Allan Fulton

Tree water status and gas exchange in walnut under drought, high temperature and vapour pressure deficit

Summary Drought reduces photosynthesis in walnut (Juglans regia L.), but it is not known whether this is due mainly to the closure of stomata, or to possible effects on leaf biochemistry. In an attempt to answer this question we studied diurnal changes in the water status and gas exchange in droughted [50% crop evapotranspiration (ETc)] and fully irrigated (100% ETc) walnut trees, over 2 d. Stem water potential ( s) ranged from –0.5 MPa in the morning to –1.2 MPa in the afternoon under drought, and from –0.1 MPa to –0.4 MPa under full watering. Net CO2 assimilation (Amax) ranged from 15 µmol CO2 m–2 s–1 in the morning to 3 µmol CO2 m–2 s–1 in the afternoon under drought, and from 25 µmol CO2 m–2 s–1 in the morning to 10 µmol CO2 mm–2 s–1 in the afternoon under full watering. At these times, stomatal conductance (gs) varied from 0.2 to 0.02 mol H2O m–2 s–1 and from 0.7 to 0.2 mol H2O m–2 s–1, respectvely. Drought reduced the internal CO2 concentration (Ci) by about 55 µmol mol–1 on day-1, and by about 100 µmol mol–1 on day-2 and increased leaf temperature (Tl) by about 2°–5°C. The reductions in gs and Ci with drought suggest that lower photosynthesis was associated with stomatal closure. However, in each treatment, Amax decreased during the day, while Ci was stable, suggesting that photosynthesis was also reduced by a direct effect of heat on leaf biochemistry. Both Amax and gs correlated with Tl and with the leaf-to-air vapour pressure deficit (VPDl), but with different relationships for droughted and control trees. However, when stomatal limitations to photosynthesis were accounted for (i.e., based on the assumption that, under stomatal limitation, photosynthesis is proportional to Ci, a single relationship between Amax and Tl described all the data (R2 = 0.81). Thus, photosynthesis was limited by the closing of stomata under drought, and by a direct effect of heat on leaf biochemistry. These results suggest that hot and dry weather reduces photosynthesis and potential productivity in walnut in the absence of soil water deficit.

Using EM and VERIS technology to assess land suitability for orchard and vineyard development

Orchard and vineyard producers conduct preplant site evaluations to help prevent planting permanent tree and vine crops on lands where the crop will not perform to its highest potential or attain its full life expectancy. Physical soil characteristics within specific soil profiles and spatially throughout an orchard influence decisions on land preparation, irrigation system selection, horticultural choices, and nutrient management. Producers depend on soil surveys to help them understand the soil characteristics of the land and may be interested in technology that provides additional information. Electromagnetic induction (EM38) and four-probe soil resistance sensors (VERIS) are being used in combination with global positioning systems to map spatial variability of soils using apparent soil electrical conductivity (ECa). The hypothesis evaluated in this study is whether rapid, in situ, and relatively low-cost methods of measuring ECa (EM38 and VERIS) can effectively identify and map physical soil variability in non-saline soils. The supposition is that in non-saline soils, ECa levels will relate well to soil texture and water-holding capacity and can be used to map physical soil variability. In turn, the information can be used to guide decisions on preplant tillage, irrigation system design, water and nutritional management, and other horticultural considerations. Two sites in the Sacramento Valley were mapped each with EM38 and VERIS methods. Site-specific management zones were identified by each provider on ECa maps for each site, and then soil samples were collected by University of California researchers to verify these zones. Results showed that on non-saline soils, ECa measured with both EM38 and VERIS correlate with physical soil properties such as gravel, sand, silt, and clay content but the relationship between conductivity and these physical soil properties varied from moderately strong to weak. The strength of the correlation may be affected by several factors including how dominant soil texture is on conductivity relative to other soil properties and on methods of equipment operation, data analysis and interpretation. Overall, the commercial providers of ECa surveys in this study delivered reasonable levels of accuracy that were consistent with results reported in previous studies. At one site, an ECa map developed with VERIS provided more detail on physical soil variability to supplement published soil surveys and aided in the planning and development of a walnut orchard. At a second site, almond yield appeared to correlate well with distinctly different soil zones identified with EM38 mapping.

Uniformity of Infiltrated Water Under a Low Energy Precision Application (LEPA) Irrigation System

ABSTRACT UNIFORMITY of infiltrated water of LEPA irrigation systems is not only affected by hydraulic losses, but also by the machine movement, the furrow check spacing, and the variability of soil infiltration rates, since water is ponded between dikes or checks along a furrow. We assessed the impact of these sources of nonuniformity using both machine movement characteristics measured during a field evaluation of a linear-move machine and soil infiltration measurements made over consecutive 1-m intervals during an unrelated project. Results showed that while the reported uniformity of the nozzle discharges may be 95%, the uniformity of infiltrated water may be between 80% to 85%. For small check spacings, uniformity was primarily affected by the machine movement, while for larger spacings, variability in the soil infiltration rate was the controlling factor. Results show need to consider all sources of nonuniformity of infiltrated water.

Midday Stem Water Potential as a Basis for Irrigation Scheduling

There has been great progress in using midday stem water potential (SWP, measured with the pressure chamber) as a basis for irrigation scheduling, particularly for the management of regulated deficit irrigation (RDI) in trees and vines. In almonds, RDI is used during a one month period when the hulls are splitting, in order to reduce hull rot and improve harvestability. A plant-based RDI experiment was performed at a number of commercial almond orchard sites to evaluate the reliability of this method, and at one site, to test whether crop consumptive water use may be reduced without significant detrimental effects to almond production. In all sites, irrigation was reduced during the hull-split period to achieve a target SWP of -14 to -18 bars. Over many sites and years, RDI showed no significant reduction, or trend in reduction of yield, and in some cases allowed a substantial savings in applied water. Under favorable soil conditions (deep, well-drained and a good water holding capacity) an annual savings in applied water of about 50% was realized compared to the estimated ETc. Even on soils with very low water holding capacity, a plant-based RDI regime was used successfully and gave a modest (10%) reduction in consumptive use. Field irrigation experiments have also been performed on other species (walnut, pear, winegrapes) and SWP has proved to be an invaluable tool to provide feedback for the adjustment of irrigation amounts and timings in order to obtain a desired plant response.