Aromatic plants contain both volatile and non-volatile fractions and the chemical composition of these two fractions may be influenced by seasonal
changes. The essential oil and solvent extracts of S. africana-caerulea, S. africana-lutea and S. lanceolata, collected at the same locality throughout the
2005 growing season, were compared in terms of essential oil composition, yields and biological activities. Mostly quantitative, rather than qualitative
variation was observed in the oil composition of each species. Major fluctuations in the composition of S. africana-caerulea oil included limonene (2–33%)
and viridiflorol (2–24%). Levels of α-pinene (1–12%), myrcene (2–12%) and α-eusdesmol (trace-13%) fluctuated seasonally in the S. africana-lutea oil.
In S. lanceolata, considerable changes were noted for β-caryophyllene (1–19%), β-caryophyllene oxide (1–21%) and ledol (3–12%). The extract prepared
from S. lanceolata harvested in winter was more active against Gram-positive bacteria. The S. africana-caerulea extract exhibited the most favourable antiplasmodial activity when harvested in winter (IC50 value: 12 μgml−1), which contrasts with the lowest anti-plasmodial activity of S. lanceolata obtained at
the same period (IC50 value: 43 μgml−1). The anti-oxidant activity of the solvent extracts also displayed variation over seasons with thewinter collection of
S. africana-lutea yielding the most favourable anti-oxidant activity (IC50 value: 10 μg ml−1). All the solvent extracts prepared from the winter collection
exhibited the lowest toxicity (
values <0.4 μg ml−1).