Effects of Defoliation on Net Photosynthesis and Regrowth of Western Wheatgrass

Abstract

Net photosynthesis (PN) and regrowth of 60-day old Agropyron smithii Rydb. plants were examined over a 10-day period following defoliation to simulate grazing. Plants grown hydroponically in full strength Hoagland's solution were moderately defoliated (1/2 tillers clipped at 5 cm), heavily defoliated (3/4 tillers clipped at 5 cm), or left as unclipped controls. Thirty minutes after clipping, PN rates of the youngest fully expanded leaf of a remaining undamaged tiller had declined by 6%-7% in both groups of defoliated plants. Rates of PN were subsequently monitored on the same leaves at 2-day intervals. By Day 2, PN (per unit of leaf area) of both defoliated groups had increased to rates 5-10% higher than those preceding treatment, while PN of control plants had decreased about 6%. From Day 2 through Day 10, PN rates of control plants averaged 90% of their preclipping PN rates, while PN rates of moderately and heavily defoliated plants averaged 106% and 114% of their preclipping rates, respectively. Defoliation had no significant effect on tiller production over this 10-day period. While total new biomass production of controls was almost twice that of either of the defoliated groups, the proportion of the new growth allocated to shoots, crowns and roots did not differ among the three groups.