Evaluation of a Non-Destructive Method for Measuring Turgor Pressure in Helianthus

The portable instrument described by Heathcote, Etherington,and Woodward (1979) for the non-destructive measurement of turgorpressure was evaluated in Helianthus annuus and Helianthus paradoxus.A good correlation was obtained between turgor pressure measuredwith the instrument and turgor pressure estimated by the pressure-volumetechnique for individual leaves allowed to dry after excision;however, variation in both the intercept and slope of the relationshipoccurred between leaves. Consequently, there was no correlationbetween the output of the instrument for individual leaves andthe turgor pressure of the same leaves estimated by conventionalmethods. Moreover, for a given leaf, the instrument had onlya limited ability to detect temporal variation in turgor pressurewhen compared with turgor pressure calculated from measuredvalues of leaf water potential and leaf osmotic potential. Theinstrument's output was influenced by its proximity to majorveins and by leaf thickness. We conclude that variability inleaf thickness and the presence of large veins limits its usefulnessfor measurement of turgor pressure in Helianthus. Key words: Leaf thickness, Turgormeter, Turgor pressure, Helianthus     

The Limits to Xylem Embolism Recovery in Pinus sylvestris L.

In this study we test the hypothesis that, when water supplyis under tension, reversal of cavitation can occur as long aswater continuity is maintained in the vicinity of tracheids.The experiments were conducted on young branches, 7–8mm diameter, of Pinus sylvestris L., freshly collected and allowedto lose water on the bench after being debarked. During dehydration,the volumetric fractions of water (V_w) and gas (V_s) changedsteadily as relative water content () declined. Meanwhile, ultrasonicemissions (UAE) started after a threshold = 90% was reachedand were maximal at = 75%. Before and after dehydration, branchsegments were connected to water-filled tubing and placed from0.2 to 3.6 m above a water source and water inflow and outflowwere recorded. These distances provided a source of water ata potential of –2.0 to –36kPa. We considered thatthe segment water potential would be a function of the surfacetension across the water meniscii at the ends of the embolizedtracheids. Thus, water potentials calculated from tracheid dimensionswould be as low as –43 kPa. Water inflow to segments declinedwhen the distance from the source was increased or the segmentswere very dehydrated. Increasing the distance above the watersource would be expected to increase the water potential differencebut to reduce water uptake. The most dehydrated segments absorbedwater faster at the beginning of the refilling period (2h),but at the end of 16h, was lower and V_g larger than in lessembolized tissue. Recovery of water flow followed a similartrend, and was lowest when embolisms increased. For a narrowrange of , hydraulic conductance was reduced sharply, indicatingthat wide tracheids were still gas-filled. Thus, the numberof tracheids remaining embolized increased when the source waterpotential was low and there was severe embolism. We concludethat embolism can be reversed in P. sylvestris at a rate dependingon the water potential of the source, severity of embolism andhydraulic conductivity. Key words: Pinus sylvestris L., cavitation induction-recovery, embolized tracheids, water content and embolism, matrix potential, capillary, hydraulic conductance     

Tissue water relations and leaf growth of tropical corn cultivars under water deficits

Abstract This study reports on the effect of water deficit on the tissue water relations and leaf growth of six corn cultivars, growing in glasshouse conditions, in order to understand growth responses to drought of tropical corn. A mild water-stress treatment was imposed slowly; plants reached a minimum pre-dawn leaf water potential of about –1.5 MPa by day 12 after watering was withheld. Analysis of the water relation characteristics of growing leaves using the pressure–volume technique demonstrated that under water deficits all the cultivars changed their moisture-release curves compared with irrigated plants. Osmotic potential at full turgor was lowered in water-stressed plants of all the genotypes and the degree of such change was between 0.34 MPa and 0.58 MPa. Thus, turgor pressure was lost at a lower water potential in water-stressed plants than in irrigated plants of all the varieties. Volumetric elastic moduli were also increased under water deficits and the increase ranged between 10% and 141% among the cultivars. In all the genotypes, the stress imposed led to a reduction of leaf area and dry matter accumulation. Leaf expansion was very sensitive to low turgor pressure and it ceased when turgor reached 0.2 MPa. Thus, varieties able to maintain a higher degree of turgor pressure (i.e. by osmotic adjustment) under water deficits may be able to prolong leaf growth.