Proline Accumulation in Water-stressed Barley Leaves in Relation to Translocation and the Nitrogen Budget

Abstract

Mobilization of N from leaves of barley (Hordeum vulgare L.) during water stress, and the role of proline as a mobilized species, were examined in plants at the three-leaf stage. The plants responded to water stress by withdrawing about 25% of the total reduced N from the leaf blades via phloem translocation. Most of this N loss was during the first 2 days while translocation of 'C-photosynthate out of the stressed blade still remained active. Free proline accumulation in the blade was initially slow, and became more rapid during the 2nd day of stress. Although a major free amino acid, proline accounted for only about 5% of the total N (soluble + insoluble) retained in severely stressed blades. When the translocation pathway in water-stressed leaves was interrupted just below the blade by a heat girdle, a cold jacket, or by blade excision, N loss from the blade was prevented and proline began to accumulate rapidly on 1st day of stress. Little free proline accumulated in the blades until after the ability to translocate was lost. Proline was, however, probably not a major species of N translocated during stress, because proline N accumulation in heatgirdled stressed leaves was five times slower than the rate of total N export from intact blades.The onset of water deficit alters the over-all N budget of crops through effects on both N transport and N assimilation. Nitrate delivery via the xylem to the shoot is depressed (17), and N03 reduction in the leaves declines (12,17). Although phloem transport of assimilated N to N sinks is eventually reduced (4, 25), it is probable that phloem transport is sufficiently resistant to water stress to permit appreciable salvage of both N and carbon from wilted leaves that are dying as a consequence of desiccation (3,6,25).Within the leaves of many plants subjected to moderate or severe water stress, one striking change in N metabolism is the accumulation of free proline as a result of net de novo synthesis from glutamic acid (e.g. 1, 2). It has been proposed that this accumulation is a metabolic adaptation which confers survival value, perhaps acting as a (phloem-mobile) reserve of N accessible for use upon stress relief (e.g. 1,18,21). This possibility cannot be evaluated without knowing: (a) how much N is exported from stressed leaves to other parts of the plant; and (b) whether proline accumulation and export are significant terms in the total N budget of a leaf during stress. Much of the published work on proline accumulation during water stress has been with detached leaves, which clearly cannot export N. Proline accumulation has also been investigated in intact plants. In Plants were grown in Perlite in plastic pots (7 x 12 cm) for 17 to 19 days after sowing (to the three-leaf stage) in a growth chamber under a regime of 16-h days, day/night temperature 22/ 16 C, day/night RH 70/85%, total irradiance 3.8 mw cm-2 (80%o from fluorescent lamps, 20o from incandescent lamps), and were watered on alternate days with half-strength Hoagland solution.Prior to the exper...

Amino Acids Translocated from Turgid and Water-stressed Barley Leaves

“…whole blade. The latter values may be compared directly with the value of 260 I&g N exported/ day-leaf, derived from N balance data for barley leaves stressed for 1 day (16). Glutamine and glutamate together accounted for about 30%o of this rate, whereas proline accounted for about 3%.…”

“…It has been reported that leaves absorb gaseous NH3 and 15NH3 (4,8), incorporate 15NH3 into amino acids and protein, and export some of the assimilated 15N to roots (12). The pathway for assimilation of NH3, which is clearly active in turgid leaves (10,12), is most probably also active in water-stressed leaves because the NH4, pools of leaves do not increase markedly during stress and little NH3 is volatilized from stressed leaves, despite extensive protein catabolism (1,16 under a bank of fluorescent tubes (15). After 30 min, the fed leaf was rapidl, excised and divided into blade and sheath portions.…”

Amino Acids Translocated from Turgid and Water-stressed Barley Leaves

“…3), there are few previous reports on the nitrogenous phloem constituents in these crops ( 18). The N-compounds moving in the phloem of cereals may originate from either or both of the following sources: (a) recent N assimilation (18,23 and refs. cited therein), and (b) hydrolysis of leaf proteins, which occurs during senescence (1) and episodes of water stress (23).…”

“…The N-compounds moving in the phloem of cereals may originate from either or both of the following sources: (a) recent N assimilation (18,23 and refs. cited therein), and (b) hydrolysis of leaf proteins, which occurs during senescence (1) and episodes of water stress (23). Although it is clear that the relative importance of sources (a) and (b) changes both with developmental stage and with environmental conditions, little is known about the chemical forms of N which these sources make available for phloem transport.…”

“…Such descriptive information on the N-compounds in the phloem of cereals is pertinent to understanding the striking capacity of these plants to remobilize N from leaves and stems, sometimes directing over 70%'o of total plant N to the grains; it is also needed for quantitative treatments of partitioning of assimi- (23), and were used at 19 days (three-leaf stage), or at 21 to 24 days (four-leaf stage). Such plants were water-stressed by flooding the roots with polyethylene glycol (PEG 6000) solution 1 day prior to use, as described previously (23). Plants for experiments with flag leaves were grown in the greenhouse in a soil mixture (7) …”