000500110000800800410001910000180006024500880007826000090016652020340017565000110220965000200222065000120224065000140225265000130226665000100227965000230228965000130231265300170232565300170234270000150235970000180237470000180239277300790241015451431999-04-08       1997    bl ---       0--  u    #d1 aCALDAS, L. S.  aLeaf heliotropism in Pterodon pubescens, a woody legume from the Brazilian cerrado.  c1997  aParaheliotropic leaflet movements were observed in Pterodon pubescens, a woody legume native to the cerrado vegetation of Central Brazil. As a result of the daily changes in orientation of the leaflets, the plants reduced the leaf area exposed to vertical radiation at midday by as much as 40%. Some leaflets were restrained in a horinzontal position from 9:00 to 14:00 h. Measurements of chlorophyll "a" fluorescence were taken in the early morning, when all the leatleats were horizontal; at midday, in vertical and restrained, horizontal leaflets; and in late afternoon, when the leaflets had returned naturally to the horizontal position. The potential quantum yield (Fv/Fm) after 30 minutes of dark adaptation was lower in the horizontal leaflets (0.66+_0.08) than in the vertical leaflets (0.74+_0.03) at midday, indicating a slight photoinhibition in the horizontal position. The effective quantum yield (F/F'm) under conditions of natural radiation at midday was reduced to 0.13+_0.06 in the horizontal leaflets, with a photosynthetic photon flux density (PPFD) of 1211 mol m-2 s-1, while the vertical leaflets maintained values of 0.05+_0.17 with PPFD of 272 umol m2 s1. The electron transfer rate calculated for the leaflets in the two positions at midday was almost as high in vertical leaflets (average of 120 and maximum of 198 umol m-2 s-1) as in horizontal leaflets (average of 152 and maximum of 190 umol m-2 s-1) that were receiving four times as much radiation. In the morning the maximum electron transfer rate (250 to 300 umol m-2 s-1) was reached at an irradiance of 500 umol m-2 s-1 whereas, at midday, there was no saturation in the horizontal leaflet up to irradiances of 2000 umol m-2 s-1 and satyration was observed at 1500 umol m-2 s-1 in the vertical leaflet with an eletron transfer rate of 400 umol m-2 s-1. The paraheliotropic movement of the leaflets of Pterodon pubescens in natural conditions maintains an elevated rate of photochemical reactions although the irradiances is drastically reduced.  aleaves  aphotoinhibition  aCerrado  aClorofila  aElétron  aFolha  aPterodon Pubescens  aSucupira  aChlorophylls  aFotoinibicao1 aLUTTGE, U.1 aFRANCO, A. C.1 aHARIDASAN, M.  tRevista Brasileira de Fisiologia Vegetal, Brasiliagv.9, n.1, p.1-7, 1997.