Mapping compensating root water uptake in heterogeneous soil conditions via neutron radiography

Water is often heterogeneously distributed in soils. Understanding roots' responses to this soil-water heterogeneity is a key parameter defining plant survival in dry climates. To determine local root water uptake for partly dry conditions in a plant's root system, we prepared soil patches with different water contents, then used neutron radiography to monitor daily changes in root structure and water uptake. Lupin plants were grown in 30 x 25 x 1 cm(3) aluminum containers filled with sandy soil. In two partitioning set-ups, the soil-root zone was divided into either two or nine hydraulically-isolated soil compartments. This was done by packing layers of coarse sand as capillary barriers, by which vertical and/or horizontal soil water heterogeneity as well as homogeneous well-watered treatments were applied for control. Daily changes in soil water content in each compartment, water uptake and root growth were monitored non-invasively and quantified by neutron radiography during a period of 15 consecutive days. In optimal homogeneously-wet soil, lateral roots in the top 10 cm of the root system showed the highest water uptake rate, up to around 10 mg/(mm. root. day), which on average was twice as much as that for younger lateral roots in lower position and taproot. In heterogeneous treatments, root water uptake declined strongly in compartments with the soil water content below 0.13-0.10 cm(3)/cm(3) while in parallel an enhanced uptake rate, rising by up to 100 %, was observed for the roots in wet compartments, presumably to compensate for roots in dry compartments and, therefore, sustain the total transpiration. Also, our observations showed that in the drying compartment a reduction of soil water content to 0.10-0.15 triggered local cluster root formation. With the experimental set-up presented the pattern of water uptake across a lupin root system can be quantified and normalized to root length. Water uptake was shown to be highly variable in different parts of the root system. A threshold for water stress to cause cessation of local water uptake was identified, and the considerable amount of compensation by water uptake in other parts identified. The dynamic trade-off among different parts of the root system seems to regulate total root uptake also during water stress to sustain the daily transpirational demand.