THE EFFECT ON SEED GERMINATION OF VOLATILES FROM Pinus radiata LITTER

Pinus radiata D. Don is an important tree in New Zealand where it is widely grown for timber and, to a lesser extent, in association with pasture species. In pure stands of P. radiata in Canterbury, poor development of undergrowth may in part be caused by volatile inhibitory substances released from the decomposing litter. In previous work it was shown that vapour from incubated P. radiata litter inhibited seedling growth. This inhibition was attributed to ethylene which was present in the vapour. Experiments were carried out to investigate the effects of litter vapour on the germination of Trifolium repens (clover) and Lolium perenne L . (ryegrass) seed. Clover germination was delayed by litter vapour, but when the vapour was diluted 10-fold a stimulation of germination was observed. Ryegrass germination was delayed by both undiluted vapour and a 10-fold dilution. When ethylene was removed from the litter vapour the stimulatory effect on clover germination was lost, but no change occurred in the inhibitory effect on ryegrass germination. Removing CO2 from litter vapour capable of inhibiting clover germination removed the inhibition and also greatly reduced the inhibitory effect on ryegrass germination. These results suggest that ethylene and CO2 in the litter vapour could be causing the observed effects. Ryegrass germination was inhibited by both ethylene (5 μ l/l) and CO2 (100 μp/p) but a mixture of ethylene and CO2 caused no additional inhibition. Clover seed treated with six concentrations of ethylene at four concentrations of CO2 showed stimulated germination both by CO2 in the absence of ethylene and by ethylene in the absence of CO2. As the CO2 concentration increased, the stimulation by ethylene was reduced and at ethylene concentrations above 0.5 μ l / 1, CO2 caused a reduction in germination rate. The lowest germination rate observed was in the absence of both ethylene and CO2.