PARENT SESSION
Poster Session #45: Gas Exchange.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Diurnal variation in the stable isotopes of transport fluids and metabolites in field-grown Lupinus angustifloius.

Cernusak, Lucas¹, Pate, John², Arthur, David², Farquhar, Graham¹, ^{1 2}

ABSTRACT- We measured the isotopic composition of xylem and phloem sap water (D and ¹⁸O), phloem sap sugars (¹⁸O, ¹³C, and ¹⁵N), and xylem solutes (¹³C and ¹⁵N) collected from field-grown Lupinus angustifolius at one to two hour intervals over a 26 hour period. Phloem sap was collected by cutting the distal tips of attached pods. Xylem sap D ranged from -16 to 3‰ and xylem sap ¹⁸O from -3.6 to -1.1‰. No diurnal pattern in the isotopic composition of xylem water was observed. Phloem sap water, on the other hand, ranged from 28 to 70‰ for D and from 7.4 to 16.1‰ for ¹⁸O. Phloem sap water was most enriched at the end of the day and least enriched late at night. The isotopic composition of phloem sap water appeared to vary in response to diurnal changes in relative humidity, but with a time lag of two to four hours. The ¹⁸O of phloem sap sugars ranged from 32.6 to 35.4‰. Sugar ¹⁸O increased from mid-morning to the end of the day and remained enriched throughout the night. In contrast, phloem sap sugar ¹³C showed little diurnal variation; values ranged from -25.6 to -23.1‰. The ¹³C of phloem sap sugar was enriched by about 2‰ relative to xylem solutes; in contrast, ¹⁵N did not differ between phloem and xylem. Results will be discussed in the context of theory relating the isotopic composition of plants to their environmental conditions.

KEY WORDS: stable isotope, phloem sap, diurnal