GROWTH AND SOLUTE ADJUSTMENT OF CALLI OF POPULUS CLONES CULTURED ON NUTRIENT MEDIUM CONTAINING POLYETHYLENE-GLYCOL

To determine whether in vitro assays can be used as a surrogate for wh ole plants in the assessment of drought tolerance, six clones in a ped igree consisting of black cottonwood (Populus trichocarpa Torr. & Gray , female) and eastern cottonwood (Populus deltoides Bartr., male) and four hybrid progeny were regenerated in tissue culture and subjected t o 0-20% polyethylene glycol (PEG) mel. wt. 8000 in the nutrient medium , providing osmotic potentials that ranged from -0.40 to -1.29 MPa. Dr ought tolerance was assessed by callus growth and solute accumulation under various stress conditions. Among the cultures grown in the light (16-h photoperiod), clones with the highest relative growth rate of c allus dry weight after 18 d in the nutrient medium without PEG include d hybrids 246 female,242 male, and the eastern cottonwood male parent; the same clones that tended to be the most drought tolerant in previo us whole-plant studies. However, there were few clonal differences in relative growth rate across PEG concentrations or among light or dark environments, and any differences were not related to solute accumulat ion. Polyethylene glycol (20%) provided a severe osmotic stress that d ehydrated callus cultures, thereby suppressing relative growth rate on a fresh-weight basis, but not on a dry-weight basis. Inorganic cation s and free amino acids constituted the bulk of the solutes in calli, w hereas concentrations of soluble carbohydrates were low. Therefore, re sults obtained from callus cultures growing under artificially induced drought regimes did not parallel those obtained from water stress stu dies on whole plants.