Calculation of Growth Yield, Growth Respiration and Heat Content of Grain Sorghum from Elemental and Proximal Analyses

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Abstract
Elemental and proximate organic compositions were determined for biomass samples from two cultivars of grain sorghum [Sorghum bicolor (L.) Moench] grown in the field with two levels of nitrogen supply. Several methods of calculating the costs of biomass synthesis were compared. Penning de Vries' Production Values (PV), based on proximate analyses and Glucose Values (GV) calculated from elemental analyses or heats of combustion, yielded similar trends in response to variation in nitrogen supply and plant age but were not different for the two cultivars. The correctness of the GV calculations was confirmed through comparisons of measured and predicted heats of combustion. The ratio of PV/GV, representing the growth efficiency (E) of the samples, was close to constant (0·84 + 0·02). Since PV is an estimate of the true growth yield, the constant growth efficiency allows the estimation of that value from elemental analyses. The true growth yield of the sorghum crops ranged from 0·73 to 0·74 g biomass g−1 glucose when grown with adequate N and from 0·77 to 0·78 with low N supply. Over the crop cycle, with adequate N, 0·19 of the C used in growth processes was calculated as lost in growth respiration; with limited N, 0·17 was lost.