I have not used a temperature gun (infrared thermometer) to measure maize temperature, but I have used a temperature gun.  It should work just fine for giving you the temperature of the maize leaves and stalks.  You could even use it to measure the temperature of the plants on the top surface of the field (perhaps using a tall step ladder to climb above the level of the maize plants) and at various levels down to the ground (for example, eye level, waist level and 1 foot (30 cm) above the ground.  I've not used a temperature gun connected to a data logger, but I suspect such equipment is available.  If you mounted three or four infrared sensors at various levels on a pole, pointed at the maize, you would get a record of the plant temperatures over the course of the days (and nights).

I would add that you will probably also want to investigate the air temperature at the various levels too.  It might be significantly different from the temperature of the plants.  For long term recording of air temperature, I would recommend just a standard weather station temperature sensors (with solar radiation shield) and data loggers, at several heights above the ground.  I would supplement the continuous air temperature record with periodic spot readings using a thermal couple with very low thermal inertia for "instantaneous" readings to correlate with the temperature gun readings that  you could make at the same time.  If you were using a data logger with the infrared thermometers, as suggested above, then you might consider using both the conventional weather station temperature sensors an the low thermal inertia thermal couples.  Using the conventional weather station sensors might seem redundant, but for me it would be important to have a solid correlation to the conventional weather records for the area and they tend to be more robust and durable.

Hello Alain,

I agree with Matthew on using multiple tools for measuring the local temperature and multiple measurements levels.  Furthermore data loggers provide you data for long term  monitoring of your sites.  I have been using weather station temperature sensors but they are costly and not always they can be easy to deploy.  So I have been using miniature sensor T and rH (relative humidity) data loggers called I-buttons. They are very small (1.6cm diameter), can be placed everywhere, are easy to be used (programmed and downloaded) and can last up to 5 years, or more.  Their cost is limited  (~100USD, or less) so you can get many of them for multiple measurements).

http://www.maximintegrated.com/en/products/ibutton/ibuttons/index.cfm

http://www.maximintegrated.com/en/products/comms/ibutton.html

They would be perfect for the application you have in mind. Off course, as for everything else, you would need to verify periodically the values they yield by using calibrated thermometers.

I hope this helps! Lemme know if you need more info.

Best,

Rosalba