Based on a physiological approach Project 7 (Root senescence) will examine the role of the root as a
source organ for nutrients after plants have entered the generative growth phase, when roots are
less supported with assimilates from the leaves. In parallel, Project 8 (Leaf senescence) will
investigate which regulatory processes are responsible for directed assimilate transport from source
leaves to either roots or grains. In a first set of experiments, barley plants will be grown
hydroponically in growth chambers under adequate or suboptimal nitrogen supply to accelerate leaf
senescence. With the onset of heading, root and leaf samples will be taken at 3-day intervals to
monitor progression of senescence using genetic and physiological markers (chlorophyll, Western
analyses of chloroplast proteins, qPCR of marker genes like SAG12, sugars, amino acids, ABA, auxin
and cytokinins) (P7+P8). In parallel, xylem exudates will be collected to monitor nutrient uptake
activity in roots via K translocation rates and meristematic root activity by cytokinin translocation
rates (P7). Photosynthetic activities during onset and progression of leaf senescence will be
measured via Imaging-PAM chlorophyll fluorometry and gas exchange measurements (P8). To
monitor assimilate export activity, leaf exudates will be collected in EDTA solution from excised leaf
blades and analyzed for sugars and amino acids. To determine the direction of assimilate fluxes, 14Clabeled sucrose will be supplied to source leaves of plants at different senescing stages, and
radioactivity will be measured in roots and developing spikes to balance carbon partitioning (P7+P8).
These experiments will be later conducted using contrasting lines from projects 1, 2, 5 or 6. In a
second approach, barley plants will be grown in the field and fertilized with either nitrate, as
senescence-retarding N form, or stabilized urea as a senescence-accelerating N form. Xylem sap and
leaf analyses will be conducted as described above, and phytohormone concentrations in developing
spikes will be measured to monitor their sink activity (P7). At harvest, grain yield and grain N and C
contents will be determined. Taken together, these two projects will characterize the
synchronization of source functions and senescence processes between roots and leaves, and
identify external triggers and endogenous signals involved in this regulation.