Ecofisiologia da interação entre feijão-caupi e plantas daninhas sob déficit hídrico

Abstract

The cultivation of cowpea (Vigna unguiculata L. Walp) in Brazil has been frequently carried out without irrigation, with frequent occurrence of dry periods during the crop cycle. The negative effects of water deficit on plant production can be intensified by weed infestations and result in lower crop performance. Therefore, the objective of the work was to evaluate the effects of temporary water deficits on the soil and of competition in the growth and dry matter partition, macronutrient accumulation and efficiency, and physiological characteristics of cowpea (Vigna unguiculata L. cv BRS Guariba) and two weeds Commelina benghalensis L. and Waltheria indica L. For that, an experiment was carried out in a greenhouse of the Department of Plant Sciences of the Federal University Rural Semi-Arid. The experimental unit corresponded to a plastic vessel with a volumetric capacity of 10 dm-3. The experimental design for the randomized blocks, with four replications. The treatments were arranged in a 5x2 factorial, with the first factor corresponding to the types of interaction between the species (V. unguiculata + C. benghalensis; V. unguiculata + W. indica; V. unguiculata, C. benghalensis and W. indica in monoculture), and the second, of water regimes (Irrigated and Water deficit). The water deficit was simulated when the the cowpea plants were with the second definite trefoil (stage V4), being maintained for eleven days, until the photosynthetic rate of the culture, at 9:00 am, reached values close to zero, when irrigation was resumed. From the date of the suspension of irrigation, the plants were submitted to the following evaluations: water potential at pre-dawn and at noon; stomatal conductance; transpiration; and net photosynthetic rate; until the CO2 assimilation rates of the plants submitted to the water deficit were similar to those of the irrigated ones. At the end of the experiment, we evaluated: the number of leaves per plant; the leaf area; the dry mass of leaves, stems, roots, and total; the dry mass partitioning; and macronutrient accumulation and partition. The water deficit reduced the water potential and the gas exchange of the plant species, cultivated alone or in interaction. Cowpea and C. benghalensis tolerated the water deficit in the soil through the strict control of the stomatal opening, being categorized as more water conservative species. Competition among plants anticipated the negative effects of soil water deficit. Waltheria indica presented greater competition capacity for water, having delayed the recovery of cowpea. The soil water deficit reduced nutrient content in the following decreasing order: cowpea - Ca (stem), K (root), Mg (leaf) and N (root); C. benghalensis - N, P and K (stem); and W. indica – K, N, P, Ca and Mg (leaf). The water deficit reduced the N and P utilization efficiency of cowpea, and of Mg of C. benghalensis. The competition provided variable effect on the accumulation and efficiency of macronutrients utilization of cowpea and weeds C. benghalensis and W. indica. The cowpea suffered greater interference with the W. indica weed when cultivated on soil not irrigated. C. benghalensis presented high potential for macronutrient cycling in the two water regimes. The water deficit reduced the growth of cowpea plants, C. benghalensis and W. indica. The competition between plants increased the effects of temporary water deficit in the soil. The W. indica leaf was the organ most affected by the water deficit. W. indica has greater capacity competition for water with V. unguiculata than C. benghalensis