I think it is also possible that there are thorny plants growing nearby, maybe even in a wild setting. Birds that eat the fruit from that thorny plant would then find your thornless plants producing fruit at the same time. At some point, the bird might poop out the seeds from the thorny plant while visiting your plants. The seeds then grew in your garden. Many weed shrubs come up in hedges because birds drop seeds in the hedge.
Dig gently around the thorny plant stems with a hand trowel to see if the thorny stems are attached by roots or underground stems to the thornless plants. If they are, then it is a thorny stem genetic variation. If they are separate plants, it is more likely to be a seedling plant.
Second, hot, dry air blowing on the plant or just plain low humidity in the house can cause the leaf edges to dry out. Raising the humidity of the whole house is better for the plants and wooden furniture in the house. Adding water to trays under the plants doesn’t work all that well.
Third, too much fertilizer may have been used. Fertilizer chemicals move through the plant and accumulate at the leaf tip and edges. Eventually, there is too much of these chemical salts in the cells along the edges and they die. This may be accompanied by white salt deposits accumulating on the top of the soil and along the edge of the pot. Scoop the salt off the soil and pot and then flood the soil with clean water several times to rinse out some of the fertilizer.
Fourth, the water may have too many minerals, too much chlorine, or too much fluoride, or it may be the wrong pH for that plant. If one of these is the problem, using rainwater or water that has been sitting out for a day to allow the chlorine to evaporate can help.
Trimming the dead areas off the leaf will improve the appearance, but unless the cause is diagnosed and treated, the dead areas will come back.
