Soil solarization for weed control in carrot.

Soil solarization is a technique used for weed and plant disease control in regions with high levels of solar radiation. The effect of solarization (0, 3, 6, and 9 weeks) upon weed populations,carrot (Daucus carota L. cv. Brasília) yield and nematode infestation in carrot roots was studied in São Luís (2o35' S; 44o10' W), MA, Brazil, using transparent polyethylene films (100 and 150 mm of thickness). The maximum temperature at 5 cm of depth was about 10oC warmer in solarized soil than in control plots. In the study 20 weed types were recorded. Solarization reduced weed biomass and density in about 50% of weed species, including Cyperus spp., Chamaecrista nictans var. paraguariensis(Chod & Hassl.) Irwin & Barneby,Marsypianthes chamaedrys (Vahl) O. Kuntze, Mitracarpus sp.,Mollugo verticillata L., Sebastiania corniculata M. Arg., and Spigelia anthelmia L. Approximately 40% of species in the weed flora were not affected by soil mulching. Furthermore, seed germination of Commelina benghalensis L. was increased by soil solarization. Marketable yield of carrots was greater in solarized soil than in the unsolarized one. It was concluded that solarization for nine weeks increases carrot yield and is effective for controlling more than half of the weed species recorded. Mulching was not effective for controlling root-knot nematodes in carrot

Successive cycles of soil drying and wetting improve tolerance to drought in mangabeira

O objetivo deste trabalho foi avaliar o acúmulo de biomassa e a fotossíntese em mangabeira, em resposta ao deficit hídrico e a ciclos sucessivos de secagem e umedecimento do solo. Mudas foram cultivadas em vasos e submetidas aos três seguintes regimes hídricos: solo a 35, 70 e 100% da capacidade de campo (CC), seguidos de um período de secagem até que a fotossíntese (medida semanalmente) se aproximasse de zero. Em seguida, as mudas foram irrigadas novamente até completar quatro ciclos de secagem-umedecimento. O tratamento-controle foi um solo permanentemente bem irrigado. Houve um declínio no acúmulo de biomassa em condições de deficit hídrico. A fotossíntese respondeu à reirrigação do solo apenas a 70 e 100% da CC e foi nula a 35% da CC. O tempo decorrido para a fotossíntese atingir um valor nulo após irrigação aumenta com os sucessivos ciclos de secagem-umedecimento. Em solo a 100% da CC, para que a fotossíntese se aproxime de zero, são necessárias cinco semanas no primeiro e oito semanas no último ciclo de irrigação. A fotossíntese melhora com ciclos sucessivos de secagem‑umedecimento, particularmente em solo a 100% de CC. A pré‑climatização da mangabeira aos ciclos de secagem‑umedecimento deve ser considerada antes do transplante das mudas no campo, e a pré‑climatização deve ser realizada inicialmente com o solo em sua CC.The objective of this work was to evaluate biomass accumulation and photosynthesis in mangabeira, in response to water deficit and to successive soil drying and wetting cycles. Seedlings were grown in pots and subjected to the three following water regimes: soil at 35, 70, and 100% of field capacity (FC), followed by a drying-down period until photosynthesis (measured weekly) was close to zero. Then, the seedlings were rewatered until completing four drying-wetting cycles. The control treatment was a   permanently well-irrigated soil. There was a decline in biomass accumulation under water deficit conditions. Photosynthesis responded to soil rewatering only at 70 and 100% FC and was null at 35% FC. The elapsed time for photosynthesis to reach a null value after rewatering increases with successive drying-wetting cycles. In soil at 100% FC, for photosynthesis to approach zero, it takes five weeks in the first and eight weeks in the last rewatering cycle. Photosynthesis improves with progressive drying-wetting cycles, particularly in soil at 100% FC. The pre-acclimation of mangabeira to drying-wetting cycles should be considered before transplanting the seedlings in the field, and pre-acclimation should be carried out initially with soil at its FC

Leaf traits and gas exchange in saplings of native tree species in the Central Amazon

