Changes in Amino Acid Content of Excised Leaves During Incubation. III. Role of Sugar in the Accumulation of Proline in Wilted Leaves

Abstract

Summary. A previously reported accumulation of proline in wilted turnip leaves has been observed in 6 additional species representing 5 different families. The results of experiments on the conditions affecting the behavior of proline in wilted leaves were interpreted to mean that the supply and metabolism of available carbohydrate was essential for proline accumulation.The role of sugar in proline accumulation was determined by incubating wilted leaves with different levels of carbohydrate (produced either by variations in conditions of preillumination or by infiltration with sucrose). The results showed that proline accumulation was greater and most prolonged in wilted leaves with higher sugar and starch contents. No proline accumulation was observed in unwilted leaves. Also some inhibitors of glycolysis and of the tricarboxylic acid cycle prevented proline accumulation in wilted leaves.Based on these results, the interpretation of the role of sugar in proline accumulation is that the oxidation of sugars furnishes a-ketoglutarate and NAD(P)H for proline synthesis in wilted leaves.In the previous paper ( 18) an accumulation (i.e., net synthesis) of proline in wilted, excised turnip leaves was reported. This accumulation was not observed in unwilted leaves. Following the period of proline accumulation, which lasted for 1 to 2 days, there was a net loss of proline corresponding in time to the diminution of metabolizable sugars. An accumulation of proline in wilted, excised perennial ryegrass plants was also observed by Kemble and Macpherson (8). In their experiment, no loss of proline was observed, the levels remaining high throughout the course of the experiment (8 days). wilted compared to non-wilted leaves have been reported in leaves of Bermuda grass (1), citrus (3), and wheat (10) but in the latter 2 cases it is not possible to tell from the data whether the increase in free proline exceeds a decrease in the proline content of proteins. Before this manuscript was revised, Routley (11) reported that wilted Ladino clover leaves were higher in uncombined proline than normal leaves. Routley suggested that there was an effect of carbohydrates on proline level but definitive data were not presented.The relationship between carbohydrate metabolism and proline accumtulation was studied following the suggestions in the previous paper (18 cised fibrous roots from corn seedlings. These observations tend to indicate further that the phenomenon is restricted to green tissue.The decrease in proline between 24 and 48 hours (fig 1) coincides with the decrease in soluble suigars. Since a similar correlation had been obtained previously (18), the behavior of sugar and starch in wilted and unwilted leaves during incubation was examined.The results (fig 2) demonstrate that, compared to tinwilted leaves, wilted leaves have an accelerated starch disappearance. When the initial starch content is high, this effect is accompanied by an increase in sugar. This effect has been observed by other investigators (7,15 fig 1). The absenc...

Effect of Water Stress on Proline Synthesis from Radioactive Precursors

“…Experiments showing greater apparent conversion of 14C-glutamate to proline do not unequivocally prove that proline synthesis is stimulated by water stress, as P5C feeding studies show that proline oxidation is inhibited under comparable conditions. This inhibition could account, at least in part, for increased proline labeling, and must be considered an alternate possibility.Although several plants have been reported to accumulate free proline during periods of water deficit (2,12,15,16,18) or when subjected to other stress (7, 8), the biochemical changes linking water stress and proline accumulation are not well understood. Barnett and Naylor (2) and Morris et al (13) have observed increased incorporation of 14C-glutamic acid into proline in wilted leaves as compared to turgid leaves of Bermuda grass and turnip.…”

“…Although several plants have been reported to accumulate free proline during periods of water deficit (2,12,15,16,18) or when subjected to other stress (7,8), the biochemical changes linking water stress and proline accumulation are not well understood. Barnett and Naylor (2) and Morris et al (13) have observed increased incorporation of 14C-glutamic acid into proline in wilted leaves as compared to turgid leaves of Bermuda grass and turnip.…”

Senescence of Rice Leaves

“…It is clear that proline increases greatly during senescence of rice leaves. Thompson et al (15) and Stewart et al (12) showed that proline did not accumulate in excised, turgid turnip leaves when incubated in darkness. In the present study, however, there was a slight but significant (about 2-fold) increase in the proline content after 1 d, but, subsequently, proline accumulated rapidly.…”

“…It seems that accumulation of proline may, at least in part, result from an increased rate of synthesis, possibly due to a disruption of the normal feedback inhibition of proline synthesis. Potassium cyanide and 2,4-dinitrophenol strongly inhibited proUne accumulation, indicating that some energy compound(s) may participate in proline accumulation during senescence of excised rice leaves.It is well established that proline accumulates rapidly in excised and intact leaves under water stress (3,7,11,12,17). However, relatively little attention has been paid to proline accumulation in senescing excised leaves.…”

“…It is well established that proline accumulates rapidly in excised and intact leaves under water stress (3,7,11,12,17). However, relatively little attention has been paid to proline accumulation in senescing excised leaves.…”