Paulo Cesar Sentelhas

Estimação da área foliar do algodoeiro por meio de dimensões e massa das folhas

ABSTRACT COTTON LEAF AREA ESTIMATES BASED ON LEAF DIMENSIONS AND DRY MASS METHODS The objective of this study was to evaluate two different methods to estimate cotton leaf area(LA), based on leaf dimensions (length - L and width - W) and leaf dry mass (DM). Two cultivars, IAC 23and Coodetec 401, were used. For leaf dimensions method, leaves were classified by age: young, heart-shape, and mature. For each age class, a leaf shape factor (LSF) was obtained by simple linear regressionbetween L*W and LA. For leaf dry mass method, leaves were classified in new and mature and a leafdry mass factor (LDMF) was obtained by simple linear regression between DM and LA. LA estimates thetwo methods were compared to LA measured in an independent sample. Good accuracy was observedwith both methods, but leaf dry mass method presented a better performance with r 2 ranging from 0.94to 0.98 and regression slopes between 0.97 and 1.00, when regression line was forced thought the origin.In this case there is and advantage since leaf dry mass method is less time-consuming.

Climate changes and technological advances: impacts on sugarcane productivity in tropical southern Brazil

The climatic projections for this century indicate the possibility of severe consequences for human beings, especially for agriculture where adverse effects to productivity of crops and to agribusiness as a whole may occur. An agrometeorological model was used to estimate sugarcane yield in tropical southern Brazil, based on future A1B climatic scenarios presented in the fourth Intergovernmental Panel on Climate Change report, in 2007. Sugarcane yield was evaluated for 2020, 2050, and 2080 considering the possible impacts caused by changes in temperature, precipitation, sunshine hours and CO2 concentration in the atmosphere, as well as technological advances. Increasingly higher temperatures will cause an increase of the potential productivity (PP), since this variable positively affects the efficiency of the photosynthetic processes of C4 plants. Changes in solar radiation and rainfall, however, will have less impact. PP will increase by 15% in relation to the present condition in 2020, by 33% in 2050 and by 47% in 2080. Regarding the actual productivities (AP), the increase observed in PP will compensate for the negative effect of the projected increase in water deficit. AP will increase by 12% in relation to the present condition in 2020, by 32% in 2050 and by 47% in 2080. The increase in sugarcane productivity resulting from the projected scenarios will have important impacts on the sugarcane sector.

Reconsidering Leaf Wetness Duration Determination for Plant Disease Management

Relationships between leaf wetness and plant diseases have been studied for centuries. The progress and risk of many bacterial, fungal, and oomycete diseases on a variety of crops have been linked to the presence of free water on foliage and fruit under temperatures favorable to infection. Whereas the rate parameters for infection or epidemic models have frequently been linked with temperature during the wet periods, leaf wetness periods of specific time duration are necessary for the propagule germination of most phytopathogenic fungi and for their penetration of plant tissues. Using these types of relationships, disease-warning systems were developed and are now being used by grower communities for a variety of crops. As a component of Integrated Pest Management, disease-warning systems provide growers with information regarding the optimum timing for chemical or biological management practices based on weather variables most suitable for pathogen dispersal or host infection. Although these systems are robust enough to permit some errors in the estimates or measurements of leaf wetness duration, the need for highly accurate leaf wetness duration data remains a priority to achieve the most efficient disease management.

Modeling monthly mean air temperature for Brazil

Air temperature is one of the main weather variables influencing agriculture around the world. Its availability, however, is a concern, mainly in Brazil where the weather stations are more concentrated on the coastal regions of the country. Therefore, the present study had as an objective to develop models for estimating monthly and annual mean air temperature for the Brazilian territory using multiple regression and geographic information system techniques. Temperature data from 2,400 stations distributed across the Brazilian territory were used, 1,800 to develop the equations and 600 for validating them, as well as their geographical coordinates and altitude as independent variables for the models. A total of 39 models were developed, relating the dependent variables maximum, mean, and minimum air temperatures (monthly and annual) to the independent variables latitude, longitude, altitude, and their combinations. All regression models were statistically significant (α ≤ 0.01). The monthly and annual temperature models presented determination coefficients between 0.54 and 0.96. We obtained an overall spatial correlation higher than 0.9 between the models proposed and the 16 major models already published for some Brazilian regions, considering a total of 3.67 × 108 pixels evaluated. Our national temperature models are recommended to predict air temperature in all Brazilian territories.

