Alterations in the activities of the enzymes of proline metabolism in Ragi (Eleusine coracana) leaves during water stress

Free proline content in Ragi (Eleusine coracana) leaves increased markedly (6 to 85 fold) as the degree of water stress, created by polyethylene gylcol treatment, was prolonged There was also a marginal increase in soluble proteins in the stressed leaves as compared to that in the controls. Water stress stimulated the activities of ornithine aminotransferase and pyrroline-5-carboxylate reductase, the enzymes of proline biosynthesis and markedly inhibited the enzymes involved in proline degradation viz., proline oxidase and pyrroline-5-carboxylate dehydrogenase. These results suggest that increase in free proline content of Ragi leaves could be due to enhanced activities of the enzymes synthesizing proline but more importantly due to severe inhibition of the enzymes degrading proline. These observations establish for the first time, the pathway of proline metabolism in plants by way of detection of the activities of all the enzymes involved and also highlight the role of these enzymes in proline accumulation during water stress.     

Leaf gas exchange,water relations,nutrient content and growth in citrus and olive seedlings under salinity

The effects of salinity on growth, leaf nutrient content, water relations, gas exchange parameters and chlorophyll fluorescence were studied in six-month-old seedlings of citrus (Citrus limonia Osbeck) and rooted cuttings of olive (Olea europaea L. cv. Arbequina). Citrus and olive were grown in a greenhouse and watered with half strength Hoagland’s solution plus 0 or 50 mM NaCl for citrus, or plus 0 or 100 mM NaCl for olive. Salinity increased Cl⁻ and Na⁺ content in leaves and roots in both species and reduced total plant dry mass, net photosynthetic rate and stomatal conductance. Decreased growth and gas exchange was apparently due to a toxic effect of Cl⁻ and/or Na⁺ and not due to osmotic stress since both species were able to osmotically adjust to maintain pressure potential higher than in non-salinized leaves. Internal CO₂ concentration in the mesophyll was not reduced in either species. Salinity decreased leaf chlorophyll a content only in citrus.     

Leaf and tepal anatomy, plastid ultrastructure and chlorophyll content in Galanthus nivalis L. and Leucojum aestivum L.

The leaf structure of Galanthus nivalis L. (snowdrop) and Leucojum aestivum L. (snowflake) is characterized by a homogeneous mesophyll tissue. The dominant characters of the leaves are cavities with mucose substance. There is a striking difference between these plants tepal anatomy. A central cavity occurs only in snowdrop tepals. Plastids from white parts of the tepals have a poorly developed membrane system. Leaves and green parts of tepals of both species possess amoeboid chloroplasts and contain chlorophyll a and b. The chlorophyll content in tepals is lower than in leaves, but the chlorophyll a:b ratio is always 2:1. Both, snowdrop and snowflake are from the family Amaryllidaceae, but their ecology is different. This paper presents common features related to systematic relatedness and differences induced by ecological factors.     

Seasonal changes in production and nutrient content ofCynodon dactylon (L.) Pers. subjected to water deficits

Seasonal changes in water relations, production and mineral composition were studied in a sward ofCynodon dactylon (L). Pers. subjected to water deficits during a dry summer, and at recovery in autumn. The experiment was carried out under Mediterranean field conditions. Water deficits during summer reduced total dry matter production by 60%, but in autumn there were no differences between treatments. Compared to well-watered sward, the sward grown under drought showed an increase in potassium, calcium and nitrogen of 55, 10 and 10% respectively. These differences decreased with the arrival of autumn rains. Leaf osmotic potential (Ψ_Π) fell during the dry summer to −2.8 MPa in well-watered plants and to −4.2 MPa in stressed plants. In autumn there were no differences between treatments. Nevertheless, relative water content (RWC) only decreased to 0.86 in droughted plants. In summer potassium contributed to the osmotic adjustment. In contrast, under water deficits a decrease of 71% in sodium and, to a lesser but significant extent decreases in phosphorus, magnesium and chlorine was observed. Nitrogen, phosphorus and sulphur showed low concentrations during summer and increased in autumn.     

Solute levels and osmoregulatory enzyme activities in reed plants adapted to drought and saline habitats

The pattern of solute accumulation and the activities of key enzymes involved in metabolism of proline and betaine were investigated in three ecotypes of reed from different habitats: swamp reed (SR), dune reed (DR), and heavy salt meadow reed (HSR). The two terrestrial reed ecotypes, DR and HSR, exhibited a higher capacity for osmotic adjustment; they accumulated higher contents of K⁺ and Ca²⁺ in the leaves in comparison with SR. DR also had the highest soluble sugar content in its leaves. HSR has higher levels of Na⁺ in its root environment and this was reflected by considerable accumulation of Na⁺ in the HSR rhizome. However, the different zones of its leaf lamina (upper, middle and lower) did not exhibit increased levels of Na⁺, suggesting that HSR has the ability to accumulate Na⁺ in the rhizome to protect the shoots from excessive Na⁺ toxicity. DR and HSR had higher levels of proline and betaine in the leaves than did SR. This difference was consistent with the activities of the various biosynthetic enzymes: betaine aldehyde dehydrogenase (BADH), pyrroline-5-carboxylate reductase (P5CR) and ornithine--aminotransferase (OAT) were enhanced in DR and HSR as compared to SR, whereas proline oxidase (PO) activities were inhibited. These findings suggest that changes in the activities of enzymes involved in osmotregulation might play important roles in the adaptation of reed, a hydrophilic plant, to more extreme dune and saline habitats. The relative contributions of the various proline synthetic pathways are also discussed.