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FORAGE & GRAZINGLANDS

Water Deficit Alters Canopy Structure but not Photosynthesis during the Regrowth of Alfalfa

^a Tasmanian Institute of Agricultural Research, Univ. of Tasmania, Private Bag 3523, Burnie, TAS, Australia, 7320
^b Dep. of Agronomy, Purdue Univ., 915 West State St., West Lafayette, IN 47907-2054

^* Corresponding author (Keith.Pembleton{at}utas.edu.au).

Growth of alfalfa (Medicago sativa L.) following the correction of a water deficit will depend in part on the characteristics of the canopy grown during the water deficit. The current study was undertaken to determine the extent to which canopy structure and photosynthetic capacity are affected in alfalfa plants regrowing following defoliation while exposed to a water deficit. The effects of five levels of water application (100, 75, 50, 25, and 0% of the replacement water requirement) during the regrowth of two alfalfa cultivars with contrasting levels of fall dormancy was investigated in a controlled environment. Mass per shoot and shoots per plant were both significantly (P < 0.05) reduced by water applications 75% or less of the replacement water requirement. Dry matter accumulation was reduced in proportion to the level of the water deficit the plants were exposed to. The rate of CO₂ assimilation and maximum efficiency of photosystem II were unaffected (P > 0.05) by water deficits except for the unwatered plants on which leaf senescence occurred. The only inhibition in the plants' capacity for photosynthesis from a water deficit was reduced leaf area. Shoot density and reduced overall canopy photosynthesis were the only canopy characteristics that were altered by water deficit in a way that could continue to inhibit plant growth following the correction of the water deficit.

Abbreviations: DM, dry matter • ET, evapotranspiration • Fv/Fm, maximum efficiency of photosystem II • MSC, mean stage count • MSW, mean stage weight • PAR, photosynthetically active radiation • RH, relative humidity