Bruce Lampinen

Silencing leaf sorbitol synthesis alters long-distance partitioning and apple fruit quality.

Sorbitol and sucrose are major products of photosynthesis distributed in apple trees (Malus domestica Borkh. cv. “Greensleeves”) that affect quality in fruit. Transgenic apple plants were silenced or up-regulated for sorbitol-6-phosphate dehydrogenase by using the CaMV35S promoter to define the role of sorbitol distribution in fruit development. Transgenic plants with suppressed sorbitol-6-phosphate dehydrogenase compensated by accumulating sucrose and starch in leaves, and morning and midday net carbon assimilation rates were significantly lower. The sorbitol to sucrose ratio in leaves was reduced by ≈90% and in phloem exudates by ≈75%. The fruit accumulated more glucose and less fructose, starch, and malic acid, with no overall differences in weight and firmness. Sorbitol dehydrogenase activity was reduced in silenced fruit, but activities of neutral invertase, vacuolar invertase, cell wall-bound invertase, fructose kinase, and hexokinase were unaffected. Analyses of transcript levels and activity of enzymes involved in carbohydrate metabolism throughout fruit development revealed significant differences in pathways related to sorbitol transport and breakdown. Together, these results suggest that sorbitol distribution plays a key role in fruit carbon metabolism and affects quality attributes such as sugar–acid balance and starch accumulation.

Plant Water Stress, Leaf Temperature, and Spider Mite (Acari: Tetranychidae) Outbreaks in California Vineyards

ABSTRACT We evaluated the relationships between plant water status and leaf temperature, and between leaf temperature and spider mite (Acari: Tetranychidae) and predatory mite (Acari: Phytoseiidae) populations in eight vineyards in California in 2006 and 2007. Temperature of south-facing leaves increased significantly by 0.8°C for every 1.0°C increase in ambient air temperature, and by 5.3°C for every one MPa drop in leaf water potential. Peak population densities of Pacific spider mite, Tetranychus pacificus McGregor, increased significantly with increasing frequency of leaf temperatures above 31°C. In contrast, peak population densities of Willamette spider mite, Eotetranychus willamettei (McGregor), showed no relationship with the frequency of leaf temperatures above 31°C. This differential relationship between the two mite species and high leaf temperatures is consistent with their upper thresholds for development, which are 40°C for T. pacificus and 31°C for E. willamettei, as identified in a previous study. Predatory mite population densities showed no relationship with peak population densities of either spider mite species during the analysis period, but decreased with the frequency of leaf temperatures above 31°C. In addition, predatory mite population densities were significantly higher on south-facing than interior leaves after adjusting for the effect of leaf temperature. These results help to explain why outbreaks of T. pacificus occur in warmer or water-stressed vineyards, whereas E. willamettei develops higher populations in cooler or well-irrigated vineyards. In addition, these results suggest that regulated deficit irrigation should be implemented with caution, especially in those vineyards with a high risk of T. pacificus outbreaks.

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.

Spur behaviour in almond trees: relationships between previous year spur leaf area, fruit bearing and mortality

In mature almond (Prunus dulcis) orchards, the majority of crop is borne on spurs (short, proleptic shoots) that can live for several years and can produce from one to five fruits. Previous research has led to the hypothesis that spur longevity is related to spur light exposure, cropping and age. However, limited quantitative data are available to substantiate these hypotheses. The objective of this study was to determine spur characteristics that were most highly correlated with spur productivity and longevity in mature, bearing almond trees. Previous year spur leaf area was strongly related to spur viability and flowering; the greater the leaf area in the previous year, the higher the probability of spur survival into the next year and the higher the probability for the spur to bear one or more flowers. Previous year bearing also appeared to influence viability and return bloom, especially in spurs with low leaf area. These results suggest that spur source-sink balance is basic to the life cycle of almond spurs. Furthermore, the results are consistent with the hypothesis that spurs are semi-autonomous organs with respect to carbohydrate balance for much of the growing season. Finally, this information provides general thresholds for maintaining spur viability and productivity that will be useful for developing and evaluating tree training systems and orchard management practices.

