Silicon (Si) has been recognized as a beneficial element to improve rice (Oryza sativa L.) grain yield. Despite some evidence suggesting that this positive effect is observed when Si is supplied along the reproductive growth stage (from panicle initiation to heading), it remains unclear whether its supplementation during distinct growth phases can differentially impact physiological aspects of rice and its yield and the underlying mechanisms. Here, we investigated the effects of additions/removals of Si at different growth stages and their impacts on rice yield components, photosynthetic performance, and expression of genes (Lsi1, Lsi2 and Lsi6) involved in Si distribution within rice shoots. Positive effects of Si on rice production and photosynthesis were manifested when it was specifically supplied during the reproductive growth stage, as demonstrated by: (1) a high crop yield associated with higher grain number and higher 1000-grain weight, whereas the leaf area and whole-plant biomass remained unchanged; (2) an increased sink strength which, in turn, exerted a feed-forward effect on photosynthesis that was coupled with increases in both stomatal conductance and biochemical capacity to fix CO; (3) higher Si amounts in the developing panicles (and grain husks) in good agreement with a remarkable up-regulation of Lsi6 (and to a lesser extent Lsi1). We suggest that proper levels of Si in these reproductive structures seem to play an as yet unidentified role culminating with higher grain number and size.

Silicon (Si) plays important roles in alleviating various abiotic stresses. In rice (Oryza sativa), arsenic (As) is believed to share the Si transport pathway for entry into roots, and Si has been demonstrated to decrease As concentrations. However, the physiological mechanisms through which Si might alleviate As toxicity in plants remain poorly elucidated. We combined detailed gas exchange measurements with chlorophyll fluorescence analysis to examine the effects of Si nutrition on photosynthetic performance in rice plants [a wild-type (WT) cultivar and its lsi1 mutant defective in Si uptake] challenged with As (arsenite). As treatment impaired carbon fixation (particularly in the WT genotype) that was unrelated to photochemical or biochemical limitations but, rather, was largely associated with decreased leaf conductance at the stomata and mesophyll levels. Indeed, regardless of the genotypes, in the plants challenged with As, photosynthetic rates correlated strongly with both stomatal (r(2)  = 0.90) and mesophyll (r(2)  = 0.95) conductances, and these conductances were, in turn, linearly correlated with each other. The As-related impairments to carbon fixation could be considerably reverted by Si in a time- and genotype-dependent manner. In conclusion, we identified Si nutrition as an important target in an attempt to not only decrease As concentrations but also to ameliorate the photosynthetic performance of rice plants challenged with As.

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We analyzed the growth and photosynthetic behavior of Gallesia integrifolia ('pau-d'alho') and Schinus terebinthifolius ('aroeirinha') under shade, seeking to obtain ecophysiological information for introducing seedlings of those species in previously established cacao agroforestry systems. Considering that light intensity under the shade of cacao trees varied between 5 and 10% daylight, 5 months old seedlings were exposed to four irradiance levels (25, 17, 10 and 5% daylight) for 92 days. With shade increase both species displayed trends of decrease leaf mass per unit leaf area, leaf area per plant (LA), relative growth rate (RGR) and net assimilation rate (NAR), and increase leaf area ratio (LAR). The mean values of light-saturated net photosynthetic rate (P nmax ) in 25 and 5% daylight were 12.8 and 8.0 lmol CO 2 m -2 s -1 for G. integrifolia and 17.9 and 7.4 lmol CO 2 m -2 s -1 , respectively, for S. terebinthifolius. Based on the measurements of photosynthetic photon flux density and estimated values of photosynthetic saturated irradiance (Is) we concluded that, in all shaded conditions, the leaves of both species were under sub optimal light conditions to reach P nmax . In spite of the lowest P nmax values, RGR and NAR were significantly higher for G. integrifolia in all irradiance levels. Differences in growth rates can be explained by the higher values of LA, LAR and leaf mass ratio (LMR), as well as by the lower values of Is, photosynthetic compensation irradiance and dark respiration rates observed for G. integrifolia. Even though seedlings of G. integrifolia presented higher capacity to adapt under conditions of dense shade, we concluded that both species were under stress conditions induced by shade in light environments below 25% daylight. On a practical point of view it is possible to conclude that seedlings of both species should be introduced in light gaps, formed after the fall of big trees, or in places in which cacao trees are cultivated using large plant spacing.

-The objective of this work was to characterize the morphoanatomy of roots and the yield traits of two corn hybrids contrasting for drought tolerance (DKB 390, tolerant; and BRS 1030, sensitive), at different stages of development. Water deficit was imposed for ten days, in a greenhouse, at three growth stages: V5, VT, and R3. These treatments were combined to generate cumulative stress during the plant cycle, as: V5VT, V5R3, VTR3, and V5VTR3. The following were analyzed: root anatomy; proportion of aerenchyma in the cortex; metaxylem number and diameter; phloem thickness; as well as morphological characteristics, such as root length, volume, and surface area, specific root length, length of fine roots, grain yield, and ear length and diameter. Development stage affected the responses to stress: DKB 390 showed the best performance for root morphoanatomy and yield traits, under drought stress, at the reproductive stages, mainly R3, and in the treatments with cumulative stress, especially V5VTR3; whereas BRS 1030 presented higher means for the studied parameters, mainly at the V5 and VT stages, but did not show a higher grain yield under water stress. The greater tolerance of the DKB 390 hybrid to water deficit is probably linked with a memory of pre-exposure to water stress at different growth stages.Index terms: Zea mays, aerenchyma, drought, endoderm, specific root length, WinRhizo. Morfoanatomia radicular em diferentes estádios de desenvolvimento e produtividade do milho sob estresse hídricoResumo -O objetivo deste trabalho foi caracterizar a morfoanatomia radicular e os atributos de rendimento de dois híbridos de milho contrastantes quanto à seca (DKB 390, tolerante; e BRS 1030, sensível), em diferentes estádios de desenvolvimento. A deficiência hídrica foi imposta por dez dias, em casa de vegetação, em três estádios de desenvolvimento: V5, VT e R3. Esses tratamentos foram combinados para gerar estresses acumulativos durante o ciclo da planta, como: V5VT, V5R3, VTR3 e V5VTR3. Foram analisados: anatomia radicular; proporção de aerênquima no cortex; número e diâmetro de metaxilema; espessura do floema; bem como características morfológicas, como comprimento, volume e área de superfície radicular, comprimento específico das raízes, comprimento de raízes finas, rendimento de grãos, e comprimento e diâmetro da espiga. O estádio de desenvolvimento influenciou as respostas ao estresse: DKB 390 apresentou o melhor desempenho quanto à morfoanatomia radicular e aos atributos de rendimento, quando submetido a estresse hídrico, nos estádios reprodutivos, sobretudo em R3, e nos tratamentos acumulativos de estresse, principalmente no V5VTR3; enquanto BRS 1030 apresentou as maiores médias para os parâmetros estudados, principalmente nos estádios V5 e VT, mas não apresentou maior rendimento de grãos sob estresse hídrico. A maior tolerância do híbrido DKB 390 ao estresse hídrico provavelmente relaciona-se à memória de exposição prévia ao estresse hídrico em diferentes estádios de desenvolvimento.Termos para indexação...