ISOPRENE EMISSION, PHOTOSYNTHESIS, AND GROWTH IN SWEETGUM (LIQUIDAMBAR STYRACIFLUA) SEEDLINGS EXPOSED TO SHORT-TERM AND LONG-TERM DRYING CYCLES

Isoprene emissions were studied in one-year old sweetgum (Liquidambar styraciflua L.) seedlings during nine drying-rewatering cycles extendi ng over five months. Each drying cycle lasted to the point of leaf wil ting. Growth was essentially stopped in response to the first drying c ycle, though seedling survival and capacity to recover turgor on rewat ering remained high throughout the entire nine cycles. Photosynthetic rates of leaves were inhibited by the drying treatments. Under severe drought, isoprene emission rates of leaves were also inhibited, though isoprene emission was generally less sensitive to drought than photos ynthesis. The lower drought sensitivity of isoprene emission compared with photosynthesis resulted in a higher percentage of fixed carbon lo st as isoprene as seedlings became more stressed. During the recovery phase of the drying-rewatering cycles, isoprene emission rates in seve ral seedlings were higher than in well-watered control seedlings. Foll owing the ninth drying-rewatering cycle, sustained daily watering resu lted in recovery of isoprene emission rates to control values within f our days. Photosynthetic rates only recovered to 50% of control values after seven days. We conclude that the mechanisms regulating photosyn thetic rate and isoprene emission rate are differentially influenced b y limited water supplies. The results are consistent with past studies that predict a protective role for isoprene emission during stress, p articularly protection from excessive leaf temperatures during drought .