Cold tolerance in beans (Phaseolus spp.) as analyzed by their exotherms

Se evaluo la tolerancia al frio en frijol comun, Phaseolus coccineus y varias lineas de frijol en condiciones artificiales. Las plantulas se expusieron en forma gradual a -6 grados C y luego rapidamente a -12 grados C en camaras de crecimiento, con pares termicas de Cu-constantan adheridos a diversas partes de las plantas. Se analizaron las curvas de congelacion de plantulas con exotermos y se midieron tres parametros basicos: tiempo requerido para presentarse el exotermo, temp. a la cual aparece el exotermo y aumento en la temp. causado por el exotermo. La aparicion del exotermo en frijol se relaciono con el dano causado por congelacion y la muerte de las plantulas. Se presentaron simultaneamente los exotermos de plantulas de frijol comun var. Bush Blue Lake 92, registrados en el tallo, en la base, apice y peciolo de la hoja primaria y en la hoja verdadera; sin embargo, su forma fue diferente. Los exotermos del tallo se presentaron a temp. superiores a los de las hojas primarias. Sus formas siempre tuvieron el mismo patron. Se encontro que el frijol con tolerancia al frio producia exotermos despues de los susceptibles. Al exponer plantulas de frijol de 3 semanas de edad a 12 grados C, se retardo el tiempo de aparicion del exotermo en el cv. Bush Blue Lake 92, susceptible al frio. Algunas plantas sobrevivieron a 2 h de exposicion a -6 grados C en la camara de crecimiento. (RA-CIAT) spa

Cold tolerance of common bean (Phaseolus vulgaris L.), runner bean (P. coccineus L.) and several bean lines was evaluated under artificial conditions. Seedlings were exposed to -6 degrees C gradually, then to -12 degrees C rapidly in the growth chamber with copper-constantan thermocouples attached to various parts of the plant. Seedling freezing curves with exotherms were then analyzed. Three basic parameters were measured: time required for exotherm to appear, temperature of the appearance of the exotherm and temperature rise caused by the exotherm. Exotherm appearance in beans was related to freezing injury and death of the seedlings. Exotherms of the common bean variety Bush Blue Lake 92 seedling recorded at the stem, primary leaf base, tip and petiole and true leaf appeared at the same time, however their shape was different. Exotherms of stem appeared at higher temperature than those of primary leaves. Their shapes always followed the same pattern. Beans with cold tolerance were found to produce exotherms later than susceptible ones. Exposing 3 weeks old bean seedlings to 12 degrees C delayed the time of exotherm appearance in the cold susceptible cultivar Bush Blue Lake 92. Some plants survived 2 hours exposure to -6 degrees C in the growth chamber. (AS) eng