The effect of pretreatment temperature on the exudation of xylem sap by detached barley root systems

Abstract

Roots of barley (Hordeum vulgare L.) plants which had been grown at 20° were cooled to temperatures of 12° C or lower for 1-5 days and then returned to 20° C when measurements were made of ion and water movement into the xylem after excision. Very marked increases in exudation were observed in cooled roots, sometimes as much as four times greater than in controls kept at 20° C throughout their life. There were no consistent increases in the concentration of K(+), Ca(2+) or H2PO4' in the exudate from cooled roots and it was concluded that the extra water flow across the root occurred without any increase in the osmotic potential of the sap.Similar changes in ion absorption and translocation were found in intact plants transpiring either rapidly or slowly.Cooled roots contained appreciably more soluble carbohydrate than controls and when they were returned to 20° C their respiration was 50-120% greater. These changes were not, however, strictly correlated with differences in exudation in cooled and untreated roots. Cooled plants which had been heavily shaded respired at the same rate as controls but exuded sap at twice the rate.It is concluded that the effects of cooling on both exudation and the accumulation of roluble carbohydrate are consequences of reduced growth and the possible alteration of the selative amounts of growth substances in the root.

Regulation of K+ Influx in Barley

“…This is perhaps contrary to expectation, given the high Qlo for K4 influx in barley (9 (4,5,7,9). These authors have argued that the observed changes represent processes which tend to compensate for the relatively slower growth of roots (due to low temperature) compared to shoot.…”

“…However, Clarkson and his co-workers (4,5,7) have clearly emphasized that under natural conditions, particularly in spring and fall, temperatures experienced by roots during daylight hours may be several degrees lower than aerial temperatures experienced by shoots. The anticipated reduction of root activity (Qo values for ion uptake, e.g.…”

“…Although shoot growth is somewhat limited under these conditions, there is also clear evidence of the acclimation of root activity which apparently reduces the severity of this temperature stress. Clarkson et al (4, 5, 7) have shown that following several days of growth at low temperatures there is increased capacity of transport processes which becomes apparent when these plants are returned to high temperatures. For example, compared to control plants grown at 200C continuously, plants whose roots had been maintained at 8°C demonstrated increased rates of nutrient absorption, accumulation, translocation, and xylem exudation when these processes were measured at 20C.…”

“…Clarkson and his co-workers (4,5,7) have emphasized that the observed responses represent a strategy which compensates for the lower root activity and higher shoot activity associated with the temperature differential. However, similar increases in ion fluxes in response to pretreatment at low temperatures have been demonstrated in a giant alga where differential temperatures were not involved (12,13).…”

In Vivo Nitrate Reduction in Roots and Shoots of Barley (Hordeum vulgare L.) Seedlings in Light and Darkness

“…This indicates that during the initial 6 h after excision, the supply of endogenous metabolites for nitrate reduction is not limited. Clarkson et al (5) also observed that in barley roots there seems to be no shortage of respiratory sugars for at least 6 h after excision and 02 uptake does not slow significantly during this time.…”