Thayne Montague

Urban tree transpiration over turf and asphalt surfaces

We used a two-layer canopy model to study transpiration of tree species as affected by energy-balance properties of a vegetated and paved surface. During several dawn-to-dusk studies, tree transpiration, stomatal conductance, leaf temperature (%), and several microclimate variables, were measured over turf and an asphalt surface. Cumulative transpiration was estimated from a leaf energy- balance equation applied to a tree crown apportioned between sunlit and shaded layers. Afternoon asphalt surface temperatures (Ts) were 20-25°C higher than turf T~ in all studies. Air-temperature differences between sites were minimal due to the size and proximity of the two surfaces that resulted in mixing of air. Trees over asphalt had consistently higher T, than those over turf, apparently due to interception of the greater upwards long-wave radiation flux from higher T s. In one study flowering pear over asphalt in a humid environment had higher T/resulting in one-third more total water loss compared to trees over turf. In other studies, however, water loss of green ash and Norway maple over asphalt in an arid environment was either equal to or less than that over turf. Less water loss was due to higher T, over asphalt causing prolonged stomatal closure. Model manipulation indicated that tree water loss over asphalt will depend on the degree of stomatal closure resulting from how interception of increased energy-fluxes and ambient humidity affect leaf-to-air vapor pressure differences. 1997 Elsevier Science Ltd.

Asexual Propagation of Grapevine Transmits Pierce’s Disease Pathogen (Xylella fastidiosa) to Rooted Cuttings

ABSTRACT An objective of this study was to investigate rooting success of grape cuttings propagated from vines symptomatic of Pierce’s disease. Additional objectives were to assess if rooted cuttings could survive and produce viable plants, and determine if Xylella fastidiosa (causal agent of Pierce’s disease) could be found in rooted cuttings. In Jan. 2008, cuttings were taken from symptomatic and asymptomatic ‘Merlot’ and ‘Cabernet Sauvignon’ grapevines growing in the Hill Country and Gulf Coast regions of Texas. Six weeks after cuttings were propagated, each cutting was uprooted and evaluated for rooting and infection parameters. Cuttings were then planted in containers and held in the greenhouse to evaluate survivability. To confirm the presence of X. fastidiosa, propagated cuttings were tested by real-time polymerase chain reaction. Data indicate several rooted cuttings tested positive for X. fastidiosa and appeared viable and healthy. Therefore, vines infected with X. fastidiosa have the ability to produce asexually propagated cuttings, and potentially contaminate non-infected vineyards.

Light-Emitting Diodes as Supplemental Lighting in Viticulture Field Research

A light-emitting diode (LED) system was evaluated as a tool for field research to investigate light effects on grapevines. Preliminary laboratory trials were conducted to develop response curves for photosynthetically active radiation (PAR) and temperature as a function of distance from the LED light source. LED panels were subsequently deployed in a three-year field study of three light-exposure treatments on developing fruit clusters of Vitis vinifera cv. Cabernet Sauvignon: 1) clusters exposed to direct sunlight, 2) clusters shaded by the canopy, and 3) canopy-shaded clusters exposed to supplemental LED light. Laboratory trials and vineyard field studies demonstrated that LED panels could provide supplemental light to grapevines over a broad range of PAR, up to and exceeding the photosynthetic light saturation point for exterior leaves, by adjusting the distance of the LED panel to the target. Although bench trials indicated significant convective heat close to the LED panel, field studies detected few temperature differences among cluster treatments. Varying results were primarily attributed to temperature measurement locations related to experimental objectives; bench trials measured target surface temperature, but field studies monitored whole cluster temperature. LED panels were effectively deployed to provide supplemental light to shaded grape clusters without significantly altering cluster temperature.