Integral water capacity (IWC) and least limiting water range (LLWR): prediction using plant growth indices and soil properties.

Soil water availability is an important field of study in soil water and plant relationship. Least limiting water range (LLWR) and integral water capacity (IWC) are two important concepts which are used for water availability to plant. LLWR is determined from four moisture coefficients ( , , , ) that are the soil water contents 10% air-filled porosity (AFP), at field water capacity (FC), 2 MPa penetration resistance (SR), and permanent wilting point (PWP), respectively. The computation is dependent on critical values, so IWC was introduced to avoid using the critical limits that sharply rises in a cut-off from 0 to 1 at the wet end of water release curve or sharply falls from 1 to 0 at the dry side in the previous concepts of water availability for plant. IWC is the integral of differential water capacity function () in the amplitude of 0 to infinity soil matric potential () multiplied by some weighting functions ( ) each considering the effect of various soil limitations on water availability to plants. Up to now, the effect of different soil attributes and the tillage treatments have been reviewed on LLWR. The effect of soil various physical and chemical limitations such as soil hydraulic conductivity (), aeration, SR, and salinity has been considered on IWC computation. LLWR and especially IWC have been seldom studied using plant real response. Results of few studies about LLWR and IWC using stomatal conductance and canopy temperature showed that their values were considerably different with those computed based on previously introduced critical limits for LLWR and weighting functions for IWC. These differences indicate that the critical limits proposed by da Silva et al. (Soil Sci Soc Am J 58:1775-1781, 1994) and weighting functions by Groenevelt et al. (Aust J Soil Res 39:577-598, 2001) may not be applied indiscriminately for all plants and should to be modified according to plant response. Physiological characteristics like transpiration and photosynthesis rate, chlorophyll index, leaf water potential, and relative water content also could be appropriate indices for monitoring plant water status and computation the real value of LLWR and IWC in the field or greenhouse for various types of plants.