Control of wheat root growth. The effects of excision on growth, wall rheology and root anatomy.

Excision and subsequent incubation of the apices (1 cm) of wheat (Triticum aestivum L.) seedling roots in simple media severely reduced elongation from 28 mm·(24 h)(-1) in intact roots to a maximum of 2 mm·(24 h)(-1) in excised roots. The reduction in growth was accompanied by a loss of cell turgor in the growing zone but was correlated with a hardening of the cell walls in this region. Rheological properties were measured as percent extensibility (both plastic and elastic) using a tensiometer, and as instantaneous volumetric elastic modulus (ε i) using the pressure probe. Excision decreased plastic and elastic properties with a half-time of some 60 min. Plastic extension was reduced from 2.5% to 0.9% and elastic from 4.8% to 2.6% for an 8-g load. By contrast, ε i was increased by excision. The observed reduction in root elongation rate was accompained by a reduction in mature cell length from 240 μm to 40 μm and a shortening of the zone of cell expansion.