REGULATION OF SUCROSE AND STARCH METABOLISM IN POTATO-TUBERS IN RESPONSE TO SHORT-TERM WATER-DEFICIT

To investigate the effect of water stress on carbon metabolism in grow ing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential between 0 and -1.2 MPa. (i) Incorporation of [C-14]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode the changes a t low water stress (up to -0.5 MPa) but not at high water stress. (ii) Although [C-14]sucrose uptake increased in water-stressed discs, less of the absorbed [C-14]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit l eads to a switch, from net sucrose degradation to net sucrose synthesi s. (iv) In parallel incubations containing identical concentrations of sugars but differing in which sugar was labeled, degradation of [C-14 ]sucrose and labeling of sucrose from [C-14]glucose and fructose was f ound at each mannitol concentration. This shows that there is a cycle of sucrose degradation and resynthesis in these tuber discs. Increasin g the extent of water stress changed the relation between sucrose brea kdown and sucrose synthesis, in favour of synthesis. (v) Analysis of m etabolites showed a biphasic response to increasing water deficit. Mod erate water stress (0-200 mM mannitol) led to a decrease of the phosph orylated intermediates, especially 3-phosphoglycerate (3PGA). The decr ease of metabolites at moderate water stress was not seen when high co ncentrations of glucose were supplied to the discs. More extreme water stress (300-500 mM mannitol) was accompanied by an accumulation of me tabolites at low and high glucose. (vi) Moderate water stress led to a n activation of sucrose phosphate synthase (SPS) in discs, and in inta ct tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked by protein phosphatase inhibitors. (vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate w ater stress, and when the sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when disc s were incubated in 300-500 mM mannitol. (ix) It is proposed that mode rate water stress leads to an activation of SPS and stimulates sucrose synthesis. The resulting decline of 3PGA leads to a partial inhibitio n of ADP-glucose pyrophosphorylase and starch synthesis. More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more of the enzymes involved in the terminal reactions of starch synthesis.