Plant Growth , Antioxidative Enzymes and Lipid Peroxidation in Sunflower Seedlings Supplemented With Eichhornia crassipes Organic Fertilizer Under Drought Stress Conditions

In the semiarid region of the Brazilian Northeast, there is still the occurrence of soils with low concentrations of organic mass and nutrients. Eichhornia crassipes (water hyacinth) is recognized as one of the top ten endemic herbs in the world. However, its accumulation capacity means it can be an alternative source of nutrients. The objective of this study was to analyse the effects of macrophyte organic residue (ROM) on plant growth, antioxidative enzyme activity and membrane lipid peroxidation in leaves and roots of sunflower seedlings submitted to drought stress conditions. The experiment was conducted under greenhouse conditions at the Instituto Federal de Educação, Ciência e Tecnologia do Ceará, Maracanaú Campus, Brazil. Samples of E. crassipes were collected in the Parangaba Lagoon, Fortaleza, Ceará, Brazil. The treatments were: 1) 100% (by volume) sand; 2) sand + fertilizer (following the recommendation of 80 kg of N/ha), and 3) sand + 100% of the recommended nitrogen in macrophytes (RN). In general, the use of ROM caused better seedling growth in relation to the other treatments in all conditions studied. Increases in antioxidative enzyme activity and reductions in the deleterious effects of drought stress on plant growth were observed.


Introduction
Water deficit is one of the main agricultural problems that reduces crop yields in arid and semiarid regions of the world, including the Brazilian Northeast (Farooq et al., 2009;Niu et al., 2017).These regions present irregular distribution of precipitation, high evaporation rates, and shallow and nutrient-poor soils (Santos et al., 2010).Despite these limiting factors, irrigation practice is the best way to ensure agricultural production, which together with organic fertilization can make soils more fertile and productive (Nobre et al., 2011;Finatto et al., 2013).catalase (CAT), ascorbate peroxidase (APX) and guaiacol peroxidase (GPX) which aim to reduce ROS concentrations (Gill & Tuteja, 2010;Gonçalves, 2017).
In the semiarid region of the Brazilian Northeast, there is still the occurrence of soils with low concentrations of organic matter and nutrients such as nitrogen and phosphorus (Esteves & Meirelles-Pereira, 2011).In this region, there is also a large presence of floating macrophytes in the water bodies, which may be related to disturbances in their short flood periods and prolonged dry periods (Pedro et al., 2006;Henry-Silva et al., 2010).
Macrophytes are plant organisms distributed throughout the world in numerous humid environments and which have high nutrient storage capacity (Bonanno & Lo Giudice, 2010).They are among the groups that produce organic matter and play an important role in the geochemistry of wetlands.In addition, because of their accelerated decomposition, macrophytes when deposited in the soil can enrich it nutritionally (Dibble, 2005;Vodyanitskii & Shoba, 2015).
Eichhornia crassipes (water hyacinth) is a free-floating macrophyte native to South America, and one of the most studied for phytoremediation purposes (Gupta et al., 2012;Melignami et al., 2015).This species reproduces sexually and asexually and can quickly colonize new areas (Villamagna & Murphy, 2010).It is present in 65 countries and is recognized as one of the ten main endemic herbs in the world.Its large accumulation can characterize problems such as eutrophication, where this species functions as a bioindicator (Shanab et al., 2010).However, this plant can remove heavy metals, nutrients and sediments from water (Buta et al. 2011).In view of this, E. crassipes could be used in the composition of substrates for plants, functioning as an alternative source of nutrients.In addition, it could be an alternative destination for this waste.
The sunflower (Helianthus annuus L.) is an oleaginous species that has been gaining ground in the Brazilian Northeast because it has a good tolerance to heat and drought.Among the characteristics of this crop, the following stand out: rusticity, high productivity and the quality of its oil, besides the potential for honey production and use in poultry rations (Lira et al., 2009;Souza et al., 2010;Santos et al., 2015).
Thus, the present work analysed the effects of using macrophyte organic residue (ROM) on the initial growth of sunflower (H.annuus L.) plants under conditions of drought stress, evaluating plant growth, antioxidative enzyme activity and membrane lipid peroxidation.

Method
The experiment was conducted under greenhouse conditions at Instituto Federal de Educação, Ciência e Tecnologia do Ceará (IFCE), Maracanaú campus, Ceará, Brazil, from October to December 2016, totalling 30 days.The mean values of temperature and relative humidity were 30 °C and 56%, respectively.
Samples of E. crassipes (water hyacinth) were manually removed from the Parangaba Lagoon, Fortaleza, Ceará, Brazil, and arranged to dry in the full sun for 20 days.Subsequently, the dry mass was crushed by a mechanical crusher and sent to the Laboratory of Biochemistry and Plant Physiology of the IFCE Maracanaú campus to finalize the drying process in an oven with forced circulation of air at 60 °C.
The material obtained was called ROM.After this process, a sample of the material was sent for analysis in the Laboratory of Soils/Water of the Federal University of Ceará (Table 1), and the results for nitrogen (N) content were used in calculations of amounts of ROM added to the substrates.For the commercial fertilizer treatment, the data of the analyses performed were provided by the manufacturer Terra Vegetal (Table 1).
ants on substra ntrol conditions In addition, both the leaves and roots of plants treated with ROM showed higher APX and GPX activity when compared to the other treatments.This may be associated with the behaviour of these species which function as a secondary mechanism in the elimination of ROS when compared to CAT, which first contributes to H 2 O 2 detoxification (Bhatt & Tripathi, 2011).
In addition to these defence mechanisms, it is important to quantify lipid peroxidation, which produces MDA which is used to determine process intensity in the lipids of plant cell membranes.Thus, an increase of this substance is directly associated with indications of oxidative stress (Hendges et al., 2015).
It should be noted that in both stress and control conditions, the treatments containing ROM had the lowest MDA levels.Moreover, similar results were found in other studies such as that of Messchmidt et al. (2015) who observed high levels of MDA in Prunus spp.under conditions of drought, and Silva (2010) who found a significant increase in the concentrations of MDA in sugarcane plants also submitted to drought.Maia et al. (2012) concluded that increases in the activity of enzymes such as SOD, APX and CAT are associated with a reduction of lipid peroxidation (MDA) in plants under conditions of drought stress.Thus, it is suggested that the use of ROM causes greater antioxidative enzyme activity and provides a reduction in lipid peroxidation and the deleterious effects of drought stress.

Conclusions
In the present experimental conditions, the use of ROM caused an increase in plant growth (SDM, RDM, TDM and leaf area) and antioxidant enzyme activity (SOD, CAT, APX and GPX) which contributed to a reduction of membrane lipid peroxidation (MDA) and the deleterious effects of drought stress, both under control conditions and drought stress, when compared to the other treatments.
Thus, the use of dried and crushed E. crassipes to add nutrients and organic matter to the soil is suggested.
Subsequent studies should be carried out to verify the use of this residue on a large scale and to act as an environmentally correct destination.