Irrigação fenológica em duas cultivares de algodoeiro herbáceo.

Abstract

Cotton was always one of the main commercially exploited crops in the semiarid of Brazilian Northeast. Due to the droughts that occur periodically in this region, it is necessary to use technologies such as irrigation practice to minimize the effect of water deficit on this crop. Despite being a tolerant crop to soil water deficit, water stress at certain stages of the cotton crop cycle can cause severe damage to it, affecting physiological processes and reproductive structures, with consequent loss of yield and quality of fiber. Thus, the objective of this work was to evaluate the response of upland cotton cultivars submitted to periods of water deficits applied at different stages of the crop cycle and its effect on gas exchange, production components, water efficiency and fiber quality. The experiment was conducted under field conditions, at the Federal University of Campina Grande, Pombal Campus, Paraíba State, Brazil. The treatments were formed in a split plot scheme consisting of 6 periods (P) of water deficit (P1 = treatment without deficit (control, 100% irrigated throughout the cycle), P2 = water deficit in the initial growth phase, P3 = water deficit at the onset of the first flower bud phase, P4 = water deficit at the onset of the first flower phase, P5 = water deficit at the onset of the first fruit phase, and P6 = water deficit at the onset of the first open fruit phase) and 2 upland cotton cultivars (C) (C1 = BRS 286 and C2 = BRS 336), in randomized block design, in split plots (deficit periods = plots; cultivars = subplots), with 4 replicates. Analysis of the gas exchange of the plants, production components, water efficiency and fiber quality were evaluated using the F test, being the means of the treatments, both qualitative, compared by the Tukey test at 5% probability using SISVAR statistical program. Water deficit reduced the gases exchanges of herbaceous cotton plants, mainly stomatal conductance, transpiration and photosynthesis. The cotton cultivars BRS 286 and BRS 336 presented similar behavior in the different water deficits applied in their different phenological phases. Gases exchanges were more sensitive to water deficiency during the apple formation phase and less affected in the initial growth stages and floral bud. The occurrence of water deficit during the phases of emergence of flowers and apples were the most detrimental phases to the components of cotton production, with substantial losses, and also the phases that produced the lowest water use efficiency. The components of production of the BRS 286 cultivars were higher than those found for the BRS 326 cultivar, except for the mean weight of buds. Water use efficiency of the herbaceous cotton was statistically similar in both cultivars. With the exception of maturity, reflectance and yellow degree of fiber, the water deficits applied in the different phenological phases of the herbaceous cotton cultivars affected the fiber quality. With the exception of the short fiber, elongation and micronaire index, the treatment without water deficit promoted the best fiber values. The herbaceous cotton was more tolerant to the deficit in the initial growth and apple stages. In general, cultivar BRS 336 was more tolerant to applied water deficits than cultivar BRS 286. The cultivars used have fiber characteristics in accordance with varietal and commercial standards of medium (BRS 286) and long (BRS 336) fiber.