young tomato plants

my tomato plants leaves have started going yellow on the leaf ends, and i have used the plastic support pots which raise them up slightly from the grow bag for the first time.Usually I just pop them into grow bag.   any ideas why leaves going yellow and crispy dry brown on ends.  its not the small young bottom leaves im talking about, its the ones above, the larger leaves.?  And Im unsure how much water to give them and whether I just water the trough part of the plastic support pots.  Inherited these support pots and no instructions with them.  unsure on how much water to give them and where i put the water....just under the plant, or in the trough part of the support posts will the roots grow down to reach the water?  any ideas would be welcome.  thks.

http://www.gardenersworld.com/how-to/projects/fruit-and-vegetables/how-to-grow-tomatoes-in-growing-bags/338.html#discuss This is the link to a video by Sarah Raven on this site in the 'How To' section in which she shows how to use the pots you're talking about. Sorry you can't click straight on link, you can copy and paste it into your browser. Or go to the How to section/ fruit and veg...it's on page 5.

Poppy, it sounds like over-watering. I've never grown in grow bags, nor am I familiar with your support pot system, but toms - however they're grown - should never be over-watered. Let the growing medium dry out between waterings.