Grazing winter wheat relieves plant water stress and transiently enhances photosynthesis

To model the impact of grazing on the growth of wheat (Triticum aestivum L.), we measured photosynthesis in the field. Grazing may affect photosynthesis as a consequence of changes to leaf water status, nitrogen content per unit leaf area (N_a) or photosynthetic enzyme activity. While light-saturated CO₂ assimilation rates (A_sat) of field-grown wheat were unchanged during grazing, A_sat transiently increased by 33-68% compared with ungrazed leaves over a 2-to 4-week period after grazing ended. Grazing reduced leaf mass per unit area, increased stomatal conductance and increased intercellular CO₂ concentrations (C_i) by 36-38%, 88-169% and 17-20%, respectively. Grazing did not alter N_a. Using a photosynthesis model, we demonstrated that the increase in A_sat after grazing required an increase in Rubisco activity of up to 53%, whereas the increase in C_i could only increase A_sat by up to 13%. Increased Rubisco activity was associated with a partial alleviation of leaf water stress. We observed a 68% increase in leaf water potential of grazed plants that could be attributed to reduced leaf area index and canopy evaporative demand, as well as to increased rainfall infiltration into soil. The grazing of rain-fed grain cereals may be tailored to relieve plant water stress and enhance leaf photosynthesis.