Rising atmospheric carbon dioxide (CO2) concentration can change crop productivity directly by increasing photosynthesis, and indirectly by positive or negative modifications of growth responses to predicted global warming and changes in rainfall. The purpose of this experiment was to determine the effects of C02, temperature, soil type, and water table depth on growth of four cultivars of sugar cane, a C4 photosynthetic pathway species (Saccharum officinarum L.). Studies were conducted in paired temperature-gradient greenhouses at ambient and enriched levels of CO2 360 and ~ 710 μηιοί (CO2) mol"1 (air), respectively] with four temperature zones along the length of each greenhouse: baseline, +1.5, +3.0, and +4.5°C. These 1.5°C steps were maintained by a combination of heat inputs (electric heaters and sunlight) and ventilation by computer-controlled fans. Other treatments were soil type (mineral vs. organic), water table depth (constant water table of 20 cm vs. ~50-cm drained profile). The four cultivars were CP72-2086, CP73-1547, CP88-1508, and CP80-2086. Doubled C02 increased the following components of plant growth of the first sampling in late June-early July of 1997: leaf number = 7%; leaf area = 15%; leaf fresh weight = 13%; leaf dry weight = 8%; mainstem length = 32%; mainstem fresh weight = 31%; mainstem dry weight = 23%; juice volume = 40%; total fresh weight = 25%; juice dry weight = 36%; total dry weight = 21%. However, total fresh weight increase of that whole-crop harvest was somewhat less at 16%. Increasing temperatures caused a slight downward trend in sugar cane yield regardless of cultivar or CO2 treatment. The order of cultivar yields for the first harvest was: CP73-1547 > CP80-1827 > CP88-1508 > CP72-2086. Doubling C02 appeared to benefit sugar cane productivity more than the anticipated 10% increase for a C4 species. The apparent increase in sugar cane dry weight, fresh weight, and juice volume indicates greater yields as global atmospheric CO2 continues to rise.


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