Quantal Response of Seed Germination inBrachiaria humidicola, Echinochloa turnerana, Eragrostis tefandPanicum maximumto Photon Dose for the Low Energy Reaction and the High Irradiance Reaction

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Abstract
Various daily photon doses (daily net number of photons per unit area), provided by varying both photon flux density and photoperiod, were applied to dormant seed populations of four graminaceous species tested for germination in alternating-temperature regimes on filter papers moistened with either water or a solution of potassium nitrate. In all four species germination was dependent upon daily photon dose, the product of photon flux density and duration, and quantified by linear relations between the probit of percentage germination and the logarithm of the daily photon dose determined by probit analysis. Photon flux density and photoperiod only influenced germination indirectly as factors of daily photon dose, except in Echinochloa turnerana (Domin) J.M. Black at high daily photon doses where lower germination was observed in continuous light than in 8 h d−1 photoperiod. The low energy reaction, in which increasing dose promotes seed germination, was observed in Echlnochloa turnerana and Eragrostis tef(Zucc.) Trotter but not in the populations of Brachiaria humidicola (Rendle) Schwcickerdt and Panicum maximum Jacq. used in this study. The high irradiance reaction, in which increasing the photon dose reduces germination, was, however, observed in all four species. In Eragrostis tef potassium nitrate influenced the response of seed germination to light, but in the remaining species its influence on this relationship was either additive (Panicum maximum) or nil (Brachiaria humidicola and Echinochloa turnerana) in the alternating-temperature regimes used in this study. The implications of the quantal response to photon dose for devising optimal light regimes to be applied in laboratory tests intended to maximize seed germination are discussed.

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