Converting Color Images to Grayscale
The first (and easiest) method is to go to Image/Mode/Grayscale. With this method, Photoshop decides what tonalities to make the grayscale image and you have no control over this. Often this technique will be sufficient but with scientific subjects it will usually be unsatisfactory. In the above example, the gray areas of hypersensitive response in A are important. When converted to grayscale (B) using Image/Mode/Grayscale, these areas blend into the healthy parts of the leaf. If this picture were published it would not make a very convincing argument for a hypersensitive response on the leaf.
Another method of converting color to grayscale which offers greater control over the final tonalities is to use a tool called the Channel Mixer. With the picture you wish to convert opened in Photoshop, click on this button (red arrow) at the bottom of the Layers palette. From the pull-down menu choose Channel Mixer.
The Channel Mixer dialog box opens and check the Monochrome box (red arrow). This will convert the image to grayscale. Now you can move the sliders in the Red, Green, and Blue color channels back and forth until the image looks the way you want. This has the same effect on the tonalities as if the object were photographed with black and white film through colored filters (what we used to do before Photoshop). If possible, it's best to use mainly the Red and Green channels in the mix because the blue channel of a digital image contains the most noise and can appear granular.
When you have a good mix, say OK. Note that the Channel Mixer creates a new layer (called an Adjustment layer) on the original colored image. If satisfied, flatten the image (Layer/Flatten Image) and then convert the image to grayscale (Image/Mode/Grayscale).
Now you can use all the adjustment tools (Levels, Curves, etc) to increase contrast and highlight the important aspects of the image. Compare this photo to the first grayscale one above (B) and I think you'll agree that it does a much better job of highlighting the hypersensitive response in the leaf.