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Seasonal variation in the accumulation and partitioning of nitrogen, phosphorus and potassium was determined in both the above-ground and below-ground dry matter of the potential energy crops Miscanthus × giganteus and Spartina cynosuroides. It is desirable from both economic and environmental perspectives that such crops should exhibit a high nutrient use efficiency and minimal nutrient losses to the environment. The N, P and K concentrations in the above-ground dry matter, at final harvest, were 5.0, 0.6 and 12.0 mg g−1 respectively in M. × giganteus and 3.0, 0.4 and 1.0 mg g−1 in S. cynosuroides. Both species exhibited the high N-use efficiency expected of C4 plants. Nitrate leaching was negligible. At the end of the growing season, nutrients were translocated to the rhizomes and, in the case of M. × giganteus, recycled to the soil in shed leaves. Consequently the nutrient content of the crop offtake was low. It was calculated that the N, P and K requirements of a M. × giganteus crop producing an above-ground harvest of 1.5 kg m−2 dry matter would be 9.2, 1.3 and 20.4 g m−2 respectively. The corresponding nutrient requirements for S. cynosuroides would be 7.5, 1.7 and 8.8 g m−2. Except for the K requirement of M. × giganteus, the N, P and K demands of both species were less than those of typical graminaceous crops, including maize.

Previous studies have shown that both photoinhibition and low temperature impairment of leaf development occur in C 4 species growing in temperate climates. These result in reductions in the maximum quantum efficiency ( ?) and the light-saturated rate of CO 2 uptake (A sat ). The perennial C 4 -grass Miscanthus x giganteus has been shown to attain high productivity in northern Europe. This study examines and analyses the seasonal variation in photosynthetic gas exchange of M. x giganteus grown in southern England. Although the crop was exposed to chilling temperatures at the start of the season neither A sat nor ? were impaired, when assessed at a measurement temperature of 24°C. Between May and July the mean values of A sat and ? were 34 μmol m -2 s -1 and 0.067, respectively, declining to 20 μmol m -2 s -1 and 0.045 in October. In the field, on clear days between May and July, the typical midday rates of CO 2 uptake ranged between 20-27 μmol m -2 s -1 , although in late-June a peak mean value of 35 μmol m -2 s -1 was attained. The study shows that under the cool temperate conditions of southern England, M. x giganteus, unlike all C 4 species previously examined, is able to realize the photosynthetic potential of the C 4 process without suffering any apparent low temperature impairment, except possibly at the very end of the growing season.

Although C4 plants are considered to have higher conversion efficiency and productivity than C3, plants, this advantage may not be realized under sub-optimal conditions. Two perennial C4 rhizomatous grasses of cool temperate origin, Miscanthus × giganteus and Spartina cyno-suroides, have been suggested as potential fuel crops for north-western Europe. The conversion efficiencies of these species were examined for 2 years in fertilized, irrigated, replicated plots in south-eastern England. In the second year, the energy conversion efficiencies for shoot and total biomass production were 0.040 and 0.051, respectively, for S. cynosuroides, with significantly higher values of 0.060 and 0.078, respectively, for M. × giganteus. The M. × giganteus crop attained shoot productivity of 2.87 kg m−2 between April and September, exceeding the highest values typically obtained with intensively managed C3, crops. Canopy development was early, and high interception and conversion efficiencies were maintained over most of the growing season. This study provides the evidence that the superior potential light conversion efficiencies associated with C4 photosynthesis can be realized under cool temperate conditions and that such climatic conditions do not Inherently impair the C4 process.