Root Respiration, Nodulation, and Enzyme Activities in Alfalfa During Cold Acclimation

This investigation was conducted to determine the effects of hardening conditions (low temperature, short photoperiod) on root respiration, nodulation, and in vitro activities of soluble enzymes in roots of alfalfa (Medicago sativa L.) cultivars which harden or fail to harden. Under hardening conditions in growth chambers root sections of Vernal alfalfa (high hardiness) had higher rates of respiration than did those from Sonora alfalfa (low hardiness). Arrhenius plots of root respiration showed that temperature kinetics for Vernal and Sonora were similar between 4 and 40 C regardless of growth conditions although Vernal had a slightly lower Arrhenius energy of activation (11.2 to 12.6 and 13.8 to 14.4 kcal mole⁻¹, for Vernal and Sonora respectively). Roamer and Vernal (both with high hardiness) grown under hardening conditions nodulated very well and had significant rates of acetylene reduction, whereas Sonora and Caliverde (low and moderate hardiness, respectively) grown under the same conditions nodulated poorly and did not reduce acetylene. Root NAD-malate dehydrogenase, NADP-malate dehydrogenase, lactate dehydrogenase, and glutamate oxaloacetatetransaminase activities were highest in hardy plants grown under hardening conditions in growth chambers. Alcohol dehydrogenase activity was usually much higher in all cultivars, hardy and nonhardy, grown under hardening conditions. Studies with six field-grown cultivars with various degrees of hardiness yielded data similar to that from growth-chamber plants for the in vitro activities of soluble root enzyme activities. Of 10 enzymes assayed, NAD-malate dehydrogenase and NADP-malate dehydrogenase activities correlated best with hardiness and levels of soluble protein. These data indicate that root functions (respiration, the ability to nodulate, and levels of various enzyme activities) increase to a greater extent or are more pronounced in hardy cultivars compared to those with lesser hardiness during the hardening process.