*3.5. H+-ATPase Activity Measurements*

Intact chloroplasts isolated from *S. oleracea L.* were resuspended in a solution of 0.35 M sorbitol, 2 mM EDTA, 1 mM MgCl2.6H2O, 1 mM MnCl2, and 50 mM Hepes medium at pH 7.6. H+-ATPase activity was measured as reported [24]. NH4Cl and DMSO were employed as positive and negative controls, respectively. Pi was quantified using a UV spectrophotometer with measurements in λ = 660 nm.

#### *3.6. Chlorophyll A Fluorescence Measurements in Spinach-Leaf Discs*

Ten 7 mM leaf discs were placed in Petri dishes with 10 mL of a modified Krebs medium containing 115 mM NaCl, 5.9 mM KCl, 1.2 mM MgCl2, 1.2 mM KH2PO4, 1.2 mM Na2SO4, 2.5 mM CaCl2, and 25 mM NaHCO3 (pH 7.4). The Petri dishes were maintained in orbital stirring for 12 h at room temperature. All alkaloids, **1**–**8**, were added to the system for a new period of stirring (12 h). The discs were dark-adapted for 30 min and chlorophyll *a* fluorescence was measured at room temperature through a Hansatech Fluorescence Handy PEA (Plant Efficiency Analyzer, King's Lynn, UK) [16,25].

#### *3.7. Plant Material for In Vivo Assays*

A suspension of *Lolium perenne* seeds prepared with 10% sodium hypochlorite solution was kept in an orbital shaker for 15 min. Then, the sodium hypochlorite solution was removed and the seeds were washed 3 times with distilled water; 100 seeds were placed in 12 cm diameter pots containing a mixture of 50:25:25 (*w*/*w*/*w*) soil/peat-moss/agrolite. All pots were watered daily and maintained in a greenhouse at 25–30 ◦C under normal day/night illumination (12/12 h). *L. perenne* plants were selected by uniformity after being 15 days old. The plants were separated in 3 groups: negative control (DMSO), positive control (50 μM of DCMU), and plants treated with each alkaloid at 150 and 300 μM [12] by being manually sprayed.
