Hardening of oleander seedlings by deficit irrigation and low air humidity

The purpose of this study was to examine the effect of irrigation and air humidity during the nursery phase on the water relations, growth and survival of Nerium oleander L. (oleander) to evaluate the degree of hardening resulting from these conditions. Oleander seedlings were planted in 1860 cm(3) plastic pots and grown in two greenhouses (nursery phase), where they were subjected to four treatments: low air humidity and deficit irrigation; control air humidity and deficit irrigation; low air humidity and control irrigation; control air humidity and control irrigation. After the nursery phase, each plant was transplanted to a 6450 cm 3 plastic pot. The plants were watered I day per week to 100% water-holding capacity for 2 weeks, after which they received no irrigation for the remaining 4 weeks of the experiment. The combined effect of both treatments (deficit irrigation and low air humidity) reduced the rate of mortality after transplant compared with the control (from 92% to 32%) as a result of acclimation processes. Such behavior was related to the morphological changes observed in the aerial part (smaller plant size and lower leaf area) and in the root (shorter, thicker, more dense and less ramified roots), and with physiological changes (development of osmotic adjustment, efficient stomatal regulation and a reordering of the assimilate gradient as the flow of solutes towards the roots intensified). This response was the result of a combination of mechanisms for reducing of water through transpiration when water levels in the soil were limiting. (c) 2004 Elsevier B.V. All rights reserved.