Fig. 1.3 Recovery to the original leaf:root ratio of common bean plants (F) after removal of the leaves (•) and the roots (■). No recovery of the ratio was found when the growing parts of the shoot were removed continuously (▲). (Redrawn from Brouwer, 1963.)
partitioned to roots and shoots in inverse proportion to the rates at which they capture resources. Davidson (1969) expressed this as:
The consequence of this statement is that the root:shoot ratio of a plant will vary to compensate for changes in root and shoot activity induced by changes in the edaphic and atmospheric environments. It explains why small root systems may be sufficient for maximum plant growth when the supply of water and nutrients is optimal (e.g. in horticultural production systems) and why managing the soil to produce more roots may be counterproductive (van Noordwijk and de Willigen, 1987). Most investigation of this hypothesis has focused on nitrogen uptake and photosynthesis in young, vegetative plants (e.g. Thornley, 1972), with subsequent refinements to allow for dynamic responses to changes in environments. Johnson (1985) found that during balanced exponential growth, a relationship analogous to that proposed by Davidson (1969) applied, but that rates of uptake influenced partitioning through effects on substrate levels. The consequence was that there was no unique relationship between shoot and root activities (cf. Davidson's  proposition), although over restricted ranges of root and shoot specific activities the linear relation implied by equation 1.1 held. A difficulty with exploring this concept further is that while shoot mass and photosynthesis can be measured relatively easily, the capture of below-ground resources and determination of the resource that is most limiting at a particular time poses many problems. Hunt et al. (1990) drew attention to some of the difficulties which include: (i)
root mass x specific root activity (absorption) = shoot mass x specific shoot activity (photosynthesis)
quantifying the fraction of a nutrient that is available at a particular time; (ii) allowing for the different nutrient contents of different soil layers; (iii) allowing for the differential rates of nutrient transport to different roots depending upon extant gradients of concentration; and (iv) allowing for the different depths and spatial distributions of roots when plants are grown in communities.
Farrar and Jones (2000) suggested that the functional equilibrium hypothesis is useful for describing how environmental factors such as light, water, N and P affect the relative growth of roots and shoots, but showed that it was inadequate for many other situations and that it lacks a physiological and mechanistic basis. Johnson (1985), too, commented on the lack of mechanistic understanding of resource and growth partitioning. Farrar and Jones (2000) proposed that acquisition of carbon by roots is determined by both the availability of, and need for, assimilate. This leads to the hypothesis that import of assimilates to roots is controlled by a range of variables in both root and shoot ('shared control' hypothesis) and that there is shared control of growth between leaves (the source of C) and roots (the sink for C). The mechanisms proposed to allow this control were phloem loading, and gene regulation by sugars and other resource compounds. However, while there is evidence for the coarse control of phloem loading in response to sink demand for carbohydrate, there is no evidence of fine control (Minchin et al., 2002), and while there is some evidence of gene up- and down-regulation by sucrose in laboratory conditions, there is little evidence for gene regulation by resources in field-grown plants. In summary, while there is evidence to support the hypothesis that control of C flux to roots is shared between the many processes contributing to whole-plant C flux, no good mechanistic model of this phenomenon currently exists.
Normal development of plants depends on the interaction of several external (e.g. light and gravity) and internal factors, with plant hormones being part of the internal factors that play a major role in regulating growth. Many hormones are produced in one tissue and transported to another where they influence the rate and nature of plant development and growth. For example, indole-3-acetic acid (IAA) is an auxin that is synthesized mainly in leaf primordia and young leaves, but plays a major part in the growth response of root tips to gravity (see section 5.2.1). Although IAA has been measured in root tips (typically in concentrations of about 150 kg-1 fresh weight of root, or 5 x 10-7 M) most evidence suggests that it is not produced there but transported from the shoot via the vascular system (Torrey, 1976; Raven et al., 1999). It has been shown that decapitated plants produce less auxin and also have decreased rates of root growth, and that application of auxin to the decapitated shoot tip restores root growth. Auxins also interact with shoot-produced gibberellins in regulating expansion of root cells (Dolan and Davies, 2004). Conversely, cytokinins (of which the most common is zeatin, 6-[4-hydroxy-3-methyl-2-transbutenylamino] purine) are synthesized primarily in root tips and transported to shoots via the xylem where they regulate cell division and, in more mature plants, the rate of leaf senescence.
In all, five groups of plant hormones (auxins, cytokinins, abscisic acid, gibberellins and ethylene) have long been recognized as regulating plant growth, but more recently other chemical signals have also been identified as important. These signals include the brassinolides (related to animal steroids) which stimulate cell division and elongation, salicylic acid which activates defence responses to plant pathogens, the jasmonates which act as plant growth regulators, and small peptide molecules such as systemin which activate chemical defences when wounding occurs (Raven et al., 1999). A practical application of the role of hormones is the use of auxin to stimulate the initiation of roots from stem cuttings. This practice is widely used by horticulturists to ensure the vegetative propagation of many plants.
There is now strong experimental evidence that root signals to the shoot modulate growth responses of the shoot. This has been most thoroughly explored in relation to soil water deficits where abscisic acid (ABA) is believed to play a major role (see section 4.2.2), but other properties of the soil, especially its strength, also play a part. For example, Passioura and Gardner (1990) investigated the effects of soil drying on leaf expansion of wheat leaves, by measuring changes in soil water potential, soil strength and phosphorus availability of plants that were either pressurized to maintain high leaf turgor or unpressurized. Their results demonstrated no significant effects of the pressurization treatment on relative leaf elongation rate (RLER), and that the plants were sensing both the water status and the strength of the soil but not the availability of phosphorus. Figure 1.4 shows that RLER decreased as the soil dried even when plants were grown in loose soil (with penetrometer resistance <1 MPa), and that RLER of plants in drying, dense soil (penetrometer resistance 2-5.5 MPa) fell below that of the well-watered controls at a much higher soil water content (0.23 g g-1, equivalent to about 100 kPa tension) than in loose soil (0.17 g g-1, equivalent to about 270 kPa tension). These results suggest that the roots were sensing both the tension and strength of the soil and sent inhibitory signals to the shoots which reduced leaf expansion as either tension or strength increased. The precise nature of the signals is still unknown, although auxin and cytokinins have both been suggested to play a part (Davies and Zhang, 1991).
At the other end of the life cycle, leaf senescence is also affected by plant hormones, with cytokinins and ABA playing a direct role in the regulation of drought-induced leaf senescence (Yang et al., 2003). Drought enhances ABA levels which increases carbon remobiliza-
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