Water Stress Induced Changes in the Amounts of Some Photosynthetic Assimilation Products and Respiratory Metabolites of Sunflower Leaves

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Abstract
Sunflower leaves, water-stressed under controlled conditions, contained greater amounts of amino acids as their water potential decreased, with glycine, serine, and glutamate increasing more than alanine and aspartate. Proline accumulated only at severe stress. Low O2 concentration altered the amounts of amino acids, principally decreasing the amount of glycine and increasing serine. The changes in total pool size are related to previous results on the accumulation of 14C and the specific activity of products. Photorespiration was large under water stress, where leaves accumulated carbon in glycine of low specific activity, and in 21% O2, where both total amount and specific activity of glycine was greater than in 1.5% O2. This suggests that there are two pools of glycine, one controlled by O2 and closely related to photosynthesis, the other non-photosynthetic and affected by water stress. The organic acids suocinate, citrate, and fumarate increased at small leaf-water potentials. Sucrose decreased in amount with stress and was absent at the most severe stress; there was less glucose and fructose. The amount of carbon lost from sugars was similar to the amount accumulated in amino acids together with the carbon lost in respiration. It is concluded that stress decreased the flux of carbon from photosynthesis for the synthesis of amino acids and sugars but more carbon from stored materials, principally sucrose, was used in the production of organic acids and amino acids.