Can I estimate leaf temperature from air temperature? What other information would be required?
One way to do this is to use the "humid operative temperature" equation, or something similar. A variant of this equation, specifically for estimating leaf temperature, is derived using a leaf energy balance equation in chapter 14.1 of the Environmental Biophysics book (cited below). 

I'll let you look up the actual equation since I'm not sure how to do math symbols here. But, in addition to air temperature, you will need to know a few pieces of information:


Vapor pressure deficit - calculated from humidity and temperature.
Incoming radiation - Measured, or you could model this using time of day, latitude, etc.
Emissivity of your leaves - you can probably look this up for your species or a similar one.
Windspeed
Characteristic dimension of leaves - measure, or look up a value for your species (Use this with windspeed to calculate boundary layer heat conductance).
Leaf vapor conductance - a combination of stomatal and boundary layer conductance to water vapor. You must know something about where the stomata are on your leaf (both sides or one side), but you can often look this value up for a given species.
A few other constants that are easy to look up.


That's about it - its a somewhat hairy calculation, but if you have standard weather data and a little knowledge of the plants you are estimating the leaf temperature of, it should be manageable.

This equation and other relevant info are in this book:


Campbell, G.S. and Norman, J.M. An Introduction to Environmental Biophysics. Springer Science, 1998