Leaf and canopy level photosynthetic responses of French prune (Prunus domestica L. ‘French’) to stem water potential based deficit irrigation

Summary The effects of full irrigation and two levels of water stress on tree physiology were investigated in a French prune orchard. The control treatment received approximately 100% of the estimated seasonal crop water requirements throughout the season. The moderate and severe water stress treatments were managed to decline from a fully watered status in late spring to a midday stem water potential of –1.5 and –2.0 MPa, respectively, by harvest. Water stress caused a clear reduction in stem water potential throughout the daylight period, as well as reductions in leaf conductance and photosynthesis, and all of these reductions were more severe with increasing water stress. In a moderate and severe stress treatment, the photosynthetic rate of fully exposed leaves was reduced to 90% and 81% respectively, of the control treatment. There was a linear decline in light saturated photosynthesis with a decline in stem water potential. However, the stress treatments also influenced leaf orientation and the resulting incident light distribution on leaves within the canopy, such that light was more uniformly distributed in canopies under water deprivation than it was under full irrigation. The overall effect of water stress was that the moderate and severe stress treatments had 101% and 93% respectively of calculated canopy photosynthesis compared with the control. This may account for the observed tolerance of prune production to deficit irrigation. A midday depression in assimilation was observed on most days in all treatments, but was not associated with feedback inhibition resulting from carbohydrate accumulation in leaves.

Perennial Crop Nurseries Treated with Methyl Bromide and Alternative Fumigants: Effects on Weed Seed Viability, Weed Densities, and Time Required for Hand Weeding

Data on the efficacy of alternative fumigants to methyl bromide for weed control in perennial crop nurseries in California are needed because few herbicides are registered for this purpose. Field studies were conducted from 2003 to 2006 in four commercial perennial crop nurseries in California. Treatments included a nonfumigated control; methyl bromide (98%) (MeBr) with high-density polyethylene (HDPE) film; iodomethane (50%) + chloropicrin (50%) with HDPE film; 1,3-dichloropropene (1,3-D) with HDPE film; 1,3-D (61%) + chloropicrin (35%) with HDPE film; 1,3-D (62%) + chloropicrin (35%) subsurface drip; and 1,3-D (61%) + chloropicrin (35%) with virtually impermeable film (VIF). All the fumigants reduced the seed viability of common purslane, johnsongrass, and tall morningglory but were not as effective on little mallow and field bindweed. Although total weed densities and the level of control provided by each fumigant differed between locations, weed density was generally reduced by all the fumigation treatments, compared to the nonfumigated control. At three locations, alternative fumigation treatments usually resulted in hand-weeding time similar to MeBr. Reductions in weed seed viability, weed emergence, and weed densities suggest that these alternative fumigants will provide weed control similar to MeBr in perennial nurseries. Nomenclature: 1,3-dichloropropene, chloropicrin (trichloronitromethane), iodomethane, methyl bromide, common purslane, Portulaca oleracea L. POROL, field bindweed, Convolvulus arvensis L. CONAR, little mallow, Malva parviflora L. MALPA, johnsongrass, Sorghum halepense (L.) Pers. SORHA, tall morningglory, Ipomoea purpurea (L.) Roth PHBPU

Relationships between spur- and orchard-level fruit bearing in almond (Prunus dulcis)

