Alteration of transpiration rate, by changing air vapour pressure deficit, influences leaf extension rate transiently in Miscanthus

A controlled environment chamber for whole plants is described in which vapour pressure deficit (VPD) and temperature can be controlled independently. Plant responses to changes in VPD at constant temperature were measured in terms of leaf extension and plant transpiration rates. Manipulation of VPD independently of temperature was shown to be capable of altering the leaf extension rates of the C-4 grass Miscanthus x giganteus grown in hydroponics. The effects of VPD on leaf extension are attributed to changes in transpiration rate and hence leaf water status. It was found that, at a temperature of 20 degrees C, the influence of a fixed change in VPD was proportionally less than those observed at temperatures which are close to the threshold for growth (between 6 and 10 degrees C), These responses are discussed in relation to our current understanding of the mechanisms of cell growth. The fact that the VPD effects on leaf expansion rates were largely transient suggest that sample models driven by temperature alone are adequate to predict leaf expansion within the temperature range 6-20 degrees C, for this genotype of Miscanthus, in the field.