I have plots of winter squash with 160 plants per plot. How should I go about deciding how many SPAD readings to take and how many plants to sample from?
Interesting question! Sad to say there is no exact answer, but there are definitely some factors to consider that we can go over. First and foremost, it matters how many plots, the difference between 3 and 30 is substantial and could add hours to data collection. It is also important to consider what is inside each of these plots, is it 160 plants with 10 genotypes and 10 treatments, or 2 treatments. This makes it hard for me to give you an exact recommendation. Maybe for you to start, pick 5 or 6 plants within a plot, and sample two leaves. Also, with any data collection, I believe most people subscribe to the "more the merrier" attitude.
With SPAD, you will not have to worry too much about the time in which it was collected, because even in total darkness you would measure the same relative chlorophyll content. What you do need to be careful about is how the time of day will effect your other parameters. Parameters like Phi2, PhiNO and others take light intensity as a factor, so measuring at high noon versus early morning can yield you very different results even on the same leaf!
I would encourage you and everyone to always try and collect the most, and useful data that you can. Maybe you will discover something new and cool about how your different treatments affect the absorbance of a certain light, who knows!
Feel free to ask more questions, or you can email me if you want to break down your project more precisely and I can work towards giving you a more solid recommendation.