Effects of Temperature Fluctuation, Red and Far-Red Light and Nitrate on Seed Germination of Five Grasses

Abstract

(1) The germination of seeds of five grass species which differ in their occurrence in seed banks beneath grasslands was investigated. The species were Agrostis capillaris, Holcus lanatus and Poa trivialis, which usually form large and persistent seed banks, and Cynosurus cristatus and Festuca rubra, which usually occur less frequently and more transiently. Germination was investigated in darkness at constant temperatures from 5 to 30 OC for 12 h, alternating with constant 20 OC for 12 h. In one series of experiments the effects of a short exposure to a fully unshaded situation or deep canopy shade was simulated by treatment with red or far-red light, respectively, before transfer to the different temperatures. In another series with older seed, the effects of different concentrations of nitrate were investigated. (2) The different temperature regimes markedly affected germination and the optimum appeared to be about 20/10 OC for all species. Minimum germination of C. cristatus and F. rubra occurred at the highest temperature (30/20 OC) but H. lanatus and A. capillaris germinated least well when there was little alternation of temperature (i.e. near-constant 20 0C). Although older seed, particularly of P. trivialis, were more germinable than younger ones, they showed a similar pattern of response to temperature. (3) In general, treatment with red light before transfer to the different temperature regimes had a persistent stimulatory effect and treatment with far-red light a persistent inhibitory effect on germination. The effects were largest in A. capillaris, especially with least temperature fluctuation. Far-red light slightly stimulated germination in P. trivialis, but neither far-red nor red affected germination of C. cristatus significantly. (4) Potassium nitrate solution (0.002 M) increased the germination of all species, except of C. cristatus, and 0.02 M solution further increased that of A. capillaris. (5) In most instances, exposure of ungerminated seeds to daylight and to a 20/10 OC alternating temperature regime resulted in almost full germination of all species, except of A. capillaris. Many seeds of this species, especially those treated with far-red light or maintained in darkness, failed to germinate under these conditions but required gibberellic acid additionally, indicating induction of dormancy by some initial treatments. (6) The results showed that the three species (A. capillaris, H. lanatus and P. trivialis) which tend to form persistent seed banks showed a larger response to the treatments (i.e. were more specific in their germination requirements) than did those species (C. cristatus and F. rubra) which usually occur only transiently in the seed bank.