The influences of temperatures on seed germination rate in Himalayan elm (Ulmus wallichiana)

The germination response of U. wallichiana seeds from three sources to constant temperature was investigated over the range 14-34°C on a thermo-gradient system. Difference between seeds in proportion and rate of germination, cardinal temperatures and thermal time required for germination was observed. The three cardinal temperatures viz., the base (Tb), the optimum (To) and the maximum (Tm) for germination rate varied slightly between seed lots as well as different percentiles of germination in a particular seed lot. The mean cardinal temperatures i.e., Tb, To and Tm were estimated as 11.3, 26.3 and 34.2°C for Chakrata, 11.6, 26.5 and 34.7°C for Manali and 12.7, 25.3 and 34.1 °C for Sangla valley seed lots respectively. The seeds from the inner Himalayan range (Sangla valley) had slightly lower To as compared to seeds from the outer Himalayan ranges (Chakrata and Manali). The mean thermal time at sub-optimal temperature for germination was estimated as 74.9, 64.9 and 71.9°Cd (degree-days), while at supra-optimal temperatures it was 35.1, 33.8 and 50.1°Cd respectively for seeds from Chakrata, Manali and Sangla valley. The thermal time both at sub and supra-optimal temperatures increased linearly as the values of the percentile germination increased in all three seed sources. The seeds germinated at supra-optimal temperatures required less thermal time to germinate than at sub-optimal temperatures.