Drought is a meteorological term: a period without rain, the length depending on the region. It varies from 2 weeks in the UK to an indefinite number of years in Australia. It is therefore difficult to mimic drought in the laboratory. The closest laboratory treatment to drought is a controlled soil water deficit with slowly drying soil.
‘Drought resistance’ is a nebulous term. There are no units for ‘drought resistance’. Likewise there are no units for ‘drought tolerance’. It is more effective to ask, given a fixed and limiting water supply, what is the best growth rate or maximum biomass that can be produced. This focuses on the efficiency of water use, that is water use when the plants have water, not when they don’t (Passioura and Angus, 2010). Desiccation tolerance is only applicable to plants that can undergo and survive a prolonged dry season, many months or many years.
The most reliable indicator of drought stress is the reduction in plant growth caused by the treatment.
The search has been going on for decades to find the best medium for growing plants in which to impose a controlled water deficit, yet there remains no clear resolution. There is no perfect medium, all have limitations including pots containing real soil, that is, soil imported from the field.
Soil drying is hard to control especially when comparing genotypes of different vigour or rates of development. Even a drying soil, as well as being very difficult to maintain at a uniform and constant water potential through the whole soil profile, may exert specific effects; for example, transmission of nutrients through the soil will be reduced at low soil water potentials. Another problem with soil in pots that are not deep is that they easily become saturated at the bottom. Pots should be tube-shaped rather than bucket-shaped, to enhance drainage. Soils should drain quickly, and the addition of perlite or vermiculite can enhance this (Passioura, 2006). However use of material with large particles and little root contact may be problematic. “Inorganic soils” such as fritted or calcined clay can overcome the problems of soil with a high clay content, which does not drain quickly or a predominantly sandy soil which holds little water and releases it quickly as the soil dries.
Hydroponics avoids problems of drainage. A variety of non-ionic osmotica have been used to mimic a decrease in soil water potential, such as mannitol. However, a percentage of these small molecules enter roots and move in the xylem to the shoots, either through cracks in roots or through membranes that are not completely impermeable to neutral solutes of this size. High-molecular-weight polyethylene glycol (PEG, MW 6000) has been examined in many early studies that attempted to impose a controlled water deficit. Its main problem is its viscosity which decreases O2 movement to roots so that the roots become O2 deficient. The latter can be overcome by bubbling with O2 rather than air, however the experiments must be limited to a short period of time as the PEG can enter the roots and reduce the hydraulic conductivity (Munns et al., 2010).
Salinity using NaCl or a balanced salt solution is an alternative to organic osmotica. NaCl is cheap and easy to impose, and can substitute for a water stress for species that are tolerant (see the following section on Salinity ). Concentrated mixed salts, such as the macronutrients used in Hoagland’s solution, are preferable to NaCl as plants are less likely to take up any one ion to toxic concentrations.
When hydroponics are used to simulate a ‘drought’ or soil water deficit, the effect on plant growth may be quite different, and a lot less, than with a drying soil. Hydroponics ensure there is no nutrient deficit, but in a dry soil the access to nutrients decreases and the plants may suffer N or P deficiency.
Ranges of values
Most plants cannot take up water when the soil dries below 1.5 MPa (‘wilting point’). With hydroponics, this also represents the solution water potential which does not support growth. Some ‘salt-tolerant’ species like barley can continue growing slowly at 300 mM NaCl, which has a water potential of about 1.5 MPa.
Health, safety and hazardous waste disposal considerations
Health and safety risks are identified for each protocol
Munns R, James RA, Sirault XRR, Furbank RT, Jones HG. 2010. New phenotyping methods for screening wheat and barley for water stress tolerance. Journal of Experimental Botany 61, 3499-3507.
Passioura JB. 2006. The perils of pot experiments. Functional Plant Biology 33, 1075-1079.
Passioura JB, Angus JF. 2010. Improving Productivity of Crops in Water-Limited Environments. In Donald L. Sparks editor: Advances in Agronomy, Vol. 106, Burlington: Academic Press, 2010, pp.37-75.