Root uptake under mismatched distributions of water and nutrients in the root zone

Most plants derive their water and nutrient needs from soils, where the
resources are often scarce, patchy, and ephemeral. It is not uncommon for
plant roots to encounter mismatched patches of water-rich and
nutrient-rich regions in natural environments. Such an uneven distribution
of resources necessitates plants to rely on strategies to explore and
acquire nutrients from relatively dry patches. We conducted a laboratory
study that elucidates the biophysical mechanisms that enable this
adaptation. The roots of tomato (Solanum lycopersicum) seedlings were
laterally split and grown in two adjacent, hydraulically-disconnected
pots, which permitted precise control of water and nutrient applications
to each compartment. We observed that physical separation of water-rich
and nutrient-rich compartments (one received 90% water + 0% nutrients and
the other received 10% water + 100% nutrients) does not significantly
stunt plant growth and productivity compared to two control treatments
(control 1: 90% water + 100% nutrients versus 10% water + 0% nutrients;
and control 2: 50% water + 50% nutrients in each compartment).
Specifically, we showed that soil dryness does not reduce nutrient uptake,
vegetative growth, flowering, and fruiting compared to control treatments.
We identified localized root proliferation in nutrient-rich dry soil
patches as a critical strategy that enabled nutrient capture. We observed
nocturnal rewetting of the nutrient-rich but dry soil zone (10% water +
100% nutrients) but not in the nutrient-free and dry zone of the control
experiment (90% water + 100% nutrients). We interpreted the rewetting as
the transfer of water from the wet to dry zones through roots, a process
commonly known as hydraulic redistribution (HR). The occurrence of HR
likely prevents the nutrient-rich soil from drying to permanent wilting
and subsequent decline of root functions. Sustaining rhizosphere wetness
is also likely to increase nutrient mobility and uptake. Lack of HR in the
absence of nutrients suggests that HR is not entirely passive,
water-potential gradient driven flow. The density and size of root-hairs
appeared to be higher (qualitative observation) in the nutrient-rich and
dry compartments than the nutrient-free and dry compartments. We also
observed organic coating on sand grains in the rhizosphere of the
nutrient-rich and dry compartments. The observations are consistent with
prior observations that root hairs and rhizodeposition aid rhizosphere
wetting. These findings were synthesized in a conceptual model that
explains how plants of dry regions may be adapted to mismatched resources.
This study also suggests that separating the bulk of applied nutrients
from the frequently irrigated soil region can increase nutrient use
efficiency and curtail water pollution from intensive agricultural
systems.