Conclusions and Future Research Directions

As global temperatures rise, both maize and sorghum will become increasingly viable at locations that currently lack adequate heat. However, as cultivation moves to higher latitudes and altitudes, these crops will still face the low temperature limitations of today at the current edges of their range. Poor germination, low-temperature-dependent photoinhibition of photosynthesis and impaired water and nutrient uptake by seedlings will remain limitations. The ability of Miscanthus x giganteus, a highly productive C4 relative of sugar cane, to grow rapidly in the cool temperate climate of southern England without the low temperature damage observed in maize shows that there is no inherent limitation to C4 plants in cold climates (Long, 1999).

Effective and rapid screens for low-temperature photoinhibition and germination at low temperature have been developed and are already being applied in the adaptation of both crops to colder climates. In addition, further adaptation or removal of the photoperiod requirement will be needed as temperature increases allow cultivation at higher latitudes. Rapid advances in methods for adapting the maize crop to cool temperate conditions have been achieved over the past two decades. Genetic transformation to enhance protection against active-oxygen radicals offers the potential for further improvements. Coupled with global climatic change, this could allow the crop to extend into yet higher latitudes.

Far more pressing, in terms of global food supply of grains for humans, will be adaptation of the crops to the warmer and drier conditions that may occur in areas of the tropics. Increased evaporative demand and increased incidence of supra-optimal or lethal temperatures may make larger areas of the semi-arid tropics unsuitable for maize cultivation, perhaps necessitating a return to lower yielding but more tolerant sorghum landraces. Sorghum is perhaps a neglected resource in considering the impacts of global climate change within the tropics. Its ability to maintain viable yields on low-fertility soils, survive transient drought and acclimate to very high temperatures may be critical to ensuring food security in the tropics under an uncertain future. Great variability exists within the sorghum germplasm. Critical to the future will be accelerating the description, preservation and utilization of this variation. Much recent effort has been placed on maximizing production; however, more important throughout the developing world may be ensuring future yield stability.

Doggett (1970) foresaw that sorghum grain yields of 20 t ha-1 'should be attainable'. Thirty years on, average sorghum yields per unit of ground area worldwide have increased little and shown no improvement over the past 20 years (Fig. 6.2b).

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