Horses have crossed the finish line. During the past week, for two of the test growth spaces, roots reached the bottom of the plate (Fig. 1). While the roots grew, once a day*, I marked the position of the root tip on the back of the square plastic plate by making a horizontal scratch with a single edged razor. Because the scratches are white, they show up well along with the roots when the plate is imaged (photographed, scanned, or as I did once upon a time—xeroxed). In contrast (well, in lack-of-contrast), marks from a black pen disappear into the black background. Loading images such as figure 1 onto my computer, I use the program ImageJ to measure the distance long the root between scratches. Dividing the distance between the scratches by the time between making the scratches (known thanks to my dutiful record of each time when I scratch plates), I get the growth rate (distance/time = rate).
Figure 1. Arabidopsis plants on plates. Left-hand plate is the “Andy” plate; right-hand plate is the “207” plate. Plates were imaged on a flat-bed scanner, which also recorded a reflection, appearing as a ghost displaced slightly from the main image. Horizontal white lines are scratches made by a razor blade at the position of the root tip, once a day. The days of the last two (Andy plate, left) or three (207 plate, right) scratches are shown beside one representative root.
In principle, I could measure the growth every day, between each set of scratches, but the early scratches are obscured by a cornucopia of lateral roots. No matter, the rate on the last day suffices: at that time, out in the clear, roots and scratches are easy to distinguish. Sometimes, two roots grow side-by-side, forming what looks like one root, a coalescence that reduces the number of roots I can measure unless I discern the tip of the shorter root snuggled up against the side of the longer one.
I image the plates by using a scanner. The scanner on-hand is dodgy for the job: you can see a reflection generating “ghost” roots (Fig. 1) . Fortunately, the light came in obliquely displacing the ghostly images: I could distinguish the real from the apparition.
In figure 1, the plate on the left marked “Andy” clearly has longer roots than the one on the right marked “207” and longer too is the distance between the last pair of scratch marks. But, appearances are deceiving**. Being a quantitative type, I measured: Roots on the Andy plates grew at 16.9 ± 0.4 mm/day compared to the 207 plates that grew at 13.5 ± 0.5 mm/day (for the record, these numbers show mean ± standard error of the mean).
The rate in the Andy cabinet, ~17 mm/day is excellent, comparable to rates that I get back at UMass. I can use it. Tomorrow, I’ll be scanning plates from two other growth spaces but I can tell from their progress thru Friday that neither of those spaces will match even 207 so they are not bound for a trophy. The dark horse, or maybe I should say bright horse, is Cabinet C, a fifth space that was made available this past week. I have set of plants there and will see how they do compared to the Andy cabinet. The advantage of Cabinet C is that no one else is using it: thus, I set the lights to be on continuously, a condition that matches conditions back home. The experiment here is independent, conditions don’t have to match; still, consistency with prior work would probably be good. Stay tuned to find out how the plants respond to the never sleeping city of Cabinet C.
*Except for the weekend. That is why figure 1 shows one very long gap between scratches.
**It is not just that appearances are always deceiving; here, there is a specific reason. The images are endpoints, scanned when the roots got near the bottom of the plate; but, they got there on different days. The Andy plate was scanned on Tuesday, the 207 plate on Wednesday. If not for the lateral root extravaganza, you could see that the roots on the Andy plate were scratched one less time. But where things get even more deceptive is that the time intervals between scratches differed on the last two days (Fig. 1). That is, you might think that I would scratch the plates more or less at the same time every day; indeed, I usually get it to within a half an hour. But on Tuesday, life intervened and I scratched plates about 5 h late. Wednesday saw me return to the usual time. Thus, the length of root between scratches made Monday and Tuesday reflects ~29 hours of growth and between Tuesday and Wednesday scratches, only ~19 h. So the distance between the last pair of scratches on the Andy plate looks longer than on the 207 plate not only because roots were growing faster but also because the time interval was longer.