A comparison of relative growth rates of different individual plants and different cultivars of onion of diverse geographic origin at two temperatures and two light intensities.
In two series of experiments, the relative leaf growth rates (RA) and relative growth rates (RW) of seedlings of 8 onion cvs of diverse geographic origin were compared at 10 deg and 20 deg C under 120 Wm-2 of irradiation and also under 40 Wm-2 at 10 deg . Different seed stocks of the cvs were used in the 2 series of experiments. There was a wide range in RA between individual plants, depending on cv. and environment. In the second, but not the first, series of experiments there were highly significant differences between cvs in mean RA. There was no relation between mean RA and the latitude of origin, normal sowing date, or whether the cv. was open-pollinated or a hybrid. Some plants were grown for 24 days at 20 deg before transferring them to 10 deg : these plants also showed a wide range of RA at 10 deg and the value of RA at 20 deg was negatively correlated with its subsequent value at 10 deg . At 10 deg and 40 Wm-2 and 20 deg and 120 Wm-2 a high RA was correlated with a high leaf area ratio, LAR. This was not the case at 10 deg and 120 Wm-2. Reducing the light intensity from 120 to 40 Wm-2 at 10 deg had little effect on RW because increases in specific leaf area (SLA) and to a smaller extent leaf weight ratio (LWR) almost compensated for a decreased net assimilation rate (NAR). Increasing temperature from 10 to 20 deg with 120 Wm-2 irradiation increased SLA and to a smaller extent LWR and NAR and all these contributed to a large increase in RW.