Water Relations of Sunflower (Helianthus annuus) Shoots during Exposure of the Root

The influence of root anoxia on the water balance of hydroponicallygrown sunflower plants was investigated in a controlled environmentroom. Leaf water potentials declined within 1 h of the onsetof anaerobic treatment of the roots. The severity of the waterstress during the initial 8 h of treatment was dependent onthe ambient humidity of the aerial environment. Recovery ofshoot water balance occurred by the beginning of the secondfull day of treatment, without stomatal closure. After 3 d of continuous anaerobic treatment, plant responsein the same experiment followed one of two patterns. In thefirst, there were declines in leaf water content, leaf waterpotentials, lamina expansion, transpiration rates and stomatalconductance. Osmotic adjustment occurred but it was not sufficientto prevent loss of turgor and the plants wilted at day 4 oftreatment. Complete rehydration took place after 6 d of treatmentif the stem was severed (under water) just below the cotyledons,indicating that shoot dehydration was a result of xylem occlusionat the root/shoot interface. Sections of the stem in this regionshowed xylem vessels to be discoloured and plate-like structuresobstructing the lumen. In the other pattern of response, plants did not develop a morenegative leaf water potential (except for an initial periodimmediately following treatment initiation) throughout the 6-danaerobic treatment period. There were no signs of water deficitalthough other symptoms such as precocious leaf senescence andepinasty occurred concomitantly with those in the first groupof plants that developed long-term water stress. Changes in shoot water relations are discussed in terms of ourearlier observations on anoxia-induced changes in the hydraulicconductivity and selectivity of the roots. Possible explanationsare offered for inter-plant variations in response to root anoxia. Key words: Anaerobiosis, water relations, xylem blockage