The influence of water stress on toxic alkaloid concentrations in threadleaf groundsel (Senecio longilobus Benth.) was investigated in a controlled environment by monitoring leaf xylem water potential during a 32-day period, and then analyzing for alkaloid concentration by plant part. Total plant alkaloid concentration increased with increasing severity of water stress. The most severe water stress (−2.0 MPa) for the 32-day period increased alkaloid concentration by 4.6 times, compared to the least stress imposition (−1.0 MPa). Averaged across watering regimes, alkaloid concentrations (percent dry weight) were highest in threadleaf groundsel roots (0.088%) and occurred in similar concentrations in stems (0.046%) and leaves (0.050%). These results indicate the importance of considering the influence of plant water stress when investigating secondary plant compounds.

Caffeic acid, chlorogenic acid, coumarin, p-coumaric acid, ferulic acid, fumaric acid, gallic acid, hydrocinnamic acid, p-hydroxybenzoic acid, juglone, and pyrocatechol were examined for effects on germination of nine crop and weed species: cotton (Gossypium hirsutum L.), cantaloupe (Cucumis melo L.), corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench], hemp sesbania [Sesbania exaltata (Raf.) Cary], sicklepod (Cassia obtusifolia L.), velvetleaf (Abutilon theophrasti Medic), prickly sida (Sida spinosa L.), and redroot pigweed (Amaranthus retroflexus L.). Germination tests with 10-3 and 10-5 M solutions were conducted under controlled conditions in petri dishes at 25 C in the dark. At 10-3 M, coumarin, hydrocinnamic acid, juglone and pyrocatechol inhibited germination, but p-hydroxybenzaldehyde and p-hydroxybenzoic acid were not effective and others had intermediate effects. There was little effect by any compound at 10-5 M. Chlorogenic acid, p-hydroxybenzaldehyde, and pyrocatechol, each combined with coumarin, inhibited germination. The combination of coumarin plus p-hydroxybenzaldehyde had an additive effect on hemp sesbania and prickly sida, inhibiting germination to a greater extent than either compound alone. The lack of inhibitory action at the higher concentration of some of these chemicals suggests they may not exhibit a high allelopathic potential.

Various physical and chemical properties of host plants influence insect larval performance and subsequent adult fitness. Tomato plants are relatively new hosts to the potato tuber moth, Phthorimaea operculella (Zeller), with the fruit being its preferred feeding site. However, it is unclear how the biochemical and physical properties of tomato fruits relate to potato tuber moth performance. Significant amounts of α-tomatine were detected in maturing green and ripening fruits of cherry (cv. Ceres) and processing (cv. Serio) types of tomatoes whereas none was detected in a fresh market variety (cv. Marglobe), at comparable stages. α-Tomatine is negatively and significantly correlated with development rate (head capsule size) of larvae reared in the fruits of the cherry and processing type tomatoes. Generally, survival, growth and development were significantly superior for larvae reared in the ripening fruits of the fresh market cultivar. At this stage, the fruits of this cultivar are also the largest. Based on these results it is concluded that fruit α-tomatine content, as wellas fruit size and maturity, all affect performance of P. operculella larvae in the fruits of cultivated tomatoes.

Geophagy (soil eating) by the forest elephant (Loxodonta africana cyclotis) and other mammal species in the Dzanga National Park, Central African Republic, has led to large treeless licks. The geological and physical properties of these licks were studied and the chemical characteristics and particle sizes of geophagical lick soils were compared with soils from the forest and from licks where evidence of geophagy was absent. Average lick size of 20 licks in the Hokou lick area was 12,900 m2 (SE = 3,400 m2). Licks were situated exclusively in areas of outcropping dolerite intrusions. Physico-chemical analysis of the geophagical lick soil showed significantly higher quantities of sodium, potassium, calcium, magnesium, phosphorus, manganese and clay compared with the other soils. Several samples of the geophagical lick soil had lower nutrient levels than the mean level of forest topsoil, indicating that nutrient supplementation is probably not the only reason for geophagy. The high clay content of the geophagical soil may help absorbing and combating secondary plant compounds. Pronounced geophagy in rainforests may be a sign for poor habitat quality for large mammals living in these ecosystems.