Effect of light and temperature on volatile compounds and growth parameters in sweet basil (Ocimum basilicum L.)
The effects of temperature, irradiance, supplementary UV-B and RlFR ratios on
volatile oil compounds and plant growth parameters in basil plants have been
determined. The base temperature for plant growth, the variation in chemical
composition between leaves, the variation in chemical composition during the day
and the effect of storage at 4°C for 24 h were also determined.
Basil is a warm climate plant and its base temperature for growth is 10.9°C. The
optimum temperature for plant growth is 25°C and this temperature also enhances the
volatile oil content in leaves. Plants grown at 25°C for two weeks were taller and
possessed more dry matter and larger leaves than plants grown at other temperatures.
The total volatile oil content in fresh leaves was three times the level compared with
plants at lSoC. Temperature also affected the composition of volatile oils. Warm
conditions resulted in the accumulation of eugenol and cis-ocimene. whereas cool
temperatures resulted in more camphor and trans-p-farnesene. There was no effect,
however, on the relative contents of 1,8-cineole and linalool. Treatments with
alternating temperature that supplied the same accumulated day degrees (ADD), but
with a different sequence of temperatures, did not affect most of the plant growth
parameters, however, volatile oil content and composition were strongly affected by
the temperature regime of the final two weeks. The higher the temperature before
harvesting, the higher the volatile oil content and the relative content of eugenol
Basil plants grow well in full sun, however they can tolerate light shade. Heavy
shade (75% and 50% shade) resulted in small plants with reduced dry matter, and the
volatile oil content in fresh leaves was five times lower than in control plants. Heavy
shade significantly increased the content of methyl eugenol, but strongly decreased
the contents of eugenol and Iinalool.
Two weeks treatment with supplementary UV-B (ultraviolet - B) light resulted in
short plants with higher dry matter and thicker leaves. It also stimulated the synthesis
of volatile oil compounds, i.e. phenyl-propanoids (eugenol) and terpenoids (notably
1,8-cineole and linalool). There was no effect, however, on volatile oil composition.
Use of supplementary light to produce a high RlFR (red / far-red light) ratio resulted
in shorter plants with less dry matter and smaller leaves. But the volatile oil content
of the leaves was greatly increased. The content of eugenol was decreased whereas
the content of ~-myrcene was increased. There were no effects on the relative
contents of 1,8-cineole, linalool and other compounds.
There were no differences in the volatile oil content and composition of fresh basil
leaves harvested during the daytime, i.e. between morning (9.00am) and late
afternoon (5.00pm). After storage for 24 h at 4°C in dark conditions, there were no
differences in volatile oil content and composition in fresh leaves. There was a great
difference, however, in the content and composition of volatile oils between young
and mature leaves.
The sensory analysis showed that trained panellists could perceive different
intensities of volatile oils and consumers preferred the stronger intensity of volatile
oils in fresh basil leaves.