Search Results

You are looking at 1 - 4 of 4 items for

  • Author or Editor: B. V. Brodbeck x
Clear All Modify Search

Abstract

Net CO2 assimilation rate (A) and plant water relations of peach [Prunus persica (L.) Batsch cv. Flordaking] leaves were monitored during development under field conditions. Leaf conductance to water vapor (gl) and transpiration rate (E) of unfolding and expanding leaves approached maximum values before maximum A values were achieved. Net CO2 assimilation rate and water use efficiency (WUE) were greatest for recently expanded leaves and gradually declined with age after full expansion. Leaf water potential (ψw) was similar for all leaf ages under field conditions. Leaf dry weight/cm2 and chlorophyll/cm2 increased with leaf age after expansion. Diurnal patterns of gl, E, and ψw were similar for expanded spring- and summer-flush leaves. Midday ψw of −2.4 MPa (ψp = about 0.3 MPa) did not reduce gl. Expanding shoots had higher osmotic potentials (ψπ) and thus maintained lower turgor potentials (ψp) than fully expanded shoots. Shoot and leaf elongation rates were related exponentially to ψp and were reduced drastically below ψp 1.0 and 0.7 MPa, respectively. The bulk modulus of elasticity (є) increased linearly with ψp, but there were no significant differences in є of expanding and nonexpanding shoots. As leaf water deficits developed, shoot and leaf expansion were inhibited prior to gl or A. Thus, a moderate level of water stress can reduce the rate of vegetative growth of peach trees without concomitant reductions in carbon assimilation.

Open Access

Abstract

Spring- and summer-flush pecan [Carya illinoensis (Wangenh.) C. Koch] leaves were evaluated to determine climatological factors affecting leaf gas exchange, biophysical factors affecting growth, and to investigate the potential impact of a summer growth flush on alternate bearing. Expanding leaves had a higher osmotic potential, lower turgor pressure (ψp), poorer stomatal control, higher cuticular conductance, and a lower bulk modulus of elasticity than expanded leaves. Stomatal closure occurred at a progressively lower leaf water potential (ψw) as leaves aged. Net CO2 assimilation rate and leaf conductance to water vapor (g1) of pecan in the field did not decline in response to high atmospheric water stress and minimum midday ψw of −1.4 to −1.9 MPa when trees were supplied with adequate soil moisture. Leaf elongation rate was exponentially related to with marked reductions in growth occurring at ψp below 0.6 MPa and a complete cessation in growth below ψp = 0.3 MPa. Net CO2 assimilation rates of expanded leaves were up to 22 μmol·s−1m−2, several times higher than previously reported. Net CO2 assimilation rate was not inhibited by 41.5°C leaf temperature, 2000 μmol·s−1m−2 photosynthetic photon flux, and 3 kPa vapor pressure deficits (VPD). Transpiration rate (E) increased greatly with increasing VPD. Values of gl and E were generally higher than those reported for woody C3 perennials. The efficient water transport system of pecans under conditions of nonlimiting soil moisture may be a consequence of evolution in a floodplain ecosystem.

Open Access

Abstract

Volume flux (Jv), solute flux (Js), and the chemical profile of xylem exudate from cut shoots of ‘Noble’ and ‘Welder’ muscadine grapevines [Vitis rotundifolia (Michx.)] were analyzed as a function of temperature and temperature preconditioning. The effects of short-term (i.e., 2-hr) temperature changes on Jv, xylem fluid osmotic potential (Ψs) and Js from bleeding ‘Noble’ grapevines were determined. The effects of 10 days of preconditioning temperature (4 to 8C or 22 to 28C) on ‘Noble’ and ‘Welder’ were monitored at 25C in relation to Jv, Ψs, Js and inorganic element, amino acid, organic acid, and sugar composition of xylem fluid. Short-term temperature changes induced marked increases in Jv (Q10 = 2.0) and Js but little alteration in Ψs. Temperature-preconditioning effects were cultivar-dependent. Js was enhanced ≈2-fold for both cultivars when preconditioned at 22 to 28C. The stimulation in Js of ‘Noble’ was a result of increased solute concentration (reduced Ψs); increased Js of ‘Welder’ was associated with increased Jv. We propose that the increase in Js with a concomitant increase in Jv of ‘Welder’ was due to a change in hydraulic conductance. Conversely, the increase in Js of ‘Noble’ was due to an increased solute concentration in cells surrounding the xylem vessels and/or to changes in membrane permeability to solutes. Temperature preconditioning had a substantial effect on inorganic ion, amino acid, organic acid, and sugar profile in xylem exudate of ‘Noble’, yet the chemical profile of ‘Welder’ was not altered. The physiological basis for this cultivar-dependent preconditioning response is discussed.

Open Access

The Cohesion Tension Theory, first in 1894 introduced by Dixon and Joly is the theory most often invoked to explain water movement in a transpiring plant. The pressure chamber technique has provided the strongest indirect evidence for this theory. However, controversy remains because 1) the necessary pressure gradients in xylem vessels have never been measured directly; 2) it is uncertain how continuous water columns under great tensions could persist in a metastable state for extended periods of time, and; 3) direct pressure probe measurements on individual xylem vessels have not been indicative of the extreme negative pressures obtained with the pressure chamber. Xylem fluid is an energy-limited resource containing the lowest available carbon (energy content = 2 to 15 J/cm3) of any plant tissue. However, many species of xylophagous leafhoppers subsist entirely on this dilute food source, despite the negative pressures thought to occur in xylem vessels. Carbon limitations of leafhoppers were underscored by 1) high feeding rates; 2) an unprecedented assimilation efficiency of organic compounds (i.e., >99%); 3) ammonotelism, and; 4) synchronization of feeding to optimum host nutrient content both seasonally and diurnally. The maximum tension that can be generated by the cibarial pumping mechanism of an insect based on anatomy and biochemistry is about 0.3 to 0.6 MPa, far below the purported xylem tensions occurring during most daylight hours. By contrast, we have shown that feeding has been usually independent of xylem tensions, as measured with a pressure chamber, and instead was a function of the amide content of xylem fluid. Moreover, the calculated net energy gain of insect feeding (or that contained within insect biomass) on xylem fluid of a given composition under a given tension have also been an a paradox. Experiments will be described that provide insight into the energetics of xylem fluid extraction.

Free access