Determination of the Least Limiting Water Range Based on Sunflower Plant Response

The range of soil water content where plant growth is least limited by water potential, soil aeration or mechanical resistance is called least limiting water range (LLWR). This study evaluated the values of LLWR determined according to the procedures proposed by da Silva et al. with those calculated on the basis of sunflower plant (Helianthus annuus L) response (LLWRP). In both methods LLWR is taken as the difference between the two soil moisture limits designated as upper (θUL) and lower (θLL). In the first method, the two limits are determined basically from the soil moisture and soil resistance characteristic curves, almost overlooking the plant type and its particular needs or behaviors. In the second method, as proposed in this research, the two limits are determined based on the stomatal response in a sandy clay loam soil packed into PVC tubes (called pots hereafter) with 30 cm diameter and 70 cm height at three compaction levels (soil bulk density equal to 1.75, 1.55 and 1.35 Mg.m-3) designated as D1, D2 and D3. Each pot was planted with three pre-soaked sunflower seeds and pots were kept under optimum condition until onset of the flowering stage. At this time two successive drying cycles were imposed and soil moisture and midday stomatal conductance were routinely measured. LLWRP were computed on the basis of relationship between soil matric suction and stomatal conductance. Results showed that on the basis of stomatal conductance behavior water uptake began at the soil matric suctions of 44, 16, 60 and continued up to 17394,31614, 39983 cm in D1, D2 and D3 treatments, respectively. Appreciable differences were observed between LLWR and LLWRP particularly when the lower limit moisture suction (equivalent to θUL for LLWR) was set at 330 cm (LLWR330). LLWR330 values of 0.148, 0.147 and 0.080 cm3cm-3 were obtained for D1, D2, D3 treatments, respectively, which were 51, 49 and 63 percent lower than the corresponding LLWRP values. This differences imply that the two moisture limits (θUL and θLL) proposed by da Silva et al. may not be applied indiscriminately for all plants and thus need to be modified according to plant needs or responses.