Global climate models predict changes on the length of the dry season in the Amazon which may affect tree physiology. The aims of this work were to determine the effect of the rainfall regime and fraction of sky visible (FSV) at the forest understory on leaf traits and gas exchange of ten rainforest tree species in the Central Amazon, Brazil. We also examined the relationship between specific leaf area (SLA), leaf thickness (LT), and leaf nitrogen content on photosynthetic parameters. Data were collected in January (rainy season) and August (dry season) of 2008. A diurnal pattern was observed for light saturated photosynthesis (Amax) and stomatal conductance (g s), and irrespective of species, Amax was lower in the dry season. However, no effect of the rainfall regime was observed on g s nor on the photosynthetic capacity (Apot, measured at saturating [CO2]). Apot and leaf thickness increased with FSV, the converse was true for the FSV-SLA relationship. Also, a positive relationship was observed between Apot per unit leaf area and leaf nitrogen content, and between Apot per unit mass and SLA. Although the rainfall regime only slightly affects soil moisture, photosynthetic traits seem to be responsive to rainfall-related environmental factors, which eventually lead to an effect on Amax. Finally, we report that little variation in FSV seems to affect leaf physiology (Apot) and leaf anatomy (leaf thickness).Os modelos climáticos globais prevêem mudanças na extensão da época seca na Amazônia, o que pode afetar a fisiologia das árvores. Os objetivos deste trabalho foram determinar o efeito da sazonalidade da precipitação e fração de céu visível (FSV) no sub-bosque da floresta nas características foliares e trocas gasosas de 10 espécies florestais da Amazônia Central. Também examinou-se a relação entre área foliar específica (SLA), espessura da folha (LT) e nitrogênio foliar em parâmetros fotossintéticos. Os resultados foram coletados nos meses de janeiro (época chuvosa) e agosto (época seca) de 2008. Observou-se um padrão de variação diurna na fotossíntese saturada por luz (Amax) e na condutância estomática (g s). Independente da espécie, Amax foi menor na época seca. No entanto, não houve efeito da sazonalidade das chuvas em g s nem na capacidade fotossintética (Apot medida em [CO2] saturante). Apot e a espessura da folha (LT) aumentaram com FSV, o contrário foi observado para a relação FSV-SLA. Também, observou-se uma relação positiva entre Apot por unidade de área e conteúdo de nitrogênio foliar, e entre Apot, por unidade de massa e SLA. Embora o regime das chuvas apenas levemente influenciou a umidade do solo, características fotossintéticas parecem responderem a fatores relacionados com as chuvas, o que repercute em Amax. Finalmente, relata-se que pequenas variações em FSV parecem afetar a fisiologia da folha (Apot) e a anatomia foliar (espessura da folha)

Características da fluorescência e fotoinibição em arvoretas de acariquara em dias nublados e ensolarados

High irradiance may reduce the productivity of tropical plants by exacerbating photoinhibition of photosynthesis, particularly in the case of shade-adapted plants. The aim of this study was to determine the effect of cloud cover on the fluorescence characteristics and photoinhibition on saplings of manwood (Minquartia guianensis Aubl.). Three-year-old saplings were exposed to full irradiance either on clear days (10, 45 and 90 min) or under overcast conditions (120, 180, and 420 min). Changes in the population of functional photosystem II (PSII), the initial (Fo) and maximum fluorescences (Fm), and the Fv/Fm ratio (maximum potential quantum yield of PSII) were monitored after plant exposure to full irradiance and during recovery (within 48 h) at low light intensity. Although photoinhibition of PSII (Fv/Fm) was determined by the number of photons reaching the leaf surface (photon fluence), cloudiness tended to reduce the photoinhibitory effect of irradiance. Fo increased with fluence on cloudy days and was unaffected by irradiance on clear days, except for a sharp rise during the first 10 min of exposure to full sunlight. For a given photon fluence, Fm was lower on clear days. Recovery from photoinhibition was similar in both light environments. Although photon fluence is the preponderant factor determining the extent of photoinhibition, cloudiness might alleviate the photoinhibitory effect of irradiance.Alta irradiância pode reduzir a produtividade de plantas tropicais por intensificar a fotoinibição da fotossíntese, particularmente em plantas adaptadas à sombra. O objetivo deste trabalho foi determinar o efeito da nebulosidade nas características da fluorescência e na fotoinibição em arvoretas de acariquara (Minquartia guianensis Aubl.). Plantas jovens de três anos de idade foram expostas à irradiância em dias ensolarados (10, 45 e 90 min) ou nublados (120, 180 e 420 min). Foram monitoradas mudanças na população de fotossistemas II (FSII) funcionais, na fluorescência inicial (Fo) e máxima (Fm) e na relação Fv/Fm (rendimento quântico potencial do FSII), antes da exposição das plantas à irradiância e durante a recuperação (até 48 h) sob baixa intensidade luminosa. Apesar de a fotoinibição (Fv/Fm) ser determinada pelo número de fótons incidentes na superfície da folha (fluência), houve tendência de a nebulosidade reduzir o efeito da irradiância na fotoinibição. Fo aumentou com a fluência em dias nublados, mas não em dias ensolarados, exceto pelo aumento nos primeiros 10 min de exposição à luz solar plena. Para uma dada fluência, Fm foi menor nos dias ensolarados. A recuperação da fotoinibição foi similar em ambos os ambientes de luz. Apesar de a fluência de fótons ser o fator preponderante na determinação do grau de fotoinibição, a nebulosidade ameniza o efeito fotoinibitório da irradiância

Photosynthesis and photoinhibition in mahogany and acariquara as a function of irradiance and leaf temperature

