Biphasic fluence‐response curves for light induced germination of Arabidopsis thaliana seeds

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Abstract
With appropriate pretreatment of the seeds fluence‐response curves for the induction of germination of Arabidopsis thaliana show two phases. A proportion of the population responds to very low fluence (VLFR), 104–10−2μmolm−2 establishing 10−4–10−2% of the total phytochrome in the far‐red absorbing form (Pfr) and a proportion of the population respond to low fluence (LFR), 1–1000 μmolm−2, establishing 1–75% Pfr. The VLFR is nol normally seen because the pre‐existing Pfr level satisfies the Pfr requirement or use of green safelight establishes more Pfr than necessary to saturate the VLFR. Endogenous Pfr was depicted by a 24 h 35°C treatment, presumably as a result of dark destruction and/or dark reversion to the red absorbing form of phytochrome (Pr), making it possible to visualize the VLFR. A short pulse of 35°C treatment in combination with an appropriate temperature regime is also able to sensitize a proportion of the seed population. The proportion of the population showing the VLFR is determined by the duration of the cold imbibition pretreatment as well as the duration of the 35°C treatment. Complex fluence‐response curves were observed in which a proportion of the seeds being promoted in the VLFR range, were inhibited at higher fluences before being further promoted in the LFR range. This was particularly clear for seed batches being sensitized by a short 35°C treatment. The VLFR may be of significance in the natural environment, enabling seeds buried in the upper layer of the soil to germinate, where the fluence rate falls off sharply and the LFR is not satisfied. A model is presented to explain the two phases in the fluence‐response curves.