Score one for older roots

Hooray! Results from this past week’s experiment matched the famous Oct. 30th run. After 5.5 h, the root growth rate stabilized, and the overall rate of growth was 2 mm / h, a speed that is healthy for a maize root (Figure 1). What the Jan 11th run shares with that of Oct 30 is shoots! That is, in both runs, the coleoptiles (seeding shoot) had emerged a centimeter or so above kernels. Shoots take longer to get growing and in all of the other trials, shoots had yet to emerge. Maybe roots proceed a little tentatively until the shoot is on its way? Or maybe this is a coincidence? As usual, have to repeat.

Figure. Root elongation versus time. Note the generally faster and more stable growth for Jan. 11 (triangles). Further details given previously.

While I am pleased with the more stable and faster growth, the Jan 11th experiment did have a problem or two. For all of these experiments where seeds are germinated beneath a doofus, when I transplant the seedlings to start the experiment, at least half of the roots are in air, not touching the surface of the wet paper substrate. This time, the ones in the air were similar in length to the ones growing on the wet paper, about 3 to 4 cm long; evidently growing in air was not setting them back. The Petri plate housing this operation is sealed with air-permeable bandage tape. Water vapor gets out too but slowly. Humidity in the air is high. Still, it seems odd that the roots ignore the nice wet surface in preference for drier air. It also creates two distinct populations of seedlings, which in principle is best to avoid.

And during the experiment the roots continued to flout the water supply. When I transplanted the seedlings onto the agar medium for imaging, I make sure to place them so the roots make good contact with the surface of the agar. When I came back the next day, after all the images had been captured, many of the of the roots had grown up off the agar surface and into the air. When I measured root growth, I could see them lifting off, about half way through the time course. This slowed the roots a little, as evident by the slope of the line for Jan 11 being tilted a little bit toward the lower right (Figure 1).

I can live with air-prone roots, but the behavior unsettles me. And as usual I have something to try. For all of these runs, I have been placing the kernels embryo up. On Friday, I set up some plates for germination with the embryos facing down, touching the wet paper surface. With this placement, the root will be contacting the wet surface from the start, rather than being a kernel’s width above. Will this really make a difference? I’ll find out.

Also, when I transplant this new set onto agar, I will likewise put the seedling face down. I want to keep things consistent, if nothing else. I have no reason to think that the kernel placement would influence whether these older roots as they grow will forsake the agar surface, but I might as well check.

And finally, I had trouble with my doofus. The foam doofus that readily holds seeds up on plates containing moist paper turns out to fail for holding seedlings on agar. The doofus is too thick; the agar is thicker and far less compressible than is the moist paper. I discovered this in the midst of setting up the Jan 11th experiment; I had no time to hike back to the lab and fashion a thinner doofus. With little alternative, I laid the doofus across the row of roots. This held them up all right, but might have stressed the little things. For the next experiment, I have now made the required thinner doofi. Monday or Tuesday, depending on when the shoots pop, stay tuned.