agronomy Article Effects of Various Environmental Conditions on the Growth of Amaranthus patulus Bertol. and Changes of Herbicide Efficacy Caused by Increasing Temperatures Hyun-Hwa Park 1, Do-Jin Lee 2 and Yong-In Kuk 1,* 1 Department of Bio-Oriental Medicine Resources, Sunchon National University, Suncheon 57922, Korea; [email protected] 2 Department of Agricultural Education, Sunchon National University, Suncheon 57922, Korea; [email protected] * Correspondence: [email protected] Abstract: Understanding the effects of climate change on weed growth and herbicide activity is important for optimizing herbicide applications for effective weed control in the future. Therefore, this study examined how climate change affects the growth of Amaranthus patulus and the efficacy of soil and foliar herbicides at different temperatures. Although the control values for A. patulus differed between herbicides and temperature, the control values increased with increasing time after the herbicide treatments. Under growth conditions in which the temperature remained constant, the efficacy of soil-applied herbicides, ethalfluralin, metolachlor, linuron, and alachlor, on A. patulus was highest when the weeds were grown at high temperature. In particular, 100% control values of A. patulus were achieved in response to metolachlor treatments at the total recommended dosage in growth chambers at 35 ◦C. The efficacy of foliar herbicides, glufosinate-ammonium, bentazone, Citation: Park, H.-H.; Lee, D.-J.; Kuk, and mecoprop, on A. patulus was also highest when the plant was grown at high temperature, Y.-I. Effects of Various Environmental except for glyphosate isopropylamine, which had similar efficacy rates regardless of the temperature. Conditions on the Growth of A. patulus was 100% controlled in response to glufosinate-ammonium, bentazone, and mecoprop Amaranthus patulus Bertol. and at the recommended dosages in growth chambers at 30 and 35 ◦C. Under growth conditions in Changes of Herbicide Efficacy Caused by Increasing Temperatures. which the temperature changed from day to night, the efficacy of soil-applied herbicides, alachlor Agronomy 2021, 11, 1773. https:// and linuron, on A. patulus was highest when the weeds were grown at high temperature. On the doi.org/10.3390/agronomy11091773 other hand, the efficacy of the soil-applied herbicides metolachlor and linuron on A. patulus was similar regardless of the temperature. The efficacy of foliar herbicides, glyphosate isopropylamine, Academic Editor: David Clements glufosinate-ammonium, bentazone, and mecoprop, on A. patulus was highest when the weeds were grown at high temperature. Although herbicide efficacy varied depending on whether the weeds Received: 19 August 2021 were grown at constant or alternating temperatures, herbicide efficacy was generally highest when Accepted: 2 September 2021 the temperature was high. Published: 3 September 2021 Keywords: Amaranthus patulus; climate change; herbicide; temperature; weed Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Among the many consequences of climate change, rising temperatures [1] and altered precipitation patterns are having the most significant impact on agriculture because of their ability to increase the probability of summer droughts [2–4]. In addition to these direct Copyright: © 2021 by the authors. consequences of climate change, indirect consequences are expected to affect sustainability Licensee MDPI, Basel, Switzerland. and food security [5]. In many ways, weather extremes associated with climate change are This article is an open access article distributed under the terms and a very serious concern for crop management. conditions of the Creative Commons The impact of climate change on weedy vegetation may be manifested in the form of Attribution (CC BY) license (https:// geographic range expansion (migration or introduction to new areas), alterations in the creativecommons.org/licenses/by/ species life cycles, and population dynamics [6]. Increasing CO2, temperature, and water or 4.0/). nutrient availability may allow new weeds to become more problematic or existing weeds Agronomy 2021, 11, 1773. https://doi.org/10.3390/agronomy11091773 https://www.mdpi.com/journal/agronomy Agronomy 2021, 11, 1773 2 of 16 to expand their geographical locations [7]. Weeds respond quickly to resource changes and are more likely to adapt and flourish in various habitats owing to their greater genetic diversity and physiological plasticity than crops [8]. Increasing atmospheric temperatures could promote the growth of some weeds in warm-season crops cultures. A 3 ◦C rise in the average temperature enhanced biomass and leaf area of itch grass (Rottboelliia cochinchinensis) by 88% and 68%, respectively [9]. An increase in temperature due to global warming might trigger weed migration. Milder and wetter winters would tend to increase the survival of winter annual weeds. In contrast, thermophile summer annuals will grow more profusely in areas with warmer summers under prolonged growing seasons, enabling them to grow further north [10,11]. Herbicides have become the major tools for weed management because of their simplicity in use, great efficacy, and, more importantly, reduced control costs by saving labor and time [12]. The successful use of herbicides depends on environmental conditions before, during, and after herbicide application. The environment influences the growth and physiology of plants, as well as herbicide activity and the interaction between plant and herbicide. Therefore, understanding how environmental conditions affect herbicide performance is important for realizing the impact of climate change on herbicide efficacy. Environmental factors, such as light, CO2, temperature, soil moisture, relative hu- midity, rainfall, and wind can affect herbicide efficacy directly by altering the penetration and translocation of herbicides within the plant or indirectly by changing the growth and physiological characteristics of the plant. While foliar herbicides are influenced by many environmental factors, soil-applied herbicides are influenced mainly by soil moisture and temperature [8]. Temperature can affect herbicide performance directly through its effects on the rate of herbicide diffusion, viscosity of cuticle waxes, and physicochemical properties of spray solutions [13]. New and effective herbicides may be needed if new weeds are introduced into a non- native area. A. patulus is a troublesome exotic weed of upland crops. Understanding the effects of climate change on weed growth and herbicide activity is important for optimizing herbicide applications for effective weed control in the future [8]. Temperature has both direct and indirect effects on herbicide efficacy. However, the underlying mechanisms responsible for varying rates of herbicide efficacy at different temperatures is poorly understood and needs investigation for a better management of weeds. Therefore, this study examined how climate change and different temperature condi- tions affect the growth of Amaranthus patulus and the efficacy of soil and foliar herbicides. 2. Materials and Methods 2.1. Plant Materials Seeds of A. patulus were collected from a field used for corn production at one of Sunchon National University’s research farms in South Korea in the fall of 2020. The seeds were refrigerated at 4 ◦C until use. 2.2. Environmental Conditions for Growth of A. patulus Fifteen seeds of A. patulus were planted in small trays (18 × 13 × 9.5 cm3) with a commercial potting mixture (Sunghwa CO. Bosung, Chonnam, South Korea). The trays were placed in growth chambers (Multi-room Incubator, VS-1203PFC-LN, Vision Bionex, Buchon, South Korea) at different temperatures (15, 20, 25, and 30 ◦C). For the CO2 study, CO2 chambers (HB-303DH-0, Hanbaek Scientific Co. Buchon, Gyeonggido, South Korea) were set to 400 or 700 ppm, and a temperature of 25 ◦C was maintained throughout the experiment. Other growth chamber conditions were 60% relative humidity, 14/10 day/night photoperiod, and 100 µmol m−2 s−1 light intensity. For seeding depths and shading degree experiments, the experiment was conducted outdoors with windbreaks. The seeds were planted at depths of 0, 0.5, 1, 2, 3, 5, and 7 cm in loam-filled small trays (18 × 13 × 9.5 cm3). Seeds that had been planted at a depth of 0.5 cm in loam-filled small trays (18 × 13 × 9.5 cm3) were used for the shading degree experiments. Agronomy 2021, 11, 1773 3 of 16 After seeding, a light-shielding polyethylene film (Morpho Inc., Dasan, Gyeongnam, South Korea) was installed to reduce natural light exposure by 20, 35, 50, 75, and 90%. The control was exposed to full natural light. During the experiment, the temperature was 28 ± 2 ◦C/18 ± 2 ◦C at day/night. There was no rainfall, and proper soil moisture was maintained. The germination rates were measured 3, 4, 5, 6, 9, 10, 11, 12, 16, and 17 days after seeding, whereas plant height, leaf area, and shoot fresh weight were measured 17 days after seeding. Leaf area was measured by an LI-3100 area meter (LI-COR, Inc., Lincoln, NE, USA). 2.3. Herbicide Efficacy in A. patulus Grown in a Growth Chamber at Different Temperatures Five seeds of A. patulus were planted at a depth of 1 cm in a loam-filled plastic cup (150 mL) and placed in a growth chamber at 25 ◦C (Multi-room Incubator, VS-1203PFC-LN, Vision Bionex, Buchon, South Korea). The other growth chamber conditions were 70% relative humidity, 14/10 h of photoperiod (day/night),