Plant nutrient uptake can be determined by measuring nutrient depletion in the root environment and by quantifying nutrient content in plant tissues.
1. Measurement of nutrient depletion in the root environment
This method is based on determining the difference in the amount of a certain ion in the root environment throughout a given period of time. This difference is associated with plant nutrient uptake, which can be calculated as [(V1-C1)-(V2-C2)] (Cabrera et al., 1995). In this equation, V1 and V2 are the volumes (L) of the nutrient solution at time 1 and 2, respectively, and C1 and C2 are the nutrient concentrations (mmol L-1) at time 1 and 2, respectively.
This method allows a good accuracy of nutrient uptake over time (Klaring, 2001) and the results are comparable to those obtained by destructive long-term N measurements (Barak et al., 1996). However, a correct methodology should be applied to avoid errors in the measurements. Obtaining samples from the root environment is difficult, and samples of the drainage might not represent the composition of the nutrient solution surrounding the roots. In that case, a soilless system with a low inertia should be used (e.g. nutrient film technique and aeroponic system). Moreover, the system should avoid evaporation so that all volume losses can be attributed to water and nutrient uptake. Finally, this method is less accurate when nutrient solution concentration is elevated (Le Bot et al., 1998a) thus, diluted solutions are recommended.
This method is based on measuring nutrient content in plant tissues. Not only can this method provide information about plant uptake, but it can also differentiate between the allocations of this uptake with different parts of the plant. This technique is very useful in crops with a growing period of several months, in which nutrient content in their tissues can be easily related to its uptake during a known period of time. However, in other crops such as woody plants in which cultivation lasts for several years, the time when the nutrient content measured in plant tissues was absorbed is more difficult to estimate. Redistribution processes among the different parts of the plant are common in woody plants. For example, in rose plants, endogenous N is redistributed within the plant during each flowering cycle (Cabrera et al., 1995). Therefore, in these cases, the measurement of nutrient content in plant tissues can be carried out by using isotopically labelled fertilizers and tracing the fate and recovery of these nutrients by the crop (Strong, 1995). Nitrogen is the element that has been most widely used as labelled N for being quantitatively the most abundant in plant tissues. NO3~ and/or NH4+ fertilizers have been used in several crops (Dong et al., 2001; Gonzalez-Mas et al., 2007; Quinones et al., 2003). The disadvantage of measuring the nutrient content in plant tissues is that it is a destructive technique and, while using labelled fertilizers, it is expensive and requires qualified personnel.
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