3 The effects on yield and phenology

Through global warming, an anticipated increase in temperature can potentially have various effects, e.g. pikelet sterility in rice, reversal of vernalisation in wheat, reduced formation of tubers in potatoes, loss of pollen viability in maize. Yields can be severely affected if temperatures exceed critical limits for periods as short as 1 h during anthesis (flowering). Flowering is a very important event in crop development, as it is a phase which is particularly vulnerable to environmental stresses (Roberts et al., 1993).

It is thought that extreme temperatures are more important than average temperatures in determining plant responses. Crop yields are affected by net primary productivity and also by the phenology of crop development. Increased temperature can speed phenological development, reducing the grain-filling period for crops and lowering yield. Crop yields were greater under elevated CO₂, but warmer temperatures reduced the duration of crop growth and, hence, the yield of determinate crops such as winter wheat and onion; but the yield of carrot, for example, an indeterminate crop, increased progressively with temperature (Wheeler et al., 1996).

In terms of temperature, a 12-day period of high temperature stress close to anthesis reduced spring wheat root biomass from 141 to 63 g m⁻² (Ferris et al., 1998) by the end of the elevated mean temperature period, whereas mean temperatures over the treatment period had no effect on either above-ground biomass or grain yield at maturity. Interestingly, it was increasing maximum temperatures over the mid-anthesis period which was related to a decline in the number of grains per ear at maturity. Grain yield and harvest index also declined sharply with maximum temperature. This study suggested that high temperature extremes may reduce yields considerably.

Elevated CO₂ can aid the recovery of plants from high temperature-induced reductions in photosynthetic capacity. Ferris et al. (1998) grew soybeans for 52 days under normal air temperature and soil water conditions at atmospheric CO₂ concentrations of 360 and 700 μmol mol⁻¹, but then subjected them to an 8-day period of high temperature and water stress. After normal air temperature and soil water conditions were restored, the CO₂-enriched plants attained photosynthetic rates that were 72% of their unstressed controls, while stressed plants grown at ambient CO₂ attained photosynthetic rates that were only 52% of their respective controls.

In the same study total biomass was 41% greater under elevated CO₂ than under ambient CO₂ but reduced by high temperature, water deficit and high temperature × water deficit under both CO₂ concentrations. At maturity, seed dry weight and number per plant under elevated CO₂ were increased by an average of 32% and 22% respectively compared with ambient CO₂. The same parameters were reduced after high temperature × water deficit by 29% and 30% respectively in ambient CO₂ and elevated CO₂. Seed filling was earlier under high temperature and high temperature × water deficit. The rate of change in harvest index was unaltered by CO₂, while it decreased under the combined effects of high temperature × water deficit. Seed number explained 85% of the variation in yield, but yield was also related linearly to photosynthesis during seed filling, suggesting both are important determinants of yields under stress.

More recently, results from some open-field experiments using free-air concentration enrichment technology have indicated a much smaller CO₂ fertilisation effect on yield than currently assumed for C3 crops, such as rice, wheat and soybeans, and possibly little or no stimulation for C4 crops, which include maize and sorghum.

• ‘Assessment of observed changes and responses in natural and managed systems’

• ‘Seed yield after environmental stress in soybean grown under elevated CO₂’

• ‘Temperature × CO₂ interaction – plant growth response (agricultural crops)’

• ‘Food-crop yields in future greenhouse-gas conditions lower than expected’