Sugar-beet, potato, and barley plants were grown in a controlled environment, for periods of up to 10 weeks from sowing, with a light intensity of 1,8oo f.c. (4·9 cal./cm.2/hr.) and a temperature of 20° C. during the 18-hour photoperiod and 15° C. during the dark period, to test whether net assimilation rate varied with age and differed between the three species. Net assimilation rate of all species based on leaf area (EA) fell approximately linearly with time. During 5 weeks EA of sugar-beet decreased by only about 20 per cent. and EA of potato decreased by 50 per cent. EA of barley remained approximately constant for 4 weeks after sowing and was halved during the subsequent 4 weeks. The average value of EA for all times was greatest for sugarbeet and least for barley. Net assimilation rates based on leaf weight (EW) and leaf N (EN) decreased at about 15 per cent. of the initial value per week for all species; this was similar to the mean rate of decrease of EA of potato and barley, but greater than that of EA of sugar-beet. Mean values of EW or EN for potato and barley were similar and less than for sugar-beet. Relative growth rate (RW), relative leaf growth-rate (RA), and leaf-area ratio (F) fell with time at similar rates for all species. Average values of RW decreased and of F increased in the order sugar-beet, potato, barley. RA was greatest for potato and least for barley.
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GILLIAN N. THORNE (1960) studied this question.