Midday Stomatal Conductance Research Articles

Establishment of White Oak Seedlings with Three Post-Plant Handling Methods on Deep-Tilled Minesoil during Reclamation

Abstract Three methods to improve first-year establishment of bare-root white oak on compacted minesoil during reclamation were investigated. We applied tree-shelter, hydrophilic-polymer, and nitrogen-fertilization treatments to seedlings planted on deep- and untilled soil. Predawn and midday water potential, and midday stomatal conductance (gs) were measured in early, mid, and late season. Leaf area, trunk diameter growth, and survival were measured in early fall. Single-row deep tillage loosened the soil to 0.7 m (2.3 ft), but outward from the midrow the effect dissipated rapidly. Water stress increased in all treatments during an extended mid-summer dry period. Deep tillage resulted in higher initial gs but differences in water stress between tillage treatments were not sustained during the dry period. Trees in shelters, however, were able to delay water stress longer than those in the other handling treatments. While there were no differences in trunk diameter growth among all treatments, trees on deep-tilled soil had greater leaf area and survival than those on unfilled soil. Among handling treatments, only trees in shelters had leaf area and survival significantly greater than the control. Both deep tillage and shelters contributed to improved survival, and the greatest effect occurred with shelters on deep-tilled soil.

Response of Cynodon dactylon to prolonged water deficits under saline conditions

Competition for available water resources in both arid and semi-arid locations has led to greater scrutiny of turfgrass irrigation. Irrigation management strategies, including deficit irrigation, need further investigation. The objective of this research was to determine the physiological response of bermudagrass (Cynodon dactylon (L.) Pers.) to prolonged water deficits under saline conditions. Bermudagrass was grown in large columns packed with three different soil types (sandy loam, silt loam and clay). Synthesized saline irrigation water was applied at three different salinity levels (1.5, 3.0 and 6.0 dS/m). Two previous experiments that were conducted with these columns over a 3.5 year period led to differential profile salinization in all 27 columns. At the end of this 3.5 year period, all irrigations were terminated and plant growth and water status were monitored over a 95 day dry-down period. Midday stomatal conductance, leaf water potential, canopy temperature, soil water in storage and stolon elongation were measured over the experimental period. On day 95, above ground tissue was harvested for dry weight and elemental tissue analysis. Midday stomatal conductance decreased around day 30 in all columns regardless of soil salinity. This decrease was not associated with a threshold leaf water potential, as midday leaf water potentials remained constant over a 60 day period. Stolon elongation also ceased before any deviation in the midday leaf water potential occurred. A concomitant reduction in evapo ranspiration was not associated with the measured decrease in stomatal conductance. This would suggest that bermudagrass may have regulated stomatal activity to compensate for lower conductances during periods of greater stress and/or that measured midday stomatal conductances cannot always be directly coupled to extended time evapotranspiration measurements.

IRRIGATION, WATER RELATIONS, AND CRABAPPLE GROWTH IN ROOT-CONTROL BAGS IN A FIELD NURSERY

Limited root development of nursery stock in root-control bags facilitates harvest but without irrigation may predispose stock to water stress. The effect of bags and irrigation on growth and water relations of field-grown Malus sieboldii var. zumi were investigated following transplanting as large liners into a silty-clay soil. Predawn leaf water potential (ψ), and midday stomatal conductance (gs) and ψ, were measured periodically through the season. Late-season osmotic potential (ψπ), caliper, leaf area, and root growth were also measured. Non-irrigated treatments exhibited water stress during an extended mid-summer drought, as predawn ψ and particularly gs were less than irrigated treatments, resulting in lower vegetative growth and ψπ. For combined bagged treatments water relations did not differ, but leaf area, root growth, and ψπ, but not caliper, were less than non-bagged trees. Growth measurements and ψπ of non-irrigated bagged trees, however, were consistently lower but nonsignificant than the other treatments. Bag-induced root reduction can limit some top growth even with optimum soil water. Moreover, in terms of potential Type-II errors extrapolated over a conventional production cycle, trees grown in root-control bags in normally non-irrigated soils may be more susceptible to water stress and subjected to further cumulative growth limitation.

