Benefits of Accumulation via Ion Trapping

Now that the herbicide is in the cell, the chloroplast can accumulate the herbicide to an even greater degree. This situation will be very beneficial for herbicides that act on photosynthesis. Here’s how: During photosynthesis, the pH of the chloroplast’s stroma can become even more alkaline than the cytoplasm (i.e. pH 8), because of the light reactions. This high pH in the chloroplast will increase its ability to accumulate the anions of weak acids, creating extremely high concentrations of weak acid herbicides near sites of action in the chloroplast. Notice in Figure 9a that the acid (X-COOH) has just traversed the chloroplast’s membrane from the cytoplasm. Then in Figure 9b, the acid has dissociated immediately into the conjugate base (X-COO-) and its H+. As a result, the conjugate base will accumulate in the alkaline stroma of the chloroplast because the charge reduces its ability to move back out across the membrane. Therefore, a weak acid herbicide, such as bentazon, can accumulate to extremely high concentrations in the stroma.

Figure 9a. A weak acid herbicide molecule has moved from the cytoplasm into the chloroplast via a concentration gradient. (Image by Dusti Duffy, Tracy Sterling, Scott Nissen, and Deana Namuth)

Figure 9b. The weak acid herbicide molecule immediately breaks down into its conjugate base and H+ upon entering the chloroplast. (Image by Dusti Duffy, Tracy Sterling, Scott Nissen, and Deana Namuth)