The leather's greatest enemy is moisture, not water. Leather becomes mouldy if it is wet for a long time. Don't let that happen.
It may lose its colour when exposed to the sun, but under the tight plastic cover it may become wet - do not leave your bike in bright sunshine all day.
Quick rain may leave spots, but won't destroy the leather.
Dry heat can lead to excessive drying - therefore shoes or other leather items must not be dried on the heater.
Take care of your wonderful saddle. I don't know what leather preservatives you can buy in your area - check what they have for leather clothes. If there is any horse riding accessories shop*, especially old fashioned, they may have best solutions.
Use this protection after drying and before winter storing.
* horses and jockeys are sweating, horse saddles and harnesses are precious - they must be good at caring for the leather.
If your bike is completely stainless and you can safely leave it outside, do not leave it with a leather saddle.
Too much fresh air and changes in the weather are bad for the leather.
If you need the best sun protection - think about an airy cotton or linen bag.
The best place to store leather is a wooden, clean, closed wardrobe.
Don't panic - I've described the best practices - you don't have to do this every day, just do the right thing.