TRANSPIRATION INHIBITION BY STORED XYLEM SAP FROM WELL-WATERED MAIZE PLANTS

There is increasing evidence that a chemical signal exists in xylem sa p of plants subjected to water deficits which influences physiological responses in plant shoots, An important method of studying this signa l is the transpiration response of excised leaves exposed to xylem sap collected from plants, However, Munns et al, [Plant, Cell & Environme nt 16, 867-877] cautioned that transpiration inhibition is observed wh en xylem sap collected from wheat and barley is stored before determin ing physiological activity. The objective of the study reported here w as to determine if transpiration inhibition develops in maize sap coll ected from well-watered plants when the sap is stored under various co nditions, It was found that storage of maize sap collected from well-w atered plants for only Id at -20 degrees C resulted in the development of substantial transpiration inhibition in bioassay leaves, Storage o f sap at 4 degrees C resulted in the development of the effect after 2 weeks, while storage at -86 degrees C showed only small transpiration inhibition after 3 weeks, The major source of the transpiration inhib ition was the development of a substance in the stored sap that result ed in physical blockage of the transpiration stream in bioassay leaves , However, a small signal component may also have developed in the sto red sap, Because of the possibility of ionic activity under freezing c onditions at -20 degrees C, calcium was studied for its potential invo lvement in the transpiration inhibition. However, the calcium concentr ations found to inhibit transpiration were nearly an order of magnitud e larger than the calcium concentrations observed in xylem sap.