Transpiration and drought stress recovery of three zinnia cultivars

Abstract

Transpiration and drought stress recovery were investigated in three container-grown zinnia cultivars [Zinnia elegans Jacq. (‘Lilliput’ and ‘Thumbelina’) and Z. haageana Reger (‘Persian Carpet’)] by measuring daily changes in the normalized transpiration ratio (NTR) of well-watered (control), water-stressed, and water-stressed/re-watered plants. Transpiration of plants grown in gradually drying substrate did not decline until the fraction of transpirable substrate water (FTSW) reached 0.16 to 0.12. Symptoms of plant-water stress (i.e. foliar wilt) were first observed on the leaves of ‘Persian Carpet’, which was also the cultivar with the highest average daily transpiration rate. By comparison, the remaining two cultivars (‘Lilliput’ and ‘Thumbelina’) exhibited lower average daily transpiration rates and took significantly longer to reach the same dry-down endpoint (NTR ≤0.15). Drought stress recovery was assessed by comparing xylem water potential and root and shoot dry weight in well-watered and in drought-stressed plants following a 7-day stress amelioration period. Xylem water potential of all three drought-stressed cultivars increased (i.e. became less negative) one week after re-watering. Root biomass and root:shoot ratio were both significantly greater in water-stressed plants than in well-watered plants of the same cultivar, a finding that suggests the likelihood of osmotic adjustment in response to drought.