Very early detection of canopy presence by seeds through perception of subtle modifications in red:far red signals

Abstract

Abstract 1. An early inhibition of germination in seeds of Silene gallica and Brassica campestris which were continuously exposed to the light environment under an establishing wheat canopy, was observed in two different experiments. Inhibition occurred c. 15 days after crop emergence, when the canopy leaf area index (LAI) was below one and the red (R):far‐red (FR) ratio recorded under the canopy was well above 0·8. 2. This inhibitory effect was either overcome by filtering FR light through a solution of CuSO 4 or could be artificially imposed by simulating the canopy with filters yeilding a R:FR ratio of 0·95 and 0·8. These results show that light subtly enriched with FR was the environmental factor regulating germination below the developing canopy. 3. Exposure to canopy‐filtered light pulses of 1 h (presumably sufficient to saturate a low fluence response, LFR) did not inhibit seed germination. Moreover, such treatment promotes germination up to an extent similar to that previously observed in the laboratory after a saturating pulse of R light. Instead, prolonged exposures were required to inhibit germination. These results, together with the relatively high R:FR ratios measured below the canopy in early stages of its establishment, suggest that a high irradiance response (HIR) would be involved in such a regulation. 4. This capacity to detect small environmental light‐quality modifications when exposed to high irradiances, would allow the seeds from these species to detect the presence of a canopy in the very early stages of its establishment and to stay in ‘safe’ pre‐germination phases when the probability of successful seedling establishment is low.