The Influence of Temperature on Root Hair Infection ofTrifolium parviflorumandT. glomeratumby Root Nodule Bacteria

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Abstract
The onset and rate of infection in root hairs of T. parviflorum and T. glomeratum inoculated with Rhizobium trifolli strain 5 varied much with root temperature. At moderate root temperature (18, 24, and 30 °C) infections were initiated earlier and in larger numbers than at low (6 and 12 °C) or moderately high (36 °C) temperatures. Both species showed a broad temperature optimum between 18 and 30 °C. The site of thread initiation (apically or laterally in a hair) was independent of temperature, as was also the proportion of successful threads penetrating the root cortex, which increased with seedling age. Threads grew more slowly at low temperatures. The size of hair nuclei near infection threads remained unaffected by temperature, but nuclei associated with laterally originating threads were larger than those associated with apical threads. Infection was non-randomly distributed along the main root at all temperatures. More zones of infection were found at moderate temperatures than at temperature extremes (6–12 or 36 °C). Probit plots of numbers of infections for individual plants were steplike, the linear sloping parts corresponding to normal distributions of infection within zones. Between 18 and 30 °C numbers of infections increased exponentially in two phases, the first more rapid phase ending at about the time nodules appeared. A model devised for the infection process and fitted to the data suggested the existence of two kinds of infections: primary ones occuring randomly at a slow rate and probably not affected by temperature and secondary infections that appeared to increase with rising temperatures in the range 12 to 30 °C. Nodule numbers were relatively more sensitive to high and low temperatures than infection. The numbers of infections and nodules and the root lengths of T. parviflorum were twice those of T. glomeratum except at the temperature extremes. Numbers of infections were otherwise unrelated to root length or cotyledon or leaf areas. The development of lateral organs (primordia, lateral roots, and nodules) was reduced at temperatures below 18 °C and above 30 °C.