E12 Growth

Topic

Plant growth kinetics describes the rate at which a plant grows; this growth rate depends not on a single process, but on at least three distinct processes occurring within the plant: cell elongation, cell division in the meristems, and secondary xylem growth. Each of these three processes proceeds at its own pace, governed by its own specific variables.

Cell elongation occurs when the turgor pressure inside a cell pushes against its cell wall with greater force than the wall can withstand without stretching; the speed at which the cell elongates depends—in addition to turgor pressure—on wall extensibility (i.e., how easily the cell wall stretches in response to that pressure). The higher the turgor pressure and the greater the wall extensibility, the faster the cell elongates—a relationship quantitatively described by the Lockhart equation.

Cell division takes place in the meristems—regions of the plant where cells retain the ability to divide—and the rate of this division is sensitive to both temperature and carbon availability. The closer the temperature is to the optimal range for the meristems, and the more carbon available to meristematic cells for building new cells, the faster cell division proceeds within the meristems.

Unlike cell elongation and meristematic cell division, secondary xylem growth does not lengthen the plant but rather increases its thickness. The xylem adds successive layers of woody tissue around the stem, and each of these layers is recorded as a growth ring; thus, growth rings serve as a cumulative record of how much the xylem grew during each season. These three processes of plant growth kinetics do not progress at the pace of calendar time, but rather at the rate determined by the accumulated temperature—measured in degree-days—that the plant has gathered. The more accumulated temperature the plant has acquired, the further its phenological stage advances, regardless of the number of calendar days that have elapsed; thus, accumulated temperature predicts the plant's phenological stage—and, consequently, the extent of cell elongation, meristematic cell division, and secondary xylem growth—with greater precision than calendar time itself.

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