Maria Manuela David

Osmotic Adjustment in Water Stressed Grapevine Leaves in Relation to Carbon Assimilation

The response of grapevine plants to severe water deficit (predawn leaf water potential of - 1.13 MPa), imposed at a rate of about 0.16 MPa day-1 was studied in terms of leaf water relations characteristics, stomatal behaviour and gas exchange. Carbohydrate status of leaves was also analysed in order to assess the contribution of soluble sugars as osmotic solutes during drought. Pressure/volume analysis showed an active osmotic adjustment in water-stressed leaves, which decreased osmotic potential at full turgor by 0.45 MPa and the apoplastic water fraction showed a reduction of 19% as compared to the well- watered plants. Cell wall elasticity was not significantly affected by water stress, and turgor loss point in stressed leaves was reached at lower water potential and relative water content values than in the well-watered controls. Photosynthesis was markedly reduced in water-stressed plants. However, well-watered and water-stressed leaves had similar concentrations of glucose and fructose. The concentrations of sucrose and starch decreased in water-stressed leaves. This accounted for a marked decrease in the ratio of leaf dry weight to area in droughted plants. The changes in concentrations of soluble carbohydrates could not account for the difference in osmotic potential between water-stressed and well-watered leaves.

Leaf age effects on photosynthetic activity and sugar accumulation in droughted and rewatered Lupinus albus plants

Changes in the photosynthetic rate (A), stomatal conductance (g), water relations, photosynthetic pigments, Rubisco and soluble sugars accumulation were studied in different aged leaves of white lupin during soil drying and following rehydration. In water-stressed plants, A and g sharply declined and recovered only partially after rewatering. The way Ci and A/gchanged with drought was strongly dependent on leaf age; only in the young leaves did A/g increase and Ci decrease. Drought induced accumulation of soluble sugars was also age dependent, decreasing as leaves aged. In response to soil drying, the contents of photosynthetic pigments, total soluble protein and Rubisco protein increased in the young leaves and were either not affected or slightly decreased in the older ones. Rehydration accentuated the losses in pigments and Rubisco in the old leaves of water-stressed plants. These results suggest that the contribution of mesophyll limitations to explain drought inhibition of photosynthesis increases with leaf age, decreasing the ability to recover after rewatering. In young leaves the tolerance of the photosynthetic apparatus to dehydration and rehydration episodes is high and it is associated with high contents of Rubisco and in soluble sugars, particularly hexoses.

Morphological and Physiological Responses of the Mediterranean Evergreen Sclerophyll, Ceratonia Siliqua, to Different Light Intensities

Mediterranean evergreen plants although basically xerophytes can exhibit several different strategies (Walter, Kreeb, 1970). Sclerophylly seems to play an important role in the “adaptation” of these plants to the natural climatic changes of mediterranean areas (Stocker, 1960). According to speculative classical work of earlier eco-physiologists, sclerophylly was envisaged as an adaptative mechanism for decreasing transpiration. However there is also evidence that sclerophylly structure vitally affects energy interception and dissipation. Moreover, sclerophylly characteristics of mediterranean plants usually show marked developmental plasticity (Thoday, 1953) leading to a canopy arrangement which ensures optimal photosynthetic capacities as long as water is available to plants. Ceratonia siliqua is a moderate sclerophyll species that seems well adapted to a wide spectrum of environmental conditions of the true mediterranean climate (Catarino, Bento-Pereira, 1976).