On a sunny day much of the carbon dioxide produced by a green plant may be

Your heart rate will decrease.

Explanation:

By breathing in and holding your breath, you are further activating the PNS  (peripheral nervous system) causing a decreased heart rate which is also known as bradycardia. In addition, holding your breath will start to create a negative suction pressure in your thorax and will commence to inflate the organs.

→ Basically, your heart rate goes up more or your bodies processes are being used and when you exercise your muscles are being used -  this makes your heart rate go up. However, when you hold your breath your heart rate steadily decreases because your lungs aren't be used and your body is trying to reserve the oxygen that you vitally need.

I hope this is helpful!