During hot days you should take care not only of your balcony plants but also of your indoor plants. Both can get sunburnt if they stay in the blazing sun too long. The sun drys out the plants which they need to live and grow. This is noticeable by brown spots or by dry leaves in spite of sufficient watering.
To avoid this, you should keep your house plants away from southern windows. You also can lower shutters slightly to shade them from strong sun rays during the midday sun.
Balcony plants which react sensitively to strong sun should be moved into the shade. Still, sufficient watering is important and should not be forgotten.