Plant developmental stability has received little attention in the past three or four decades. Here we review differences in plant and animal development, and discuss the advantages of using plants as experimental subjects in exploring developmental stability. We argue that any type of developmental invariant may be used to assess developmental stability and review the use of fluctuating asymmetry in studies of plant developmental stability. We also examine the use of deviations from translatory, radial, and self-symmetry as measures of developmental instability. The role of nonlinear dynamics and epigenesis in the production of the phenotype is also discussed.

Developmental stability in plants: symmetries, stress and epigenesis

Mature trees of Ailanthus altissima produce one or more potent inhibitors of seed germination and seedling growth. Inhibitor activity is highest in bark, especially of roots, intermediate in leaflets, and low in wood. Crude extracts of Ailanthus root bark and leaflets corresponding to 34 and 1 9 mg water extractable material/L, respectively, caused 50% inhibition of cress radicle growth. Ailanthus seeds also contain one or more inhibitors. These are bound within the seed by the pericarp but diffuse into water agar when the pericarp is removed. The inhibitor(s) could readily be extracted from Ailanthus tissues with methanol, but not dichloromethane, indicating polar characteristics. Ailanthus leaflets had highest inhibitory activity during expansion in spring, whereas activity of trunk bark peaked just before emergence of leaves. This pattern suggests transport of allelochemicals from bark into new leaves. A comparison of seven plant species for sensitivity to the inhibitor(s) from Ailanthus root bark showed little selectivity, although velvetleaf was somewhat more resistant. The inhibitor(s) from Ailanthus root bark exhibited strong herbicidal effects when sprayed pre- and postemergence on plants in soil in the greenhouse. Postemergence effects were striking, with nearly complete mortality of all species, except velvetleaf, at even the lowest doses tested. The results suggest the allelochemical(s) from Ailanthus may have potential for development as natural-product herbicides. PLANTS PRODUCE many compounds that have no apparent metabolic, physiologic, or struc

Allelopathic and herbicidal effects of extracts from tree of heaven (Ailanthus altissima).

Pharmaceutical monotherapies against human malaria have proven effective, although ephemeral, owing to the inevitable evolution of resistant parasites. Resistance to two or more drugs delivered in combination will evolve more slowly; hence combination therapies have become the preferred norm in the fight against malaria. At the forefront of these efforts has been the promotion of Artemisinin Combination Therapy, but despite these efforts, resistance to artemisinin has begun to emerge. In 2012, we demonstrated the efficacy of the whole plant (WP)--not a tea, not an infusion--as a malaria therapy and found it to be more effective than a comparable dose of pure artemisinin in a rodent malaria model. Here we show that WP overcomes existing resistance to pure artemisinin in the rodent malaria Plasmodium yoelii. Moreover, in a long-term artificial selection for resistance in Plasmodium chabaudi, we tested resilience of WP against drug resistance in comparison with pure artemisinin (AN). Stable resistance to WP was achieved three times more slowly than stable resistance to AN. WP treatment proved even more resilient than the double dose of AN. The resilience of WP may be attributable to the evolutionary refinement of the plant's secondary metabolic products into a redundant, multicomponent defense system. Efficacy and resilience of WP treatment against rodent malaria provides compelling reasons to further explore the role of nonpharmaceutical forms of AN to treat human malaria.

https://www.pnas.org/content/pnas/112/3/821.full.pdf

Dried whole-plant Artemisia annua slows evolution of malaria drug resistance and overcomes resistance to artemisinin

Artemisia tridentata ssp. tridentata is the dominant and defining shrub in the Great Basin Desert, with well-documented allelopathic tendencies that have generally been ascribed to its most abundantly released secondary metabolites. However, as a minor component, sagebrush releases a highly biologically active substance, methyl jasmonate (MeJA), which is known to function as both a germination inhibitor and promoter in laboratory studies. Nicotiana attenuata is a tobacco species native to the Great Basin Desert and grows in newly burned juniper–sagebrush habitats for 2–3 yr following a fire. With a combination of field and laboratory studies, we examined the role of MeJA release from sagebrush by both air and water transport in inhibiting N. attenuata seed germination. We demonstrated that sagebrush interacts allelopathically with the seed bank of N. attenuata through its release of MeJA. In the field, seeds buried 0–40 cm from sagebrush plants for 4 months in net bags had significantly reduced germination compared to seeds buried similarly but protected in plastic bags. Moreover, germination on soils collected from underneath sagebrush plants was reduced by 60% compared to seeds placed on soils collected between sagebrush plants or outside of the sagebrush population. Exposure to A. tridentata seeds and seedlings did not affect N. attenuata germination, suggesting that established sagebrush plants only influence the tobacco's seed bank. In the laboratory, exposure of seeds to sagebrush emissions resulted in germination delays of up to 6 d. Exposure to volatile and aqueous MeJA also inhibited germination of N. attenuata seeds at quantities that are released naturally by sagebrush: 3.5 μg/hr and 1.12 μg/seed cup (56 ng/seed), respectively. A. tridentata seeds were significantly more resistant to MeJA, being inhibited at 336 μg MeJA (16.8 μg/seed), 300 times greater than the level of aqueous MeJA required to inhibit N. attenuata seeds. MeJA inhibited N. attenuata germination regardless of the seed's dormancy state and the specific epimer (trans- or cis-) of MeJA. Germination on sagebrush chaff that had been heated to reduce MeJA content was negatively correlated with the amount of MeJA remaining in the chaff. Germination of a nondormant, conspecific tobacco, N. trigonophylla, which grows in the same area but is not associated with fire, is less sensitive than N. attenuata to the extracts of sagebrush litter, but similarly sensitive to MeJA. Additionally, four of five other tobacco species that are not known to be associated with sagebrush are less sensitive to MeJA, suggesting an evolved sensitivity to MeJA. To determine the proportion of germination inhibition of a sagebrush extract that could be attributed to MeJA, we serially diluted sagebrush extracts with water and restored the quantity of MeJA of the original extract by adding appropriate quantities of synthetic MeJA; 16–60% of the inhibitory activity of the original extract could be attributed to the MeJA. We conclude that MeJA release from sagebrush plays an allelopathic role for N. attenuata seed banks, but other unidentified compounds are also involved.

Methyl Jasmonate as an Allelopathic Agent: Sagebrush Inhibits Germination of a Neighboring Tobacco, Nicotiana Attenuata

During the recent past, the importance of allelopathy in nature as well as in agroecosystems has attracted worldwide attention. In the beginning, allelopathic studies were limited to the effect of one plant on the other, without much knowledge about the chemicals responsible for such effects. Later, active involvement of scientists belonging to various disciplines, made allelopathy a multidisciplinary subject. Consequently, several allelochemicals which were at the helm of the entire allelopathic affair were isolated, identified and their effects were studied not only on germination, growth or metabolism but also at the molecular level. These developments slowly transformed allelopathic research from basic to applied, and ideas were floated to exploit allelopathy/allelochemicals in various ways in agriculture and forestry (Rice, 1984; Thompson, 1985; Putnam and Tang, 1986; Waller, 1987). We started working with allelochemicals during 1976 and remained concerned mainly with exploring various aspects of allelopathy that may lead to increased crop production. In this chapter we shall discuss some of our work on the potential of allelochemicals in pest control, crop rotation and also discuss the implications of allelopathy in agroforestry, a relatively younger aspect of allelopathic research.

S. Mark Lee

Papers

Terpenoids from the GenusArtemisiaas Potential Pesticides