Use of the Plastochron Index to Evaluate Effects of Light, Temperature and Nitrogen on Growth of Soya Bean (Glycine max L. Merr.)

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Abstract
The plastochron index (PI) has been compared with leaf growth and biomass accumulation in young soya bean plants of several cultivars that were grown in controlled environments with different irradiance levels and durations, temperatures, and nitrogen (N) regimes. Increasing the photoperiod from 10 to 16 h day−1 increased the plastochron rate (PR) and the proportion of axillary growth. Doubling the photosynthetic photon flux density (PPFD) to 1000 μmol m−2S−1, increased PR and the proportion of roots to total plant weight, but decreased the proportion of stems plus petioles to total. In a series of experiments, the plants were grown in an 8 h photoperiod at constant temperatures of 17, 20, 26 or 32 °C. As temperature increased, PR increased, but the duration of leaf expansion decreased. Leaves were largest at 20 and progressively smaller at 26, 32 and 17 °C. Biomass was greatest for a given PI at 20 °C and decreased in the order of 26, 32, and 17 °C. The proportion of axillary growth also was greatest at 20 °C. When plants were grown in a 15 h photoperiod at temperatures from 17.1 to 26.6 °C, leaf size continued to increase up to the highest temperature. At 17 °C, the PR in the 15 h photoperiod (PPFD 390 μmol; m−2S−1) was about threefold greater than in 8 h (500 μmol m−2 S−1); biomass accumulation per day was about fivefold greater. Increasing N from 3 to 36 mM increased PR about 10 per cent, altered biomass partitioning among plant parts, and increased the biomass of the plants. The NO2 form of N markedly stimulated axillary growth as compared with the NH4+ form. Environment or cultivar had little influence on the duration of leaf expansion in terms of PI. Cultivars did not differ consistently in biomass production and allocation in the different environments.

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