Looks like they may be exposed to too much light and/or just old leaves that are ready to fall off. I would've removed them when they got a little old looking. The other ones you mentioned are the same begonia? What do the newest leaves look like on this begonia?

Here are some ideas on how to treat chlorosis

Wikipedia Reference for Chlorosis

Is that a Lucerne?

Pics can be deceiving but that pot looks really small for that plant, difficult to maintain an even level of moisture. If the roots have run out of room, the foliage will suffer.