The soybean yield gap in Brazil – magnitude, causes and possible solutions for sustainable production

Brazil is one of the most important soybean producers in the world. Soybean is a very important crop for the country as it is used for several purposes, from food to biodiesel production. The levels of soybean yield in the different growing regions of the country vary substantially, which results in yield gaps of considerable magnitude. The present study aimed to investigate the soybean yield gaps in Brazil, their magnitude and causes, as well as possible solutions for a more sustainable production. The concepts of yield gaps were reviewed and their values for the soybean crop determined in 15 locations across Brazil. Yield gaps were determined using potential and attainable yields, estimated by a crop simulation model for the main maturity groups of each region, as well as the average actual famers’ yield, obtained from national surveys provided by the Brazilian Government for a period of 32 years (1980–2011). The results showed that the main part of the yield gap was caused by water deficit, followed by sub-optimal crop management. The highest yield gaps caused by water deficit were observed mainly in the south of Brazil, with gaps higher than 1600 kg/ha, whereas the lowest were observed in Tapurah, Jatai, Santana do Araguaia and Uberaba, between 500 and 1050 kg/ha. The yield gaps caused by crop management were mainly concentrated in South-central Brazil. In the soybean locations in the mid-west, north and north-east regions, the yield gap caused by crop management was 2000 kg/ha. For reducing the present soybean yield gaps observed in Brazil, several solutions should be adopted by growers, which can be summarized as irrigation, crop rotation and precision agriculture. Improved dissemination of agricultural knowledge and the use of crop simulation models as a tool for improving crop management could further contribute to reduce the Brazilian soybean yield gap.

Obtaining weather data for input to crop disease-warning systems: leaf wetness duration as a case study

Os sistemas de alerta fitossanitario sao ferramentas de suporte a decisao desenvolvidos para ajudar os agricultures a determinar o melhor momento da aplicacao das medidas de controle para combater as doencas de plantas. As variaveis meteorologicas sao dados de entrada quase que obrigatorios desses sistemas. Este trabalho apresenta uma revisao sobre os meios pelos quais as variaveis meteorologicas sao coletadas para serem usadas como dados de entrada em sistemas de alerta fitossanitario e sobre os desafios associados a logistica de obtencao desses dados. Essa revisao compara o monitoramento meteorologico ao nivel do produtor, nas propriedades agricolas, com aquele feito ao nivel de redes de estacoes meteorologicas, assim como discute as vantagens e desvantagens entre medir e estimar tais variaveis meteorologicas. Especial enfase e dada a duracao do periodo de molhamento foliar (DPM), nao somente pela sua importância como dado de entrada em diversos sistemas de alerta fitossanitario, mas tambem pelo desafio de se obter dados acurados dessa variavel. Pode-se concluir, apos ampla discussao do assunto, que nao ha um metodo unico e melhor para se obter os dados meteorologicos para uso em sistemas de alerta fitossanitario; por outro lado, as circunstâncias a nivel local, regional e nacional provavelmente influenciam a estrategia de maior sucesso.

Regiões homogêneas e funções de distribuição de probabilidade da precipitação pluvial no Estado de Táchira, Venezuela

The objective of this work was to determine regions of monthly pluvial precipitation based on the seasonal and the probability distribution that best fit to precipitation of those areas in the State of Tachira, Venezuela. Long-term series of 24 throughout 62 years of monthly precipitation data of 25 climatological stations were used. The Wards clustering methods of analyses was used to group the months with similar monthly pluvial precipitation and also the climatological locations with similar precipitation (homogeneous regions). The adjust of the probability density functions exponential, Gamma, Gumbel, normal, log-normal were evaluated with three parameters as well as Weibull distributions in order to observe monthly precipitation data. The precipitation seasonal variation at the State of Tachira presents three periods statistically defined as: dry, transition, and wet. For the dry and wet periods, four homogeneous regions of locations with similar monthly precipitation could be identified and, in the transition period, three regions. In the dry period, the recommended probability distribution is the exponential, except for the homogeneous regions with the largest rainfall values during that period, when Gamma distribution is better. In the wet period, in all regions, normal distribution prevails, except for August, when Gamma prevails. As far as transition periods are concerned, Gamma distributions are better in April and normal distribution, in November.

