Cowpea (Vigna unguiculata (L.) Walp.) is a leguminous species widely cultivated in northern and northeastern Brazil. In the state of Pará, this crop still has low productivity due to several factors, such as low soil fertility and climatic adversity, especially the water deficiency. Therefore, the present study aimed at evaluating the physiological parameters and the productivity of cowpea plants under different water depths. The experiment was conducted in Castanhal/Pará between 2015 and 2016. A randomized block design was applied with six replications and four treatments, represented by the replacement of 100%, 50%, 25% and 0% of the water lost during crop evapotranspiration (ETc), starting from the reproductive stage. The rates of net photosynthesis (A), stomatal conductance (gs), leaf transpiration (Eleaf), substomatal CO2 concentration (Ci), leaf temperature (Tleaf) and leaf water potential (Ψw) were determined in four measurements at the R5, R7, R8 and R9 phenological stages. Cowpea was sensitive to the water availability in the soil, showing a significant difference between treatments for physiological variables and productivity. Upon reaching a Ψw equal to −0.88 MPa, the studied variables showed important changes, which allows establishing this value as a threshold for the crop regarding water stress under such experimental conditions. The different water levels in the soil directly influenced productivity for both years, indicating that the proper water supply leads to better crop growth and development, increasing productivity.

Due to the sensitivity of the cowpea to water deficit during the dry season in the Amazonian environment, there is a need for studies related to water management and the optimization of annual yield. Thus, the objective was to estimate the crop evapotranspiration (ETc) and the actual evapotranspiration (ETr), whilst also evaluating the effect of different irrigation depths on the yield response factor (Ky) and dry biomass (Kss) of the cowpea cultivar BR3-Tracuateua under edaphoclimatic conditions in the Amazon region. The experiment was carried out in randomized blocks, with six replicates and four treatments: T1 (100% ETc), T2 (50% ETc), T3 (25% ETc), and T4 (0% ETc), commencing in the reproductive phase with a drip irrigation system. The total evapotranspiration recorded for the cowpea cultivar BR3-Tracuateua across the four treatments was 337.5, 284.35, 258.62, and 219.82 mm with an average consumption of 4.6, 3.90, 3.54, and 3.01 mm d-1 for T1, T2, T3, and T4, respectively. The emergence phase showed evaporation rate of 5.19 mm d-1 and the reproductive, vegetative, and senescence ETc phases showed evaporation of 4.87, 4.84, and 3.32 mm d-1, respectively. The flowering stage had the greatest water demand (5.88 mm d-1). There was a significant difference in the crop yield among all treatments, with decreases of 18.91, 33.12 and 57.17% for T2, T3 and T4 in the grain yield, with a Ky of 1.48, and Kss of 2.03, 1.08, and 0.87 for the flowering, grain filling, and physiological maturation stages, respectively.

