Biomedical subjects

J. S. Pereira

Publications and source records attributed to J. S. Pereira.

4 recordsLinked to original sources

PubMed1998

Water deficits are more important in delaying growth than in changing patterns of carbon allocation in Eucalyptus globulus.

Potted cuttings of three Eucalyptus globulus Labill. clones (AR3, CN44, MP11) were either well watered or subjected to one of two soil water deficit regimes for six months in a greenhouse. Reductions in lateral branching, leaf production and leaf expansion were the leading contributors to the large differences observed in biomass production between well-watered and water-stressed plants. Although no significant differences among clones were observed in dry matter accumulation or in the magnitude of the response to soil water deficits, sensitivity of lateral branching, leaf initiation and whole-plant foliage to water stress was significantly lower in CN44 than in AR3 and MP11. When the confounding effect of differences in plant size resulting from the different watering regimes was removed, allometric analysis indicated that the genotypes differed in biomass allocation patterns. In addition to a drought-induced reduction in leaf number, water deficits also resulted in smaller leaves because leaf expansion was inhibited during dehydration events. Resumption of leaf expansion following stress relief occurred in all of the clones, but was particularly evident in severely stressed plants of Clone AR3, possibly as a result of the osmotic adjustment observed in this genotype.

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PubMed1990

Plant area index in Eucalyptus globulus plantations determined indirectly by a light interception method.

Plant area index (PAI) was estimated in 3-year-old plantations of Eucalyptus globulus Labill. from (i) allometric relationships obtained from biomass harvesting, and (ii) canopy transmittance measurements. Diffuse light canopy transmittance was measured with a multi-cone sensor consisting of five inverted cones to scan a hemisphere. Plant area index was calculated from transmittance measurements by the inversion method assuming an ellipsoidal angle distribution of uniformly distributed canopy elements, or by application of the Beer-Lambert Law using an empirically determined extinction coefficient (k) for diffuse light of 0.5. Plant area index values of the experimental plantations varied as a consequence of diffferent water and mineral nutrient application rates. The PAI estimated from biomass harvesting ranged from 1.61 to 4.04. The estimates of PAI obtained by the ellipsoidal angle distribution method were significantly different from those based on the allometric relationships or from transmittance measurements assuming a value of k for diffuse light of 0.5.

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PubMedDate not supplied

Genotypic differences in water use efficiency and (13)C discrimination in Eucalyptus globulus.

Cuttings of three Eucalyptus globulus Labill. clones (called SM, VC, DG henceforward) were grown for 56 days in 10-l pots in a greenhouse. Every other day, eight pots of each clone were watered to field capacity (HW treatment), whereas the other eight pots of each clone received only 25% of the water needed to maintain the soil at field capacity (LW treatment). Transpirational water loss, biomass production, leaf gas exchange and water potential (at predawn and midday) were determined at different times during the experiment. Leaf tissue formed by the LW plants after the onset of the treatment was analyzed for (13)C/(12)C ratio against the PeeDee Belemnite standard (delta(13)C). The three clones differed significantly in growth capacity (SM > DG > VC) and in their response to water stress. Even though leaf water potentials were not significantly modified by withholding water, total biomass and plant leaf area were affected significantly by soil water deficits by the end of the experiment. The fastest growing clone (SM) was the most affected by water deficit. Long-term water use efficiency (i.e., biomass per unit of water transpired, WUE) was significantly increased by water deficit and was positively correlated with delta(13)C. Long-term WUE was well correlated with the quotient of the daily integrals of carbon assimilation rate and stomatal conductance. The value of delta(13)C was negatively correlated with the variables normally positively related with growth, such as specific leaf area (SLA) and the biomass/intercepted light quotient (epsilon), and it was positively related to the amount of carbon per unit leaf area, which is usually negatively correlated with relative growth rate.

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PubMedDate not supplied

Diurnal changes in photoprotective mechanisms in leaves of cork oak (Quercus suber) during summer.

Daily variations in photoprotective mechanisms were studied in sun and shade leaves of 40-year-old cork oak (Quercus suber L.) trees during early summer in Portugal. Although trees were not severely water stressed because predawn leaf water potentials remained high, photosynthesis and stomatal conductance decreased at midday. The midday depression in gas exchange was not reversed by short-term exposure to "optimal" conditions of temperature, light and vapor pressure deficit. Chlorophyll a fluorescence, maximum photochemical yield of photosystem II and the quantum yield of noncyclic electron transport showed midday depressions, but recovered by the evening. Both short-term changes in the components of the xanthophyll cycle (reversible de-epoxidation of violaxanthin during the day) as well as long-term changes (higher xanthophyll content in sun compared with shade leaves) were detected and may play a role in the dissipation of excess energy at midday. Because the activities of enzymes of the antioxidant system, superoxide dismutase and ascorbate peroxidase, were high enough to cope with the increase in oxygen reactive species likely to arise under the stressful conditions of midday, we conclude that these enzymes may provide an additional mechanism for energy dissipation.

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