A fotoinibição pode ser intensificada por fatores de estresse como alta temperatura. O objetivo deste trabalho foi determinar os efeitos da luminosidade e temperatura na fotossíntese e na recuperação da fotoinibição sob baixa irradiância em mogno (Swietenia macrophylla King) e acariquara (Minquartia guianensis Aubl.). As plantas cresceram sob baixa irradiância: 0,20 (acariquara) e 1,43 mol m-2 dia-1 (mogno). A fotossíntese líquida e as características da fluorescência foram avaliadas em duas irradiâncias, 1.000 e 1.700 mmol m-2 s-1, e duas temperaturas foliares, 32 e 38ºC. A 32ºC, a fotossíntese máxima (Amáx) foi maior em mogno do que em acariquara. Em mogno, a exposição a 38ºC causou redução gradativa na fotossíntese, sendo a taxa fotossintética após 50 min similar à encontrada em acariquara. A 32ºC, o aumento na irradiância, de 1.000 para 1.700 mmol m-2 s-1, intensificou a fotoinibição, mas Amáx não foi alterada. Em 1.700 mmol m-2 s-1, o aumento da temperatura, de 32 para 38ºC, não afetou a fotoinibição. Após 72 horas em baixa irradiância, a recuperação da fotoinibição foi de 80% em acariquara e 89% em mogno. Não houve efeito da temperatura da folha na fotoinibição.Photoinhibition may be exacerbated by stress factors such as high temperature. The aim of the work was to determine the effects of temperature and irradiance on photosynthesis and photoinhibition in mahogany (Swietenia macrophylla King) and acariquara (Minquartia guianensis Aubl.). Also to assess the effect of irradiance and temperature on the recovery from photoinhibition at low irradiance. Sapling were grown at low irradiance: 0.20 mol m-2 day-1 (mahogany) and 1.43 mol m-2 day-1 (acariquara). Photosynthesis and fluorescence characteristics were assessed in plants subjected to two irradiances 1,000 and 1,700 mmol m-2 s-1, and two temperatures, 32 e 38ºC. At 32ºC, light saturated photosynthesis (Amax) was greater in mahogany than in acariquara. Mahogany plants exposed to 38ºC showed a consistent decline in carbon uptake, and after 50 min both species showed the same photosynthetic rates. In saplings exposed to 32ºC, increase in irradiance, from 1,000 to 1,700 mmol m-2 s-1, exacerbated photoinhibition, but without reducing Amax values. At 1,700 mmol m-2 s-1, the rise in temperature, from 32 to 38ºC, did not increase photoinhibition. After 72 hours at low irradiance, recovery from photoinhibition was 80% in acariquara and 89% in mahogany. There was no effect of leaf temperature on photoinhibition

Successive cycles of soil drying and wetting improve tolerance to drought in mangabeira

Abstract The objective of this work was to evaluate biomass accumulation and photosynthesis in mangabeira, in response to water deficit and to successive soil drying and wetting cycles. Seedlings were grown in pots and subjected to the three following water regimes: soil at 35, 70, and 100% of field capacity (FC), followed by a drying-down period until photosynthesis (measured weekly) was close to zero. Then, the seedlings were rewatered until completing four drying-wetting cycles. The control treatment was a permanently well-irrigated soil. There was a decline in biomass accumulation under water deficit conditions. Photosynthesis responded to soil rewatering only at 70 and 100% FC and was null at 35% FC. The elapsed time for photosynthesis to reach a null value after rewatering increases with successive drying-wetting cycles. In soil at 100% FC, for photosynthesis to approach zero, it takes five weeks in the first and eight weeks in the last rewatering cycle. Photosynthesis improves with progressive drying-wetting cycles, particularly in soil at 100% FC. The pre-acclimation of mangabeira to drying-wetting cycles should be considered before transplanting the seedlings in the field, and pre-acclimation should be carried out initially with soil at its FC

SPAD-502 readings in response to photon fluence in leaves with different chlorophyll content

The chlorophyll meter (SPAD-502) is widely used to estimate chlorophyll content, but non-uniform chloroplast distribution can affect its accuracy. This study aimed to assess the effect of photon fluence (F, irradiance x time of illumination) in leaves with different chlorophyll content and determine the effect of chlorophyll a/b on SPAD values of four tropical tree species (Croton draconoides Müll. Arg., Hevea guianensis Aubl., Hymenaea courbaril L. and Matisia cordata H.B.K.). There were also determined calibration equations for the chlorophyll meter and assessed the effect of F on SPAD values between 07:00 h and 17:00 h. Calibration equations were obtained after determining leaf chlorophyll content in the laboratory. Increases in F with time caused a reduction in SPAD values in species with a high chlorophyll content, with reductions of 20% in M. cordata and 10% in H. guianensis. Leaves of C. draconoides and H. courbaril had lower chlorophyll content and showed no changes in SPAD values with increase in F. The chlorophyll a/b ratio increased with SPAD values and the SPAD/chlorophyll relationship was best described by an exponential equation. It seems that F may affect SPAD values in leaves with high chlorophyll content, probably due to non-uniform chloroplast distribution at high irradiance. This indicates that SPAD values tend to be more accurate if recorded early in morning when irradiance is low