BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

7. DEVELOPMENTAL PHYSIOLOGY

7.5. Systemic Control of Development

The term "correlations" encompasses interactions that ensure the coordination of developmental processes within a phytomer; in other words, these are systemic processes that unify a multicellular plant Organism into a harmonious whole. They are present even in lower plants, and correlations are particularly pronounced in the well-developed vegetative parts of higher plants. Correlative interactions represent not so much a competition for nutrients as an information exchange often mediated by phytohormones (see 7.6). Plant HORMONES are transported through the conductive strands of the xylem and phloem. Interestingly, however, macromolecules themselves can move over long distances as signaling molecules within the plant body and could therefore participate in correlative regulation. According to these currently still highly hypothetical concepts, regulatory Proteins (such as METABOLISM/31.html">Transcription factors, see 7.2.2.3) or even their mRNAs should not only be capable of moving from Cell to Cell via plasmodesmata (see 7.4.4.1), but in certain cases also of being transported over long distances through the sieve tubes of the phloem, thereby contributing to the correlative control of development.

Correlations occur both in the form of correlative inhibition and correlative stimulation. Stimulation can be based on the supply of nutrients, Vitamins, and growth-stimulating hormones. For instance, an intensively assimilating SHOOT, by providing an adequate supply of assimilates, will stimulate The Development of The ROOT System, which in turn, through its vigorous growth, will once again optimally supply the shoot with Water and mineral salts. However, the shoot also supplies the roots with vitamins and certain phytohormones, such as Auxins, which stimulate root elongation and The formation of lateral root primordia (see 7.6.1), while the root system, in turn, provides the shoot with Cytokinins (see 7.6.2).

Girdling experiments (in which the bast, including the cambium, is cut around the entire circumference of the stem) demonstrate tissue Swelling and often the formation of adventitious roots above the girdled zones. Here, assimilates and basipetally transported auxin (see 7.5.1.3) accumulate, which stimulates radial growth and the formation of adventitious root primordia.

The regulation of fruit set and growth in relation to practical agricultural use has been investigated with particular intensity. Many fruit trees (e.g., apple, pear, peach, plum) initially set significantly more fruits than ultimately mature. In the early phase of development, plants shed numerous fruits. This is a correlative phenomenon; on a short shoot, the fruit that begins development first (the so-called "king fruit")¹ generally inhibits the development of other fruits initiated later. Removal of the "king" fruit leads to the cessation of this inhibition.

¹ The German term "Königsfrucht" literally translates as the largest fruit. — Editor's note.

Correlative inhibition can also be mediated either via nutrient supply or through hormonal interactions. In the first case, this may involve competition for nutrients, for example: an individual fruit becomes smaller if numerous fruits develop, just as individual seeds within a fruit become smaller when several seeds mature on the plant (e.g., horse chestnut). Furthermore, as soon as a plant begins producing fruits and seeds, vegetative growth often noticeably declines.

A widespread example of correlative inhibition is apical dominance. This refers to the preferential growth of the apical bud to the detriment of the lateral buds, even though the latter, due to their position, should experience no shortage either of assimilates supplied via exporting leaves or of mineral salts provided by the roots. Apical dominance varies widely in expression among different species. It is absolute, for instance, in sunflowers (only the apical bud develops), whereas in tomatoes apical dominance is relatively weak: branching begins already at a short distance from the apical bud. Often, the dominance of the apical bud also weakens during the course of plant development; thus, for example, many trees grow in length unbranched at first and branch only after several years.¹

¹ Here the authors refer to the visible growth of lateral shoots, which begins late in ontogeny. However, many trees may develop weakly branched sympodia in the early stages that appear as a single trunk. The issue of main shoot branching must be examined in detail on a case-by-case basis. — Editor's note.

If the apical bud is removed (under natural conditions this happens, for example, due to wind or snow, or As a result of animal grazing), one or more previously suppressed lateral buds will sprout. As a rule, the most rapidly developing and vertically growing lateral bud then takes over dominance and suppresses the further growth of the remaining lateral buds.²

² The given example applies only to vertically growing shoots (orthotropic shoots). In horizontally growing shoot systems (lateral branches of spruce, coffee, rhizomes), the direction of growth of the lateral buds must also be horizontal. — Editor's note.

Apical dominance is explained by the fact that auxin is synthesized in the apical bud and transported downward (see 1.7.6.1): if the apical bud is removed and replaced with an auxin paste (concentration in the µg · g^-1 range), the lateral buds remain suppressed. The Mechanism of this auxin effect is not yet fully understood; however, it appears that the high auxin concentration maintained by the shoot's apical bud inhibits the formation of new conductive elements between the lateral buds and the vascular bundles of the main axis, thereby limiting the nutrient supply to the lateral buds. Following Decapitation, the formation of conductive elements is accelerated. Auxins are also involved in the correlative inhibition of fruit growth. The dominant fruit exports more auxin than other fruits, which induces the correlative suppression of growth. Consequently, in developing-delayed fruits, an abscission zone differentiates at the Base of the fruit stalk, leading to premature shedding (see 7.6.1.4).

Cytokinins supplied from the root system to the lateral buds stimulate their growth (see 7.6.2.3) and can thus, to some extent, counteract apical dominance; however, sustained development of lateral buds also requires the synthesis of auxin within the lateral shoot itself.

Complex correlative control is also characteristic of potato stolon growth (see Fig. 4.11). Typically, they grow horizontally underground, with leaves remaining rudimentary and internodes greatly elongated. If the apical bud and all lateral branches are removed, the stolons turn upward and develop into normal leafy shoots.

Apical dominance also occurs in lower plants. Isolated thallus fragments of the liverwort Lunularia cruciata, for example, regenerate from proliferating thallus Cells, whereas fragments containing the apical tip regenerate strictly from the tip itself. Here, the auxin indole-3-acetic acid (see 7.6.1.1) similarly suppresses regeneration from thallus cells and thereby functionally replaces the apex.

The following correlatively regulated developmental processes will be discussed below:

• organ abscission¹ (see 7.6.5.3): leaves, flowers, fruits, and (for instance, in poplars) sometimes also branches; a process belonging to the normal developmental course of perennial plants;

• senescence² (see 7.6.2.3): age-related changes and, ultimately, the death of the organism.

¹ Abscission in foreign literature. — Editor's note.

² Senescence in foreign literature. — Editor's note.