BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

9. ALLELOPHYSIOLOGY

9.5. Allelopathy

Allelopathy refers to the chemical effect of one plant upon another. This interaction can be stimulatory or, more frequently, inhibitory. The effect may be mediated, on the one hand, by volatile compounds that reach sufficient concentrations to become active within a diffusion or convection chamber. For instance, Ethylene acts both as a pheromone (signaling molecules that coordinate activities

between individuals of the same species) and as a kairomone (signaling molecules that operate between individuals of different species). However, whether sufficiently high concentrations of ethylene actually build up in the ambient air in nature remains an open question. Nevertheless, during the TRANSPORT AND STORAGE of fruits (especially when co-storing varieties or species with differing sensitivities), one must account for accelerated senescence driven by released ethylene (see

7.6.5.2). Another subject of ongoing debate is whether the release of alarmones by insect-damaged plants observed in experiments (see 9.4.2) serves a signaling function, alerting neighboring, undamaged plants to activate their defenses against herbivores. Furthermore, many plants release Water-soluble compounds into the soil via their ROOT systems, which can limit the growth of competitors. Finally, in some species, active compounds are leached out by precipitation and enter the soil via runoff.

Although it is well established that allelopathically active inhibitors frequently play a role in plant communities characterized by intense competition, providing unequivocal proof is difficult because these effects overlap with others (such as competition for light and nutrients). Moreover, many compounds identified as allelochemicals cause highly unspecific inhibition in microorganisms and animals as well (at least in experimental settings). These include numerous simple phenols (such as cinnamic acid and its derivatives, e.g., coumarin, see 6.16.1) and terpenoids (see 6.16.2). Such compounds predominantly inhibit the germination of certain species; it therefore seems plausible that their accumulation in the upper soil layers (particularly through leaf leaching or The breakdown of membranous scales) reduces intra- or interspecific competition for the recruitment of newly emerging individuals.

For example, 1,4,5-trihydroxynaphthyl-4-glucoside is leached from the leaves and fruits of the walnut tree (Juglans regia) and converted in the soil through Hydrolysis and subsequent oxidation into juglone, a potent germination inhibitor (Fig. 9.23). Consequently, In addition to the factors of light (canopy shading) and nutrients (soil depletion in the root zone), unfavorable conditions are created for seed germination, leaving only a sparse herbaceous understory beneath the tree's crown.

Class="center">Fig. 9.23. Allelopathic effect of juglone from the walnut tree (Juglans regia). Juglone blocks prolyl oligopeptidases, which are required for the correct conformation of Cell Cycle Proteins, thereby arresting The Cell cycle at the G₂ phase. This mechanism likely accounts for the direct Toxic Effect of juglone on vigorously growing seedlings

A clear-cut example of allelopathy can be observed in the shrub communities of southern California in the Santa Ynez Valley (see 15.2.7), known as chaparral. The vegetation here is dominated by dense stands of Salvia leucophylla and Artemisia californica, which increasingly displace herbaceous plants. A zone about 1–2 m wide surrounding the shrubs is entirely devoid of other plants; within a 3–8 m radius, plant growth is stunted; and only at greater distances do unaffected plants appear, primarily grasses (e.g., Bromus hordeaceus, Festuca megalura, and Avena fatua). This suppression of grass growth is exclusively attributed to monoterpenes toxic to herbaceous plants—chiefly camphor and 1,8-cineole (see Fig. 6.123)—which are released by Salvia leucophylla and Artemisia californica, especially under high air temperatures. These monoterpenes accumulate in the upper soil layers through adsorption onto the colloidal fraction of loamy soils. From there, these highly

lipophilic compounds (via gas-phase equilibrium distribution!) penetrate the cell membranes of germinating seeds. A similar accumulation occurs in the lipophilic cuticles of grasses, from which the compounds subsequently cross into The plant cell membranes. The exact mechanism of toxicity for camphor and 1,8-cineole remains unclear, as does the self-protection mechanism of Salvia or Artemisia. The accumulation of monoterpenes in dense shrub stands significantly increases the probability of spontaneous auto-ignition of the air–terpene mixture during periods of high heat. Chaparral is subject to fires at roughly 25-year intervals, during which the stands of Salvia and Artemisia, as well as the soil-bound Terpenes, are destroyed by fire. Following this, an herbaceous cover develops first, eventually followed by the regeneration and spread of shrubs.