Responses of temperate plants to drought conditions |
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Effects on: growth and allocationPlant growthGrowth results from the interaction of all other processes within a plant: photosynthesis, respiration, transport, water relations, and nutrient balance. Growth can include any increment in dry mass, volume, length or area of cells (Lambers et al. 1998). Cellular growth appears to be one of the plant processes most sensitive to water stress, and is usually reduced long before photosynthesis or stomatal conductance (Hsiao 1973). Cellular growth has two components:
Cell divisionCell divisions lead to creation of new cells from meristematic tissues. Although growth clearly depends on cell divsion, cell division itself does not constitute growth. Growth occurs as the new cells undergo expansion. In general, cell division is reduced less than cell expansion under drought conditions (Lambers et al. 1998). However, since division also depends somewhat on expansion, anything that slows down cell expansion will limit division (Kozlowski and Pallardy 1997). Cell expansionCell expansion is dependent on cell turgor (hydrostatic pressure) being above a critical threshold, as well as the cell wall being expandable. For most plants the critical value is around 15-50% of the turgor pressure under normal (non-stress) conditions (Lambers et al. 1998). Therefore, under drought conditions with lower cell water potentials, expansion of developing cells can be limited and growth can be severely reduced. However, some research suggests that turgor is fairly well maintained even under moderate stress, and that a reduction in cell wall expansibility (plasticity) is what actually leads to the decrease in expansion frequently seen . Much like the discussion on stomatal conductance, the trigger for the stiffening of cell walls seems to be a combination of hydraulic and chemical (ABA) signals (Zhang and Davies 1990). For more information on how ABA effects various plant processes. One result of smaller cells may be smaller leaves, which reduces the potential for carbon gain (Nilsen and Orcutt 1996). Allocation of carbon within plantsAllocation is the distribution of growth within a plant, particularly between above and below ground organs (Lambers et al. 1998). Allocation can have tremendous effects on whole plant growth, sometimes even more so than the photosynthesis rate (Nilsen and Orcutt 1996). Leaf growth allows plants to increase their photosynthetic capacity, while root growth allows plants to further exploit the soil for water and nutrients. However, these organs are in competition with each other for receiving photosynthetic assimilates. It is unknown exactly how a balance is reached between the different organs, particularly in stress situations. However, this may be able to be explained by the differential sensitivities of growth of roots and shoots to water stress (Hsiao and Xu 2000). Differential sensitivities of leaf and root growth to soil water deficitIt is frequently observed that drought increases allocation to roots. It is also known that leaf growth is usually far more sensitive to water stress than root growth. This does not appear to have any relation to roots having a closer proximity to the water source, but rather that at the same cell water potential, leaf growth will be reduced more than root growth. In part this appears to reflect a greater ability of roots to maintain turgor through osmotic adjustment. It may also be that ABA plays a role in inhibiting shoot growth, while maintaining root growth (Hsiao and Xu 2000). Finally, the reduction in leaf growth leaves more assimilates free to go to roots (Lambers et al. 1998).
Clearly there are many other plant processes which are affected by drought
stress. This work does not claim to cover
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