Development and phenology

Climatic change may influence crop phenology by changes in [CO2], air temperature and drought. The expected increase in [CO2] may potentially affect crop phenology in two different ways. It may cause an increase in the surface temperature of the crop, and higher photosynthetic rates that can be realized under elevated [CO2]. Higher photosynthetic rates will likely occur because of a higher CO2 gradient from the source to the chloroplast and a higher reduced-carbon gradient from the leaves to the sink organs. This will cause faster filling of the sinks and perhaps the occurrence of earlier leaf senescence.

Changes in surface temperature of a crop grown under elevated [CO2] are caused basically by the existence of a so-called physiological feedback effect of stomatal conductance on the surface energy balance of the crop. It is known that an increase in the external [CO2] causes an increase in the intercellular or substomatal [CO2] (Ci) (Farquhar et al., 1980) and that increased Ci reduces the aperture of the stomatal pores (Morison, 1987). It has been theoretically demonstrated that, for most weather conditions, this physiological feedback effect favours increased surface temperature of the crop (Raupach, 1998).

Phenological development of a crop is strongly determined by changes in canopy temperature. It has been widely shown that, for most plant species, changes in air temperature affect their development rate, with generally an acceleration of development at higher temperatures. In most crops, development is associated with the production of leaves. Leaf production or, better, the initiation of leaf primordia in the apices is a temperature-driven process, but it may also be sensitive to severe nutrient or water deficiencies. Potato is no exception and its leaf production appeared to be accelerated when the temperature increased from 9°C to 25°C (Kirk and Marshall, 1992). However, the same study showed that the leaf production rate did not increase further when the temperature was above 25°C. Varietal differences were not investigated specifically, but it may be assumed that temperature increases, at least in warmer climates, may have less influence on potato phenology than on the development rates of other crops such as winter cereals (Miglietta and Porter, 1992; Miglietta et al., 1995). This suggests that temperature increases in cooler climates are more likely to hasten crop phenology than in warmer climates. Confirmation of such an effect comes from the results of the potato FACE experiment that was made in a warm climate during the summer. In that experiment, surface temperature of the crops exposed to elevated [CO2] increased (Miglietta et al, 1998), but phenological development and leaf production rates were not affected, at least until flowering. Tuber initiation in potato is determined largely by photoperiod, being retarded by long days (Kooman, 1995). Temperature increases that result from climatic change may allow earlier planting dates and this may result in initial growth during shorter days, and thus accelerated crop development and earlier crop senescence.

Finally, the length of the growing season in cool areas may limit the growth period of a crop such as potato and thus its production potential. With climatic change in such areas, increased temperature may result in a longer growing season and higher production for potato and other crops (Carter et al., 1996; Harrison and Butterfield, 1996). Of course, generalization of the data shown and considerations made from potato to other root and tuber crops are not possible. Each species deserves specific investigations, because the functional similarity in the mechanisms of phenological development among these crops is probably not that strong.

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