THE
ACTION SPECTRUM FOR MAXIMAL PHOTOSENSITIVITY OF GERMINATION
Karl M.
Hartmann, Botany I of the University, Staudtstr. 5, 91058
Erlangen,
Germany
Three
times repeated night-time cultivation of agricultural fields was
proposed to reduce the weediness (Hartmann & Nezadal 1990).
This method of lightless tillage proved to be unreliable if only applied
once.
One reason might be that the photosensitivity of the seeds in the soil seed-bank
may shift. Freshly sown seeds of typical spring germinators, like Garden
Lettuce, do not respond to natural night- or moonlight, but short exposures to
weak day- or twilight are needed. However, germination responds to the level of
moon- or nightlight if seeds are chilled for one week in 0.01 molar nitrate (Hartmann
et al. 1998). Hence, the action spectrum for maximal photosensitivity of
germination was determined.
Fruits of Lactuca sativa cv. Grand Rapids were used, imbibed in 0.01 molar
potassium nitrate, depleted of maternal phytochrome B by saturating deep-red
exposure, and photosensitized by 1 week of chilling at 4 °C. Twenty fluence-response
curves for germination between 300 and 800 nm for exposures from 6 to 600 s at
22.5 °C show linear and closely parallel regression in the logarithmic
probability net. The reciprocals of the half-response fluences gave the apparent
conversion spectrum and this was corrected for the transmittance of the
seed-coat. It is a phytochrome spectrum of phyAr with photoconversion
cross-sections of 1.2 billions and 4500 m2 per mole at 666 and 800
nm, respectively. This means that for half-saturated germination of sensitized
seed, fewer than 1 out of 200,000 phyA molecules have to become phyAfr, and no
photoreversibility by deep-red was found. Therefore, all spectral colours of
nightly moon- and skylight should stimulate the germination of sensitized weed
seeds. Consequently, exposure periods of several minutes between timely
separated nightly tillage operations should be avoided.
The better strategy is using light-shielded tillage equipment, because the
photosensitivity of the seed-bank may reversibly shift over 8 orders of
magnitude within 1 week (Hartmann & Mollwo 2000; see Naturwissenschaften 87,
398-403).
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