SOIL WATER DEFICIT EFFECTS ON PLANT WATER RELATIONS AND LEAF GROWTH OF CUCUMBER

Cucumber (Cucumis sativus L. cv. Marketmore 80) plants were exposed to a soil water deficit and subsequently rewatered. Maximum stress intensity was -1.5 MPa midday leaf water potential compared to -0.6 to -0.8 MPa in the well watered control, eight days after withholding water. Midday stomatal conductance {ks), leaf turgor potential and water potential decreased in the stress treatment compared to the control beginning at the first sampling, two days after withholding water. The decrease in all three was approximately linear with time over the stress. Decreased leaf elongation was observed at the second sampling, three days after the initial decline in ks and five days after withholding water. At similar relative water content {RWC), osmotic potentials of the stress and control treatments were the same throughout most of the stress. Further, there was no difference in osmotic potential, at the same RWC, between the stress and control treatments 12 - 16 hours after rewatering. Split-root experiments were also conducted to examine a possible role of a non-hydraulic signal from roots in drying soil in the regulation of ks and leaf elongation in cucumber. No conclusive evidence of a signal was found despite significant decreases in soil water potential of one-half of the root system of the stress plants. However, fluctuating vapor pressure gradients (vpg) may have obscured evidence of a signal.

NITROGENASE ACTIVITY, PHOTOSYNTHESIS AND TOTAL NONSTRUCTURAL CARBOHYDRATES IN COWPEA DURING AND AFTER DROUGHT STRESS

Greenhouse experiments were conducted to determine whether depressed nitrogenase activity (NA) of cowpea (Vigna unguiculata (L.) Walp.) nodules during drought stress is associated with altered carbohydrate supply to the nodules. Nitrogenase activity of the nodules, midday abaxial stomatal conductance, leaf net photosynthesis and mg total nonstructural carbohydrate (TNC) g−1 dry weight of several plant parts were measured in cowpea subjected to and recovering from drought stress periods of 0, 4 and 8 days. In addition, cowpea plants were shaded or partially defoliated (75% leaf removal) to limit carbohydrate supply to the nodules of well-watered plants. Stomatal conductance, leaf net photosynthesis and nodule NA declined 60, 62 and 90%, respectively, within 4 d of withholding water. After 8 d of drought stress, leaf net photosynthesis and nodule NA were near zero. Stomatal conductance and leaf net photosynthesis returned to the level of unstressed plants within 24 h following rewatering while recovery of NA was delayed. Shading and defoliation of cowpea plants under well watered conditions decreased NA 62 and 44%, respectively. TNC concentrations of leaves, petioles, stems, roots and nodules did not differ appreciably between drought stressed and control plants. In contrast, TNC concentrations of shoot plant parts from shaded or defoliated plants declined significantly compared to controls and nodule TNC concentrations declined in shaded plants. The decline in NA by cowpea nodules during drought stress did not appear to be directly associated with carbohydrate supply to the nodules, but rather the result of a new equilibrium attained with overall limited plant growth under the stress.Key words: Vigna unguiculata (L.) Walp., nitrogenase activity, drought stress recovery, assimilate stress, shading, defoliation, total nonstructural carbohydrate

A comparison of water relations, visual symptoms, and changes in stem girth for evaluating impact of Phytophthoracinnamomi dieback on Eucalyptusmarginata

Water relations, stem girth, and crown vigour were measured in jarrah (Eucalyptusmarginata Donn ex Sm.) trees in adjoining areas of healthy and dieback-affected forest. The soil-borne fungus Phytophthoracinnamomi Rands was recovered from soils in the dieback areas, but not from neighbouring healthy forest. Trees on dieback sites were found to be more water stressed (with lower dawn water potentials and daytime stomatal conductances) than healthy trees on nearby areas free of P. cinnamomi. Severe water stress usually preceded crown deaths by several weeks to months. Dawn water potential and midday stomatal conductance were the most useful nondestructive indicators of dieback severity in declining trees. Subjective assessment of crown form was slightly less reliable than water relations as a separator of dieback and healthy trees. Changes in trunk girth were the least sensitive indicators of dieback severity. Midday stomatal conductance is recommended as a useful tool for monitoring short-term changes in vigour of dieback-affected trees.