Climatic effects on sugarcane ripening under the influence of cultivars and crop age

The lack of information about the effects of cultivars, crop age and climate on the sugarcane (Saccharum ssp.) crop yield and quality has been the primary factor impacting the sugar-ethanol sector in Brazil. One of the processes about which we do not have a satisfactory understanding is sugarcane ripening and the effects of cultivars, crop age and climate on that. Sugarcane ripening is the process of sucrose accumulation in stalks, which is heavily influenced by several factors, mainly by climatic conditions such as air temperature and water deficits. Because it is a complex process, studies of the variables involved in sugarcane ripening can provide important information, resulting in a better use of commercial cultivars, bringing advantages to growers, processing units, breeding programs and scientific community. In this review, we discuss the available knowledge of the interaction between climate conditions and sugarcane ripening, under the influence of genotypic characteristics and crop age. In several studies, the main conclusion is that sugarcane ripening depends on a complex combination of climate variables, the genetic potential of cultivars and crop management. Soil moisture and air temperature are the primary variables involved in sugarcane ripening, and their combination stimulates the intensity of the process. In addition, the need for studies integrating the effects of climate on plant physiological processes and on the use of chemical agents to stimulate sugarcane ripening is highlighted.

Class A pan coefficients (Kp) to estimate daily reference evapotranspiration (ETo)

O coeficiente do tanque classe A (Kp) tem sido empregado para se estimar a evapotranspiracao de referencia (ETo), importante componente no manejo de agua de culturas irrigadas, a partir da evaporacao do tanque Classe A (ECA). Existem diversos metodos para se determinar os valores de Kp, cuja maioria e baseada nas informacoes de velocidade do vento, umidade relativa e extensao e condicoes da area de bordadura, ao redor do tanque. Neste trabalho foram analisados os seguintes metodos de estimativa do Kp: Doorenbos & Pruitt (1977), Cuenca (1989), Snyder (1992) e Pereira et al. (1995); Raghuwanshi & Wallender (1998) e FAO/56 (Allen et al., 1998). Os valores de Kp, estimados e calculados pela relacao entre a ETo medida em lisimetro de pesagem e a ECA, foram comparados por meio de analise de regressao. O mesmo procedimento foi utilizado para se avaliar o desempenho dos diferentes metodos de determinacao do Kp na estimativa da ETo, a partir da ECA, cujos resultados mostraram que a concordância entre os valores de Kp calculados e estimados pelos diferentes metodos, foi pequena (R2< 0,2), o que resultou em estimativas de ETo com alta dispersao (R2< 0,8). Os melhores metodos de determinacao do Kp, para a estimativa da ETo, foram os de Pereira et al. (1995) e Cuenca (1989), ambos apresentando alta eficiencia. O uso de um Kp arbitrario e constante (0,71) na estimativa da ETo resultou na mesma precisao e exatidao das estimativas feitas com os valores de Kp determinados pelos metodos de Pereira e de Cuenca. O uso de um valor constante de Kp e uma opcao simples e pratica para se estimar ETo a partir da ECA, porem este valor necessita ser calibrado para cada local, sob diferentes condicoes climaticas.

EVAPOTRANSPIRATION AND IRRIGATION REQUIREMENTS OF A COFFEE PLANTATION IN SOUTHERN BRAZIL

Crop evapotranspiration (ETc) was measured as evaporative heat flux from a drip-irrigated coffee ( Coffea arabica ) plantation with 5-year-old trees using the Bowen ratio-energy balance technique. Crop transpiration (T) was determined with the stem heat balance method. Irrigation requirements were determined by comparing the ETc and T with reference evapotranspiration (ETo) derived from the Penman-Monteith equation and expressed as the ETc/ETo (Kc) and T/ETo (Kcb) ratios. Also, relationships were established between ETc and T and class A pan evaporation (ECA). The influence of inter-row vegetation on ETc was analysed, since the measurements were taken in a period of transition between dry-wet seasons. The average Kc value obtained was 1.00. The strong coupling of coffee plants to atmospheric conditions and high sensitivity of coffee plants to large vapour pressure deficits and air/leaf temperatures caused variations in Kcb in relation to ETo. Kcb ranged from 0.67, when ETo exceeded 4 mm d −1 , to 1.27 when ETo was less than 2 mm d −1 . When vegetation did not occupy the inter-row ground spaces, T represented about 0.87ETc, but 0.68ETc when ground vegetation filled the inter-row spaces.