*Article* **Evaluation of Alkaloids Isolated from** *Ruta graveolens* **as Photosynthesis Inhibitors**

**Olívia Moreira Sampaio 1, Lucas Campos Curcino Vieira 2, Barbara Sayuri Bellete 3, Beatriz King-Diaz 4, Blas Lotina-Hennsen 4, Maria Fátima das Graças Fernandes da Silva <sup>5</sup> and Thiago André Moura Veiga 6,\***


Academic Editor: John C. D'Auria

Received: 12 September 2018; Accepted: 6 October 2018; Published: 19 October 2018

**Abstract:** Eight alkaloids (**1**–**8**) were isolated from *Ruta graveolens*, and their herbicide activities were evaluated through in vitro, semivivo, and in vivo assays. The most relevant results were observed for Compounds **5** and **6**–**8** at 150 μM, which decreased dry biomass by 20% and 23%, respectively. These are significant results since they presented similar values with the positive control, commercial herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Based on the performed assays, Compound **5** (graveoline) is classified as an electron-transport inhibitor during the light phase of photosynthesis, as well as a plant-growth regulator. On the other hand, Compounds **6**–**8** inhibited electron and energy transfers, and are also plant-growth inhibitors. These phytotoxic behaviors based on acridone and quinolone alkaloids may serve as a valuable tool in the further development of a new class of herbicides since natural products represent an interesting alternative to replace commercial herbicides, potentially due their low toxicity.

**Keywords:** *Ruta graveolens*; photosystem II; Chl *a* fluorescence; Hill reaction inhibitors; acridone alkaloids

## **1. Introduction**

*Ruta graveolens* L. (Rutaceae) is a medicinal plant whose roots and aerial parts contain more than 120 special metabolites as coumarins, flavonoids, acridones, and furoquinoline alkaloids [1,2]. Many of these metabolites have attracted biological and pharmacological interest, demonstrating antifungal, phytotoxic, and antidotal activities [3–9]. In this context, the effect of the natural products as photosynthesis inhibitors has been efficiently evaluated [10–12]. The photosynthetic process is divided into three parts: the initial light-harvesting process and local charge separation, proton-coupled electron transfer, and multielectronic redox catalysis [13]. During the phenomenon, light absorption by antenna molecules is followed by efficient charge separation across the membrane via photosynthetic reaction centers [14]. The antenna system absorbs and converts light into chemical energy at P680. Accordingly, charge recombination is prevented by the presence of an electron-transport chain driving electrons towards P700; a second light-harvesting process occurs at photosystem I (PSI),

providing additional energy to electrons for their final purpose: production of adenosine triphosphate (ATP) and dihydronicotinamide-adenine dinucleotide phosphate (NADPH), which are used for CO2 fixation through the Calvin cycle (biochemistry phase) [13,14]. Therefore, we analyzed chlorophyll *a* fluorescence kinetic transients to verify the damage on photosynthetic apparatus, demonstrating the quantitative and qualitative effects of herbicides on both photosystems [15,16]. From this perspective, the main goal of this report was to investigate the effects of alkaloids (**1**–**8**) isolated from *Ruta graveolens* L. (Figure 1) on photosynthetic activities through polarography, chlorophyll (Chl) *a* fluorescence, and in vivo plant-growth experiments. Our results suggested that these techniques are powerful and sensitive enough to localize, in detail, the mechanisms of action related to such a complex target, photosynthesis.

**Figure 1.** Alkaloids isolated from *Ruta graveolens*.
