For example, when Arabidopsis plants are transformed with a gene that encodes cyclin, a key component of the cell cycle regulatory machinery (see Chapter 1), the cells of the apical meristem progress through their cell cycles more rapidly, so more cells form per unit time. However, the largest component of plant growth is the rapid cell expansion that occurs in the subapical region after cell division ceases.īecause all the cells of the plant axis elongate under normal conditions, the greater the number of cells produced by the apical meristem, the longer the axis will be. Therefore, an increase in cell number, such as the increase brought about by the activity of the apical meristems, does contribute to plant growth. Because the zygote is an especially large cell, this lack of correspondence between an increase in cell number and growth may be unusual, but it points out the potential problem in equating an increase in cell number with growth.Īlthough cell number may not always be a reliable measure of plant growth, under most circumstances dividing cells, particularly in meristems, double in volume during their cell cycle. Reaches the eight-cell stage does the increase in volume begin to mirror the increase in cell number. Then comes the stationary phase, in which the cell number remains constant or even declines as nutrients are exhausted from the medium. This period of rapid growth is followed by a period of slowing growth in which the cell number increases linearly. An initial lag period during which cells may synthesize enzymes required for rapid growth is followed by a period in which cell number increases exponentially. Temperature, light, and nutrients provided are optimal for growth. Growth is assessed by a count of the number of cells per milliliter at increasing times after the cells are placed in fresh growth medium. Only after itįIGURE 16.33 Growth of the unicellular green alga Chlamydomonas. In multicellular plants, however, cell number can be a misleading growth measurement because cells can divide without increasing in volume.įor example, during the early stages of embryogenesis, the zygote subdivides into progressively smaller cells with no net increase in the size of the embryo. In these situations, measurements of dry weight are often more appropriate.Ĭell number is a common and convenient parameter by which to measure the growth of unicellular organisms, such as the green alga Chlamydomonas (Figure 16.33). However, the fresh weight of plants growing in soil fluctuates in response to changes in the water status, so this criterion may be a poor indicator of actual growth. Growth also can be measured in terms of change in fresh weight-that is, the weight of the living tissue-over a particular period of time. However, size is only one criterion that may be used to measure growth.
During this process, cells increase in volume manyfold and become highly vac-uolate. The largest component of plant growth is cell expansion driven by turgor pressure. Growth in plants is defined as an irreversible increase in volume.
0 kommentar(er)
