Water stress, carbon dioxide, and light effects on sucrose-phosphate synthase activity in Phaseolus vulgaris

Abstract

The characteristics of sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity in leaves of Phaseolus vulgaris L. cv. Linden was studied in plants subjected to water stress and various CO2 and light treatments. When water was withheld for 3 days causing mild water stress (–0.9 MPa), the activity of SPS measured in crude extracts was reduced ca 50%. The effect of water stress was most evident when the enzyme was assayed with saturating amounts of its substrates fructose 6-phosphate and UDP glucose. Placing a water-stressed plant in an atmosphere containing 1% CO2 reversed the effect of water stress on SPS activity over 5 h even though the water stress was not relieved. Holding unstressed leaves in low CO2 partial pressure reduced the extractable activity of SPS. After 1 h of low CO2 treatment the effect of low CO2 could be reversed by 20 min of 5% CO2. However, after 24 h of low CO2 treatment, less SPS activity was recovered by the 20 min treatment. The cytosolic protein synthesis inhibitor cycloheximide prevented the slow recovery of SPS activity, but did not affect the rapid recovery of SPS. We conclude that the effect of water stress on SPS activity was a consequence of the inhibition of photosynthesis caused by stomatal closure. Responses of Phaseolus vulgaris SPS to light were similar to the response to low CO2 in that the effects were most pronounced under Vmax assay conditions. This is the first report of this type of light response of SPS in a dicotyledonous species.