The effects of root temperature on water flux, potential and root resistance in sunflower

Abstract

— We observed the effects of a sharp fall in temperature of the root medium over a 24-h period, on flux and water status in sunflowers, under controlled conditions using a device which simultaneously recorded the absorption, the transpiration, the water potential of the leaves and the temperature of the nutrient solution. For the lowest temperatures, we observed that the water fluxes were temporarily uncoupled and that a water deficit appeared in the plant inducing a fall in leaf water potential. Then transpiration itself decreased, following a very similar pattern to that of absorption. We were able to observe the fall in root conductivity below temperatures of 10.0 C. For the lowest tem- peratures, water resistance increased dramatically and rapidly, and decreased a few h later. On the contrary, above the threshold temperature of 6 C (where both phenomena occurred successively) the water root resistance increased after a few h. Finally, raising the temperature to 20.0 C caused the rise in leaf water potential to nearly the control value. However, although the water fluxes increased, they remained lower than their initial values. root temperature / water flux / potential / root resistance / acclimatization Rsum — Effets de la temprature racinaire sur le flux hydrique, le potentiel hydrique et la rsistance raci- naire chez le tournesol. Nous avons suivi l'influence d'un abaissement de la temprature de la solution nutritive (20 C 2,5 C), pendant 24 h, sur les flux et l'tat hydrique d'un tournesol en conditions contrles, grce un dispo- sitif mesurant simultanment et en continu (fig 1): l'absorption, la transpiration, le potentiel hydrique foliaire et la tem- prature du milieu racinaire. Pour les tempratures les plus froides nous observons un dcouplage du flux d'absorption par rapport la transpiration, conduisant au dficit hydrique du vgtal (fig Le potentiel hydrique foli- aire chute. Ce dcouplage n'est que temporaire, la transpiration venant rapidement s'aligner sur l'absorption. Nous pouvons alors quantifier la baisse de la conductivit hydraulique des racines partir d'une temprature de 10 C (fig Pour les tempratures les plus froides (2,7 C et 4,2 C), la rsistance hydraulique (fig 4) augmente fortement et trs rapidement pour diminuer sensiblement quelques h aprs le choc (acclimatation). Par contre, au-dessus d'une temprature charnire de 6 C (o l'on retrouve les 2 tendances), la rsistance hydraulique racinaire n'augmente qu'aprs plusieurs h. Enfin le potentiel hydrique foliaire retrouve un niveau proche de celui qui prcdait le choc, alors que les flux hydriques tout en augmentant, restent infrieurs leurs valeurs initiales. temprature racinaire / flux hydrique / potentiel / rsistance racinaire / acclimatation