Mechanism of High Temperature Adaptation in Maize.

High temperature (HT) stress severely limits plant productivity and causes extensive economic loss to US agriculture.  Understanding HT adaptation mechanisms in crop plants is crucial to the success of developing HT tolerant varieties to alleviate the negative impact of HT stress on plant growth and development, especially on the reproduction tissues.  Maize inbred lines vary greatly in HT tolerance based on field observations. Two contrasting maize inbred lines, B76, heat-tolerant and B106 heat-sensitive, were selected to conduct detailed physiological, biochemical, and genetic analyses to identify mechanisms accounting for HT tolerance between the two lines.  Leaf firing and tassel blasting are the common heat-injury symptoms observed in sensitive B106 plants.  Preliminary results indicate that the HT tolerance mechanism in these lines is independent of the induction of HSPs and EF-Tu.  Photosynthetic capability measured as quantum yield of PSII was significantly decreases in B106 when temperature reached 33-34°C or higher, whereas no significant reduction was observed in B76 at temperatures of 37 to 38ºC.  Electrolyte leakage results revealed that high temperatures caused cell injury and eventually cell death in B106 tissues, indicating that the cell membranes of B106 are more sensitive to high temperature stress than those of B76.  Lipid profiles revealed a close correlation between heat sensitive of B106 tissues and an extremely low amount of phosphatidic acid (PA) of these tissues.  Recent studies indicate that PA acts as a lipid mediator and is involved in multiple signal transduction pathways and membrane trafficking.  It also plays regulatory roles in plant response to ABA, biotic and abiotic stresses.  For genetic study, a recombinant inbred line population of B76 and B106 has been developed. Major QTLs associated with HT tolerance will be mapped and molecular markers tightly linked to the QTLs will be identified for marker-assisted-selection of HT tolerant varieties in maize.