Cowpea Ecophysiological Responses to Accumulated Water Deficiency during the Reproductive Phase in Northeastern Pará, Brazil

Abstract

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.

Crop Water Stress Index of Cowpea Under Different Water Availability Levels in Castanhal-Pa

“…This response to the effect of water deficit was also observed by Costa et al (2019) and Ferreira et al (2021) in cowpea, since the low availability of water in the soil causes stomatal closure by the plant in order to reduce water losses by transpiration aiming at the maintenance of cellular turgor, which contributes to the decrease of CO 2 assimilation, as it is a common route to both (COSTA et al, 2019).…”

“…According to Ferreira et al (2021), this same cultivar when subjected only to the rainfed condition in the study region (0% irrigation) in the reproductive stage showed an increase in its leaf temperature of up to 4 °C throughout the stage, while when 100% irrigated was maintained they noticed a variation of only 2 °C in leaf temperature. According to the authors, stomatal closure to prevent water loss due to transpiration is the main responsible for the increase in leaf temperature, which also contributed to the reduction in photosynthetic capacity as the decrease in CO 2 influx suggests serious consequences due to water stress (FERREIRA et al, 2021).…”

“…The highest CWSI values were observed when the plant was under water deficit, regardless of phenological stages (Figure 2b). This result is due to the low availability of water in the soil (Figure 1b), which affects the stomatal opening and transpiration of the plant, favoring the increase in leaf temperature due to lower heat dissipation (LIU et al, 2020;FERREIRA et al, 2021). High CWSI values for plants under water deficit conditions were also found by Silva et al (2018) in tomato (1 <CWSI <0.75) and by Alghory and Yazar (2019) in wheat (CWSI = 0.90).…”

“…At 47 DAS, the T0 treatment promoted the consumption of soil water whose volumetric content monitored reached 0.117 m 3 m -3 , close to the wilting point for the type of soil studied. Ferreira et al (2021) report, however, that each species differs in response to soil moisture and that PWP alone is not an adequate criterion to establish water availability for the plant.…”

“…In Brazil, cowpea (Vigna unguiculata (L.) Walp.) is widely found in the North and Northeast regions and commonly produced by family farmers, and its importance is linked to the composition of the food base of these populations and the contribution to the generation of employment and income (FERREIRA et al, 2021).…”