Almond is often considered to be a moderately alternate-bearing species but historical yield data typically do not exhibit clear patterns of alternate bearing at the orchard level, while research has indicated that spurs (the main fruit bearing unit in almond trees) rarely produce fruit in two subsequent years. The objective of the present work was to analyze the bearing behavior of almond trees at both the orchard level and the individual spur level over multiple years to explain this apparent paradox. The 10-year yield patterns of three almond cultivars grown at three different sites within California were analyzed for tendencies of alternate bearing at the orchard level. At the individual spur level, data on spur viability, and number of flowers and fruits per spur were collected on 2400 individually tagged spurs that were observed over 6 years to characterize bearing at that level. At the orchard level one cultivar (Nonpareil) did exhibit a tendency for alternate bearing at one site (Kern) but other cultivars and sites did not. The orchard and the individual trees in which the spur population study was conducted showed tendencies for alternate bearing but the spur population did not. Only a relatively small percentage of the total tagged spur population bore fruit in any given year and therefore while individual fruiting spurs exhibited a high level of non-bearing after fruiting the previous year the spurs that did produce fruit in any year generally did not constitute enough of the total spur population to exhibit alternate bearing at the whole population level. Our results suggest that annual bearing fluctuations in almond are probably mainly due to year-to-year variations of parameters affecting fruit set and that high rates of fruit set in a given year may involve a larger-than-normal percentage of a spur population in fruit bearing. This would limit the size of the spur population available for flowering in the subsequent year and could cause alternate year bearing. However, from historical records, this would appear to be the exception rather than a normal circumstance. Therefore, almond should not be considered to be a strictly alternate-bearing species.

Fruit development in almond is influenced by early Spring temperatures in California

Summary The period from full bloom (FB) to fruit maturity for individual cultivars of peach, nectarine, plum, and prune is influenced by daily temperatures between the start of FB and 30 d after FB (DAFB). Typically, warm Springs accelerate fruit development. Almond is closely-related to peach, but the date of fruit maturity is not always closely-related to the date of harvest. Normally the date of “hull-split” (HS) signals the beginning of fruit maturity. The aim of this study was to determine if the length of the period between FB and HS in several important Californian almond cultivars was related to temperatures shortly after the start of FB. Data on the dates of FB and HS from three locations in the Central Valleys of California (North, Central, and South) were analysed over 8 years to determine the effect of Spring temperatures on the duration of fruit development. Data on 28 cultivars were evaluated, but only the results for 12 of the most important cultivars are reported here. The length of the period of fruit development from FB to HS was negatively correlated with the accumulation of degree-days between FB and 90 DAFB (mean R2 = 0.51 ± 0.3), with generally poorer correlations with degree-days to 30 or 50 DAFB (mean R2 = 0.31 ± 0.02 and 0.36 ± 0.3, respectively). These results suggest that temperatures in the first 90 DAFB are the primary factor influencing the time of nut maturity in almond cultivars in California. This information will be used to develop a harvest prediction model to assist growers in planning harvest dates. To facilitate this, we are in the process of developing a webpage on the UC Davis Fruit and Research Information Website similar to the one for peach and plum growers (http://fruitsandnuts.ucdavis.edu/Weather_Services/Harvest_Prediction_About_Growing_Degree_Hours.htm).

Spring bud growth depends on sugar delivery by xylem and water recirculation by phloem Münch flow in Juglans regia

AbstractMain conclusionDuring spring, bud growth relies on long-distance transport of remotely stored carbohydrates. A new hypothesis suggests this transport is achieved by the interplay of xylem and phloem. During the spring, carbohydrate demand of developing buds often exceeds locally available storage, thus requiring the translocation of sugars from distant locations like limbs, stems and roots. Both the phloem and xylem have the capacity for such long-distance transport, but their functional contribution is unclear. To address this ambiguity, the spatial and temporal dynamics of carbohydrate availability in extension shoots of Juglans regia L. were analyzed. A significant loss of extension shoot carbohydrates in remote locations was observed while carbohydrate availability near the buds remained unaffected. This pattern of depletion of carbohydrate reserves supports the notion of long-distance translocation. Girdling and dye perfusion experiments were performed to assess the role of phloem and xylem in the transport of carbohydrate and water towards the buds. Girdling caused a decrease in non-structural carbohydrate concentration above the point of girdling and an unexpected concurrent increase in water content associated with impeded xylem transport. Based on experimental observations and modeling, we propose a novel mechanism for maintenance of spring carbohydrate translocation in trees where xylem transports carbohydrates and this transport is maintained with the recirculation of water by phloem Münch flow. Phloem Münch flow acts as a pump for generating water flux in xylem and allows for transport and mobilization of sugars from distal locations prior to leaves photosynthetic independence and in the absence of transpiration.