Concepts: Conditions for growth and exercise practice

5.1 What growth needs

A plant can show its genetic potential only when the conditions around it allow growth. The chapter groups the requirements under a few headings. Water is needed because cell enlargement depends on the uptake of water into the vacuole, and a shortage of water slows or stops growth before it visibly harms the plant. Oxygen is needed for respiration, which supplies the energy for every growth process, so roots in waterlogged soil grow poorly even though water is plentiful. Mineral nutrients supply the elements used in proteins, nucleic acids, chlorophyll and cell walls, and their deficiency changes both the rate and the pattern of growth.

Temperature sets the speed of the enzyme reactions that drive growth. Every plant has a range within which growth is fastest, and above or below that range the rate falls. Light plays two roles: it supplies the energy fixed in photosynthesis, and it acts as a signal that directs development. The response of a plant to light and temperature is often called the extrinsic control of growth, while the genetic and chemical controls inside the plant are the intrinsic controls. Temperature and light reach development by changing the levels and the effects of plant growth regulators, so the same plant can answer a season with flowering, dormancy or germination. The chapter lists vernalisation, flowering, dormancy, seed germination and plant movements among the events that these factors influence.

5.1.1 Growth is generally indeterminate

A plant retains the capacity for unlimited growth through its life in some regions, which is why its growth is called indeterminate. Meristems at the shoot and root tips and the lateral cambium keep producing new cells, so a tree does not stop growing at a fixed size. This is also the reason pruning a hedge or mowing a lawn produces denser growth: the apical dominance is removed and the lateral meristems are released.

5.1.2 Reading growth in the field

A second practical point is that the conditions interact. Plenty of water is of no use to a waterlogged root that has no oxygen, and a high temperature is of no use to a leaf that is short of water and has closed its stomata. The chapter's warning that growth is measurable but not simple follows from this interaction: a single factor rarely controls the rate on its own, and the limiting factor is the one nearest its minimum value. Two measurements are enough for most field questions. The increase in length or height per unit time gives the absolute growth rate and is easy to record with a ruler. The increase in dry weight gives the rate of addition of new material and is the better measure when water content changes. In the field the rate also depends on the season, so a plant may grow fast in spring, slow in summer and stop in winter. That seasonal pattern is a sigmoid curve spread over a year.

5.1.3 Exercise practice

The chapter has one exercise section of ten questions and all are represented across the concept pages. The remaining items are collected here.

Exercise 1. The eight terms asked here are defined on the second concept page of this chapter, and every definition should be written with one example.

Exercise 4. The five groups are auxins, gibberellins, cytokinins, abscisic acid and ethylene; the note on auxin is on page 3.

Exercise 6. Higher plant growth and differentiation are open because new cells and organs keep forming from meristems through life, and the pattern of differentiation depends on position and environment.

Exercise 7. The two kinds of plants respond to different night lengths, so a place whose day length lies between the two critical values can satisfy both at once. The short day plant flowers because its dark period is long enough, and the long day plant flowers because its dark period is short enough. The exercise asks the student to explain this overlap rather than to pick one type as the winner.

Exercise 9. A defoliated plant does not respond to the photoperiodic cycle because the leaf is the organ that receives the light signal. Without leaves the plant cannot read the day length, so the signal that would reach the shoot apex and start flowering is not produced.

Exercise 10. (a) GA3 on rice seedlings makes them grow taller because gibberellin promotes elongation. (b) If dividing cells stop differentiating, an undifferentiated mass forms instead of organised organs. (c) A rotten fruit mixed with unripe fruit ripens the lot faster because it releases ethylene. (d) Without cytokinin in the culture medium a good callus does not form.