COLD TOLERANCE OF SOYBEAN (GLYCINE-MAX (L) MERR) DURING THE REPRODUCTIVE PHASE

Ten indeterminate MG00 and MG000 soybean genotypes of Swiss and North American origin, representing different levels of cold tolerance, were studied during 3 years in a total of five environments. The results s howed that cold tolerance during flowering is based on three independe nt and synergistic traits, viz: number of flowers produced on central racemes; capacity to retain central raceme flowers despite cold stress ; and capacity to compensate a loss of central raceme flowers by rapid and sustained flower development on lateral racemes. Within the mater ial studied large genotypic variability exists for each of these trait s. Particularly interesting is a 3 degrees difference between tolerant and susceptible genotypes in the threshold below which temperatures c an be considered to be damaging (15 and 18 degrees C, respectively). T he relative importance of different compensation mechanisms is discuss ed and the vulnerability of genotypes showing synchronous flowering of central and lateral racemes is confirmed. At harvest, susceptibility of soybean genotypes to low temperatures occurring during flowering ca n be distinguished by an irregular distribution of pods and seeds alon g the stem. Low yielding or barren nodes were the result of stress ind uced flower abscission. Results showed that narrow sense tolerance, i. e., reduced abscission of flowers following cold stress, can be assess ed at maturity by rating pod set on central racemes of the median part of the stem.