Evaluation location: Iowa, United States
PLANTS FROM 11 ACCESSIONS OF 9 ALNUS AND BETULA TAXA PROPAGATED FROM SEED (1999), STEM CUTTINGS (1999), OR SHOOT CULTURE (2000) WERE SUBJECTED TO THREE CYCLES OF DROUGHT TO EVALUATE THEM FOR DROUGHT TOLERANCE. PLANTS WERE GROWN SINGLY IN 700 CUBIC CM OF SOIL: SPHAGNUM PEAT MOSS: PERLITE (1:3:1) IN PLASTIC CONTAINERS (TOP DIAMETER 15 CM). ONE ACCESSION PER TAXON WAS USED, EXCEPT FOR A. JAPONICA AND A. MARITIMA, WHERE TWO ACCESSIONS OF EACH WERE BULKED. ROOT ZONES OF CONTROL PLANTS WERE SATURATED DAILY WITH TAP WATER; DROUGHT WAS IMPOSED BY WITHHOLDING IRRIGATION, BASED ON DIFFERENTIAL EVAPOTRANSPIRATION RATES AMONG THE TAXA. WATER USE WAS ESTIMATED BY TRACKING THE TOTAL WEIGHT OF THE SEVEN REPLICATES OF EACH TAXON IN THE DROUGHT TREATMENT WITHIN ALL DROUGHT CYCLES. DECREASES IN WEIGHT WERE ASSUMED TO BE DUE TO EVAPOTRANSPIRATION. TREATMENT CYCLES WERE SEPARATED BY A 7-DAY RECOVERY PERIOD BETWEEN CYCLES 1 AND 2, BUT CYCLE 3 BEGAN THE DAY AFTER CYCLE 2 ENDED. DROUGHT CYCLES WERE ENDED THE FIRST DAY WHEN LEAVES OF AT LEAST FIVE OF THE NINE TAXA WERE WILTED BEFORE A DAILY IRRIGATION. DROUGHT CYCLES 1, 2, AND 3 EACH LASTED 7 DAYS. TREATMENTS WERE APPLIED SIMULTANEOUSLY. PHOTOSYNTHETIC RATE OF A STANDARD LEAF ON EACH PLANT WAS MEASURED WITH A LI-COR LI-6400 BEFORE IRRIGATION ON THE DAYS CYCLES 1 AND 3 ENDED. MOISTURE CONTENT OF UPPER 6 CM OF THE ROOT MEDIUM OF EACH PLANT WAS MEASURED (MODEL HH1 THETA PROBE/ML1 SENSOR). HEALTH OF SHOOT SYSTEMS WAS SUBJECTIVELY RATED BY TWO EVALUATORS. A RANGE OF RATINGS (1 = DEAD PLANTS THROUGH 5 = EXCELLENT HEALTH) WAS USED. TOTAL LEAF SURFACE AREA OF EACH PLANT WAS DETERMINED (LI-COR MODEL 3100 AREA METER). DATA ON PHOTOSYNTHESIS AND LEAF AREA WERE TRANSFORMED FOR GRIN BY PRESENTING THEM AS PERCENTAGES OF THE MEAN VALUES FOR THE CONTROL TREATMENTS.