USING MAIZE (ZEA MAYS L.) AS A SUGAR CROP

The increased demand for homegrown energy has created a market for new feedstocks for the growing biofuel industry. Plants with C4 photosynthesis are particularly suited as biofuel crops because of their high radiation, water, and nitrogen (N) use efficiency. C4 species that store high levels of sucrose in their stalks such as sugarcane (Saccharum spp), sorghum (Sorghum bicolor L.), and maize are especially useful. Maize has been repeatedly evaluated as a sugar crop during the last century, and prevention of pollination or ear removal is typically associated with the highest concentrations of stalk sugar. Elimination of the reproductive phase, however, usually results in accelerated leaf senescence, which is expected to limit sugar accumulation. We have developed a series of hybrids that exhibit photoperiod sensitivity as an approach to simultaneously increase biomass and sugar production by crossing seven tropical inbreds with the historic temperate inbred B73. We used a tropical parent to confer photoperiod sensitivity and to greatly delay flowering and increase the anthesis-silking interval, resulting in low seed set. When grown in temperate regions these hybrids produce abundant biomass and do not exhibit accelerated leaf senescence without grain, but rather remain green and accumulate sugars in their stalks. Total biomass (stover and grain), sucrose accumulation, and the response to N of these hybrids was determined and compared to a similar number of locally grown commercial grain hybrids. On average the tropical hybrids produced 20% more total biomass than the commercial hybrids, and they showed a smaller response to the addition of fertilizer N. Total biomass yields of tropical hybrids ranged from 16.3 to 27.5 Mg/ha (average of 23.5 Mg/ha) and the stalk contained from 1.7 to 3.2 Mg/ha of sucrose (average of 2.6 Mg/ha). Increasing the N supply from 0 to 225 kg/ha increased the average biomass production of tropical hybrids by only 2.2 Mg/ha, while decreasing stalk sucrose by 0.3 Mg/ha. Conversely, similar increases in N supply raised the average biomass yield of commercial hybrids from 14.5 to 19.5 Mg/ha, with most of this increase associated with the grain (increase from 7.0 to 10.7 Mg/ha). Estimates of the total potential ethanol yield (i.e. from grain, stover and sucrose) showed that tropical hybrids have the potential to produce as much ethanol without N as do the commercial grain hybrids with 225 kg/ha of fertilizer N. Although additional research is needed to optimize the genetic potential and the cultural management of tropical maize hybrids, our research shows there is considerable potential in using maize as a sugar crop