This study aims to determine the cowpea efficiency in absorbing and using solar radiation according to different irrigation depths under the climatic conditions of the northeast of Pará State. The experiment was carried out on 2014 and 2016 in an experimental design of randomized blocks, which consisted in six blocks with four treatments, in which different irrigation depths the reproductive phase were applied, as follows: T100, T50, T25 e T0, that corresponded to 100%, 50%, 25% e 0% of the crop evapotranspiration, respectively. The absorbed photosynthetically active radiation, leaf area index (LAI), total aerial dry matter (TADM) and grain yield were measured. The extinction coefficient (k) was obtained by nonlinear regression between the fraction of absorbed PAR (fPARinter) and the LAI. The radiation use efficiency (RUE) was calculated by linear regression between the TADM and the accumulated absorbed PAR. The water deficit imposed by the treatments had a significant influence on the LAI, TADM and cowpea yields. The water deficit did not significantly influenced k À it ranged between 0.83 for T100 and 0.70 for T0. The RUE showed significant behaviors regarding the treatments with adequate water supply and treatments under water deficit, ranging from 2.23 to 1.64 g·MJ -1 , respectively. ResumoObjetivou-se neste trabalho determinar a eficiência do feijão-caupi em interceptar e usar a radiação solar quando submetido a diferentes lâminas de irrigação nas condições climáticas do nordeste paraense. O experimento foi realizado em 2014 e 2016 em delineamento experimental de blocos casualizados, com seis blocos e quatro tratamentos, que consistiram diferentes laminas de irrigação na fase reprodutiva, sendo T100, T50, T25 e T 0 correspondente a 100%, 50%, 25% e 0% da evapotranspiração da cultura. Realizou-se medição da radiação fotossinteticamente ativa (PAR) interceptada, do índice de área foliar (LAI), da matéria seca total da parte aérea (TADM) e produtividade. O coeficiente de extinção (k) foi obtido através de regressão não linear entre a fPARinter e o LAI. A eficiência de uso da radiação (RUE) foi obtida pela regressão linear entre a TADM e a radiação PARinter acumulada. O déficit hídrico imposto pelos tratamentos influenciou significativamente as respostas no LAI, TADM e produtividade do feijão-caupi. O k não foi influenciado significativamente pelo déficit hídrico, apresentando variação de 0,83 no T100 e 0,70 no T 0. A RUE apresentou resposta significativa entre os tratamentos com suprimento hídrico adequado e os tratamentos sob déficit hídrico, variando de 2,23 a 1,64 g·MJ -1 , respectivamente.Palavras-chave: vigna unguiculata, radiação interceptada, coeficiente de extinção, déficit hídrico.

The present work evaluated the Aquacrop model as a tool for climate risk analysis and yield prediction of cowpea, cultivated in a dystrophic yellow oxisol. The model was previously calibrated and validated for two harvests, in order to simulate the biomass and yield of cowpea, considering four applied water blades over its reproductive period. The good achieved results prove the model’s efficiency for this kind of simulation. After validation, the yield simulation of cowpea based on the meteorological data (2003 to 2014), soil and crop management of 12 harvests was performed. Two scenarios were given: the potential yield without water restrictions; and the actual yield, considering to pluvial availability conditions of the inserted series. The results suggested that the optimum sowing dates are between April 1st and 20th, in which there was a low yield loss (< 10 %) considering the potential yield, high probability (> 90 %) of achieving high yields (above 1300 kg ha-1) and a low risk of getting crop harvesting in the rainy period. After all, the model proved to be a feasible tool for predicting cowpea yield in the region and also over regions with similar characteristics.

Resumo O feijão-caupi é uma leguminosa bastante cultivada no Norte e Nordeste do Brasil, mas no Pará, a cultura apresenta baixa produtividade, devido à baixa fertilidade dos solos e especialmente à deficiência hídrica. O trabalho teve como objetivo avaliar o efeito de distintos níveis de deficiência hídrica acumulada durante a fase reprodutiva nas trocas gasosas do feijão-caupi. O experimento foi conduzido em Castanhal-Pa, em 2015 e 2016, em um delineamento de blocos ao acaso, com seis repetições e quatro tratamentos correspondentes à reposição de 100% da água perdida pela evapotranspiração da cultura (T1), a 50% de reposição (T2), a 25% de reposição (T3) e sem irrigação (T4). As trocas gasosas foram medidas no estádio fenológico R9, correspondente aos 64 dias após a semeadura (28 dias após o inicio dos tratamentos). A deficiência hídrica foi calculada pelo balanço hídrico diário considerando a capacidade de água disponível (CAD) e a profundidade efetiva do sistema radicular observada em campo. Não houve diferença significativa entre os anos experimentais além do próprio efeito dos tratamentos. O feijão-caupi mostrou-se sensível ao aumento da deficiência hídrica acumulada, diminuindo sua condutância estomática e taxa de fotossíntese, as quais diferiram significativamente entre os 4 tratamentos. Para o potencial hídrico foliar e carbono interno houve diferença significativa apenas entre os tratamentos extremos. Tal comportamento sugere que o feijão-caupi consegue preservar o potencial hídrico foliar mesmo em condições cumulativas de deficiência hídrica agindo como uma espécie conservadora ao priorizar a manutenção de seu status hídrico ao invés da sua atividade fotossintética. As menores produtividades ocorreram em função do déficit hídrico imposto pelos tratamentos com menores lâminas de irrigação.