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Abstract
Water availability is a concern for agricultural production and sustainability, escalating the need for an efficient weed control. Weeds are in direct competition with crops for water and its unsuccessful control risks water management across the crop season. However, little is known regarding the impact of herbicide on weed water use behavior. Thus, the objective of this study was to unveil the effect of glyphosate doses on Merremia aegyptia (L.) Urb. water transpiration, water use efficiency (WUE), and intrinsic water use efficiency (iWUE). Two greenhouse studies were conducted using the troublesome weed hairy woodrose (M. aegyptia) in four glyphosate doses (1.4; 15; 111; and 740 g a.e. ha−1) plus an untreated control. Water transpiration was measured in a 3-day-interval until 21 days after treatment (DAT), determining WUE based on its dry biomass and water use. iWUE was determined at 21 DAT through IRGA analysis. Glyphosate label dose (740 g a.e. ha−1) effectively reduced water transpiration 3 DAT and ceased whole water transpiration by 12 DAT. Lower doses (111 and 15 g a.e. ha−1) reduced 39 and 20% of water transpiration, in comparison to the untreated control, and increased its iWUE. Therefore, label dose application of glyphosate successfully controlled hairy woodrose and ceased its water transpiration.
Introduction
Weeds are the greatest biotic restraint for crop production (Chauhan, 2020), competing directly with crops for nutrients, water, space, and light (Patterson, 1995). Considering climate changes and water scarcity, water availability has become a great concern among growers to maintain high yields and quality production (Cai et al., 2015). Brazilian agriculture fields are widely spread throughout the country, which comprehends different environments and weather conditions. Among these environments, water availability as well as best practices in water management are a common concern, which also involves good practices in weed management, considered one of the key points for water sustainability (Ziska, 2020).
Most weed species have high photosynthetic activity due to a survival characteristic, consequently requiring great amount of water in a short period of time (Aguiar et al., 2016). Therefore, an efficient weed control is necessary to maintain yield quality and to avoid water losses due to weed competition. For instance, a plant from the genus Ipomoea, considered a troublesome weed throughout agriculture fields in Brazil, has shown the potential to use as much water as the crop itself during its development growth (Araldi et al., 2012). Hairy woodrose [Merremia aegyptia (L.) Urb] is an annual climbing herb, well adapted to Brazilian weather conditions, and is neutral photoblastic, showing a very adaptive growth within the no-tillage systems, being reported within maize, soybeans, and sugarcane crops (Araldi et al., 2012; Martins et al., 2010). This weed is more problematic after canopy closure, with potential to reduce sugarcane yield in 60% as well as interfere the mechanized harvesting process (Beluci et al., 2018).
Hairy woodrose is commonly managed by glyphosate spraying, especially considering that great part of maize and soybean used are glyphosate-resistant crops. Glyphosate inhibits the synthesis of aromatic amino acids and secondary compounds derived from these amino acids by acting at the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) (Duke, 1988). Thereby, glyphosate will affect plant photosynthesis and can reduce plant transpiration by stomatal closure (Shaner, 1978). However, there is a lack of studies on the effect of glyphosate on weeds water use as well as regarding how it affects plant's water use efficiency over time of application.
Herbicides may drastically affect plant transpiration as it acts into the plant, ceasing its photosynthetic activity (Araldi et al., 2011). However, in the cases which herbicide does not control the weed, such as in herbicide resistance or insufficient amount of the active ingredient, the herbicide can still affect plant water use (Garlon et al., 2010). One of the ways to evaluate plants water use is by determining its water use efficiency (WUE): determining the ratio of plant yield or plant biomass to the amount of water used (i.e., plant transpiration) (Mega et al., 2019). Another way of assessing plant WUE is performed by determining the ratio of the fluxes of net photosynthesis, through CO2 assimilation, and conductance for water vapor, known as intrinsic water use efficiency (iWUE), usually performed with infrared gas analyzers (IRGA) (McAusland et al., 2013; Li et al., 2017).
A better understanding of the effect of herbicides on water use over the time of application is still required, exploring its impact on the whole agricultural system sustainability and supporting growers on key management decisions. The hypothesis is that glyphosate affects plant water transpiration soon after application and before any symptoms are shown as well as those lower doses, although it does not control the weed, it will affect water transpiration. Therefore, considering the importance of crop water use as well as to determine the effect of glyphosate on hairy woodrose water transpiration, the objective of this study was to determine the effect of glyphosate doses on M. aegyptia water transpiration, its water use efficiency and intrinsic water use efficiency.
Section snippets
Materials and methods
Two greenhouse studies were conducted to evaluate the effect of glyphosate doses on M. aegyptia water transpiration and its water use efficiency in September/2020 and January/2021, located at Sao Paulo State University, Botucatu-Sao Paulo (22o50′32″S, 48o25′29″W). The greenhouse was kept at 25 ± 3 °C under natural light conditions during the whole conduction of the studies.
The effects of glyphosate doses on hairy woodrose water transpiration and WUE was performed by adapting two
Results
Glyphosate label dose (740 g a.e. ha−1) resulted in satisfactory M. aegyptia control at 21 DAT (Fig. 2), causing over 90% of plant injury. Moreover, at this evaluation date, glyphosate application at 111 g a.e. ha−1 and 15 g a.e. ha−1 resulted in 39% and 20% of injury, respectively (Fig. 3). Plants treated with 740 g a.e. ha−1 of glyphosate presented a reduction in water use 3 DAT, consuming 25% less water in comparison to the untreated control, whereas the treatment of 111 g a.e. ha−1 resulted
Discussion
This study investigated the effect of glyphosate doses on hairy woodrose water use. Crop-weed competition will affect not only crop yield, but scarce primary resources, such as water. As the profitability of a farm is accounted for its net balance of costs and benefits, water-related costs are one of the main concerns in many agricultural scenarios (Singh et al., 2021). Plant water transpiration increases as plants develops and grows in size, making it necessary to rapidly control the weeds to
Conclusions
Glyphosate at label dose effectively reduced hairy woodrose water use 3 days after treatments and ceased its water use 12 days after treatment. These results highlight the importance of an efficient weed control in order to avoid losses and reducing costs with water management. The glyphosate doses tested did not affect WUE. In addition, glyphosate at 111g a.e. ha−1 increased plant's carbon assimilation (A) and increased iWUE. Further studies are needed to understand the effect of herbicides on
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 and CNPQ (Project code: 140170/2020–0). We thank Bryan Dias, Gustavo Gamboa, and Gabriel Moretti for their assistant in the project.
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