Crop Water Relations and Global Climate Change

The major effects of global climate change on crop water relations that are relevant to plant breeding are likely to occur due to changes in rainfall and soil drought for crops grown without irrigation. Where rainfall is reduced, breeding may have to give greater emphasis to enhancing adaptation to drought and this can be a difficult task (Blum, 1988; Eastin et al., 1988). In regions where global climate change substantially reduces rainfall, it may be necessary for farmers to grow alternative crop species that have enhanced adaptation to dry environments.

Crop water relations are also influenced by transpiration rates under both well-watered and dry soil conditions (Schulze and Hall, 1982). Plants that transpire faster develop lower leaf water potentials in the short term and more rapidly deplete the water in the root zone on a long-term basis. The effects of elevated atmospheric [CO2] on transpiration are difficult to predict, because increased leaf growth rate results in greater interception of solar radiation and could result in faster transpiration, whereas the partial stomatal closure and earlier leaf senescence that can result from elevated [CO2] could reduce seasonal transpiration. Due to different magnitudes for these effects, elevated [CO2] has resulted in greater seasonal water use for cotton (Samarakoon and Gifford, 1996) but less seasonal water use for spring wheat (Kimball et al., 1995). The reduced water use of spring wheat could have been responsible for their greater grain yield response to elevated [CO2] under water-limited compared with well-watered soil conditions. Any increases in air temperature would cause plants to transpire faster and experience lower leaf water potentials. Effects on crops of changes in transpiration rate, due to global climate change, do not appear to warrant developing special plant breeding procedures.

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