Revisiting integral water capacity on the basis of stomatal conductance at various soil and root length densities in sunflower plant

Computation of integral water capacity (IWC), as an alternative criterion for soil water availability to plant, requires developing of weighting function (ω(h)) for each limiting factor may prevail within a particular water or suction range (e. g., hd and hw) in the root medium and diminish water availability to plant. So far defining hd and hw limits for a particular limiting factor has been primarily soil property based. We investigated the possibility of plant response based ω (h) as a proxy. For this purpose, a sandy clay loam soil was evenly compacted to three bulk densities (Db) of 1.35, 1.55 and 1.75 Mg m-3 in large (30 cm diameter and 70 cm depth) PVC tubes (called pots hereafter). Sunflower (Helianthus Annuus L) seedlings were planted in the pots and after their full establishment (at late vegetative growth stage) root pruning was implemented by cutting off approximately 75%, 50%, 25% and 0% of the root branches in the 0-25 cm section along the root crown. Two periods of wetting and drying cycles were subsequently imposed and soil water contents and midday leaf stomatal conductance were routinely measured. At the end of the experiment total root length in the samples separately taken from three sections (5-25, 25-50 and 50-70 cm) of each pot was measured using GSA image analyzer software and converted to root length density (RLD). Relative stomatal conductance (g/gc), presumably varying between 0 to 1 and comprising impact of all water availability limiting factors in the root medium, as function of h were developed and substituted for ω(h) and the plant response based integral water capacity (IWCP) was then computed and compared between various imposed treatments and with least limiting water range (LLWR) and plant available water (PAW). For the treatments without root pruning (L4), increase in soil compaction (from D1 to D2 and from D1 to D3) reduced IWCP by about 17.5% and 20.5%, respectively. The corresponding decreases for LLWR were, 22% and 66%, respectively, and negligible for PAW. These results showed that PAW is almost irresponsive and LLWR is over responsive to soil compaction or bulk density while IWCP seems reflect more realistic influence of soil compaction on soil water availability to plants. Decrease in IWCP from L4 to L1 was by 43%. In contrary to LLWR and PAW, our proposed method showed the effect of root length density on plant available water.