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In the previous chapter we introduced some of the com-plexity and subtlety of the functioning of plants in diverse, variable and unpredictable environments. Their sessile nature makes it a necessity for plants, if they are to succeed as individuals and populations through many generations, to have the resourcefulness to cope with and adjust to environmental change, especially at the extremes. Remember that plant physiologists are probably the only people on earth who routinely grow plants under constant environments in growth chambers! In this chapter we examine some of the internal mechanisms that plants use to coordinate development. Let us start by considering the concept of a plant not as a packaged collective of independent processes but as a highly interactive network of perception, control and feedback. A plant has a genetic blueprint that specifies its normal morphology and physiology throughout the whole life cycle, but every individual is also shaped, sometimes literally, by the environment it experiences. Think of the bent-over shape of trees growing on coasts with a prevailing on-shore wind, or the ability of pasture plants to recover repeatedly from grazing of their shoot tips.

We might first ask whether plants really need internal communication. The answer lies with multicellularity. With multicellularity comes almost invariably differentiation. Dif-ferentiation is in effect specialisation, which can also be thought of as division of labour. The particular physiological and developmental facets of an organ, tissue or cell type (say, a leaf, a phloem bundle and a guard cell, respectively) make it more efficient at carrying out its set of functions. But with specialisation comes a dependency on the rest of the organism, and a need for coordination among its component tissues. Some of the control is attributable to resource limitations: water, light, CO2 and inorganic nutrients in the environment; water, carbon, nitrogen and mineral fluxes inside the plant. Many of these factors are discussed in Part IV of this book. Depending on quantities and types of resources available, and their mobility in the plant, there are undeniable limits placed on the scope of development. A shoot system can develop only as rapidly as the root mass can supply water and minerals for the shoot structure; a root can grow only if fed with fixed carbon, normally from the shoot. These ubiquitous molecules function as integral parts of cell structures and core metabolism. What we find in addition is another layer of control: information-rich mobile molecules that serve as an integrating communication system throughout the plant.


Table 9.1

Animals have a central nervous system and a suite of specific hormones each with highly defined functions. Plants lack the former, but do possess a quite different set of chemical signals called plant hormones (Table 9.1). Plant hormones are sometimes called ‘plant growth substances’ or ‘plant growth regulators’ partly to distance them from mammalian concepts of hormone action. However, these alternative terms undervalue the repertoire of functions of plant hormones: they affect so many processes other than just growth, so we continue to refer to endogenous regulatory substances as ‘plant hormones’. We also talk later (see Section 9.3) about plant growth regulators as a broader group of active substances applied to plants which includes more than just plant hormones.

Before describing specific plant hormone functions, we need to consider how hormone signals might operate effectively. In any signalling system there is a source and a target, and in between a mode of transmission — in radio parlance, the transmitter and receiver with signals travelling as electro-magnetic airwaves. In animals, the conventional system is a source gland, mass-flow transport (e.g. blood circulation) and a target tissue. Plants are harder to diagnose, but we can make the following generalisations:

• Each hormone can be synthesised in more than one location in a plant. Indeed, all living cells may produce all hormones, but some generate larger quantities and others almost undetectable amounts.

• Each hormone has many functions, at least by deductions from experiments with applied hormones and from phenotypes of hormone-deficient and hormone-insensitive mutants.

• Plant hormones are small molecules and are mobile, both over short (diffusive) and long (mass-flow) distances.

• Many cell types respond to each hormone class.

• Some hormone functions occur in the same cells or tissue in which they are synthesised.

From this, we conclude that a plant’s hormones are indeed quite different from those in animals (Table 9.1). There are relatively few classes but each is multifunctional, they are not synthesised in glands, they move in several channels and affect several tissues in a multitude of ways. A recipe for crossed wires and confusing ambiguity of signals? Perhaps, but as we introduce the major hormones, an overall picture of plant communications will emerge. We now examine signal sources and signal mobility, and then consider how signals are perceived and translated into altered physiology and development.