The lowest I50values were found in lettuce for root length (4.4 g a.i. ha-1), shoot fresh biomass (8.4 g a.i. ha-1), root fresh biomass (3.5 g a.i. ha-1), and total fresh biomass (7.6 g a.i. ha-1). Otherwise, the lowest I50 value for plant height was estimated for sorghum (9.7 g a.i. ha-1). Khalil et al. (2018b) found I50 of 2.4 and 18.4 g a.i. ha-1 of pyroxasulfone for shoot height of annual ryegrass (Lolium multiflorum) and cucumber (Cucumis sativus), respectively. The I50 values for shoot height of cucumber found by the authors are close to those obtained in this work (21.2 g a.i. ha-1).
The I50 indexes provide evidence that lettuce was most susceptible species to pyroxasulfone, followed by cucumber and sorghum. Assuming the highest rate (100 g a.i. ha-1) did not affect the development of tomato plants, we were not able to estimate I50, and thus suggesting that tomato is not an effective option as a bioindicator. In summary, the order of sensitivity among the species evaluated in this research was lettuce > sorghum ≥ cucumber>tomato.
In order to identify the most useful bioindicator for pyroxasulfone soil activity, it is critical not only to consider the species susceptibility but also its adaptability. In this research, for example, lettuce was the most susceptible species to pyroxasulfone. However, some difficulties in the use of this species in practical tests can be encountered due to sensibility to adverse climatic conditions. Conditions such as high temperatures and tropical rain regimes can hinder their development and damage plants (Bezerra Neto et al. 2005; Gomes et al. 2005; Brzezinski et al. 2017). Because of some of these issues, it is extremely important to evaluate more than one species in bioindicator studies. Here, we demonstrated that sorghum may also be a suitable species to detect pyroxasulfone soil activity, manly based on plant height measurements (I50 = 9.7 g a.i. ha-1). In addition to ryegrass (Khalil et al. 2018b) and sugar beet (Szmigielski et al. 2014), the present work provides at least three additional species (lettuce, sorghum, and cucumber) that can be used as a bioindicator for pyroxasulfone.
Lettuce was the most useful species selected for pyroxasulfone soil studies. Sorghum and cucumber also showed to be potential candidates as bioindicators. Tomato was considered tolerant to pyroxasulfone, and therefore not considered a good option as a bioindicator for pyroxasulfone.
This research was funded in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) - Brazil, and Núcleo de Estudos Avançados em Ciência das Plantas Daninhas of Universidade Estadual de Maringá (NAPD / UEM) - Brazil.
Conflicts of Interest
The authors have declared no conflicts of interest.
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