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J.L. Catley

The influences of temperature and irradiance on vegetative growth of two species of Leucocoryne (Leucocoryne coquimbensis F. Phil and L. ixioides (Hook.) Lindl.) were examined in controlled environment growth rooms. The growing environments had day/night temperatures of 10/5, 15/10, or 20/15 °C, providing mean temperatures of 7.5, 12.5, or 17.5 °C, and photosynthetic photon fluxes (PPF) of 497 or 710 μmol·m-2·s-1. Leaf emergence data were recorded up to three times a week, and measurements of vegetative growth were made in the rooms twice weekly. Destructive harvests were carried out at intervals up to four weeks apart. Leaves of L. ixioides emerged first in all mean temperatures. As mean temperature decreased from 17.5 to 7.5 °C, the differences in first emergence dates became more apparent between species. Appearance of the second leaf of both species occurred in less than half the number of days the first leaf took to emerge. The time taken for further leaves to develop increased as temperature decreased, particularly for L. ixioides and at mean temperatures below 12.5 °C. Although leaves of L. ixioides emerged first, days to emergence of further leaves increased to lag behind production of L. coquimbensis leaves, particularly when mean temperatures dropped below 12.5 °C. Temperature also significantly affected growth of other plant parts. As mean temperature increased, maximum leaf, root and main bulb dry weights increased for both species, along with secondary bulb dry weights of L. coquimbensis. As irradiance increased, maximum leaf dry weights decreased and maximum bulb dry weights increased of both species, and maximum dropper dry weights of L. coquimbensis increased. Leucocoryne coquimbensis appears to have the greatest capacity to multiply vegetatively and this is enhanced by high mean temperatures. These results suggest that mean temperatures higher than those used in this study are required for sustained leaf emergence, particularly for L. ixioides although this species has the capacity to emerge at low temperatures. High mean temperatures are also likely to promote vegetative mass of all plant parts of both species, whereas higher irradiance levels than used in this study would enhance main bulb growth.

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Rui Zhou and Bruno Quebedeaux

In order to determine whether the changes in the demand for the transported carbohydrates in apple source leaves are associated with specific carbohydrate enzyme changes, we made source–sink manipulation by girdling or defoliation. The girdle was applied to side branches with several fully expanded leaves, and the defoliation was conducted by removing about 90% of source leaves in apple seedlings. 3-year-old apple (Malus domestica Borhk. cv. Gala) seedlings were grown in a 15/9-h light (≈700 μmol photons/m2 per s, 22 °C)/dark (18 °C) in the growth chamber. When the demand for transported carbohydrates from a particular source leaf is limited by girdling, carbohydrates including starch, sorbitol and sucrose accumulated in the source leaves, meanwhile girdling reduced net photosynthetic rates (Pn) dramatically from 12.8 initially to 4.6 μmol CO2/m2 per s over a 7-day period. When the demand for transported carbohydrate in the remaining source leaves was increased by defoliation, all carbohydrate levels decreased while Pn of individual leaves increased from 13.6 initially to a maximum of 19.8 μmol CO2/m2 per s after 7 days. These Pn changes in the carbohydrate depleted and accumulated leaves were due mainly to changes in the photosynthetic capacity as indicated by Pn-Ci curve measurements. The carbohydrate enzyme activities were also dramatically changed during the 7-day experimental period. The activity of aldose-6-phosphate reductase (E.C. 1.1.1.200), an important enzyme in sorbitol biosynthesis, increased significantly from 27.5 to 39.2 μmol/h per g FW in the carbohydrate depleted leaves while it remained unchanged in the girdled leaves, the activity of sucrose-6-phosphate synthase (SPS, E.C. 2.4.1.14), a key enzyme for sucrose biosynthesis, increased from 15.4 to 23.0 μmol/h per g FW in the depleted leaves and declined from 17.4 to 8.2 μmol/h per g FW in the girdled leaves, the activity of fructose 1,6 bisphosphatase (E.C. 3.1.3.11), another key enzyme for sucrose biosynthesis in non-Rosaceae species showed a similar pattern as SPS, ADPglucose-pyrophosphorylase (E.C. 2.7.7.27), a key enzyme for starch biosynthesis, decreased a small amount in the girdled leaves but increased markedly from 42.9 to 56.0 μmol/h per g FW in the depleted leaves. These results indicated the specific roles of the enzymes in the partitioning of carbon between sorbitol, sucrose and starch in apple source leaves.

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Hui-lian Xu, Laurent Gauthier, and André Gosselin

Tomato plants were grown in peat bags in greenhouse to examine the effects of variation of the nutrient solution electrical conductivity (EC) and substrate water potential (Ψsub) on photosynthesis in leaves, fruits, stem, and petioles. EC of the nutrient solution delivered to peat bags varied between 1 to 4 dS·m–1 with Ψsub of either –5 kPa or –9 kPa as the setpoint for starting the irrigation. The EC variation was adjusted by a computer system according to potential evapotranspiration. Gross photosynthetic capacity (PC) decreased as the leaf age developed. PC in the 10th, 15th and 18th leaves from the top was only 76%, 37%, and 18% of PC in the 5th leaf, respectively. However, low quantum use efficiency (QUE) was only observed in the 18th leaf and low dark respiration (RD) was only in 15th and 18th leaves. Net photosynthesis (PN) was only observed in young fruits (≈10 g FW) or young petioles and no PN was observed in large fruits (50 g or more FW) and stems. Both PC and RD were lower in older fruits and petioles or in the lower part of the stem compared to the younger ones or upper parts. EC variation increased PC, QUE, and RD in most parts. Low Ψsub increased RD in most parts and decreased PC in fruits, stem, and petioles. It is suggested that EC variation increased plant physiological activity of tomato and low Ψsub increased carbon consumption, although it was not severe enough to depress leaf PC.

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Hui-lian Xu, Laurent Gauthier, and André Gosselin

Tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in peat bags, rockwool slabs, and NFT in a greenhouse to examine the effects of nutrient solution electrical conductivity (EC) and potential evapotranspiration (PET)-dependent EC variation on plant water relations. Peat bags were irrigated by a PET-dependent irrigation system. EC was varied from 1 to 4 mS·cm-1 according to PET under –5 and –9 kPa of substrate water potential setpoints (SWPS). The plants in rockwool and NFT were treated with ECs of 2.5, 4, and 5.5 mS·cm-1. Peat bags and rockwool slabs were overwatered once a week to wash out the accumulated salts. Leaf water potential (ψ1) and relative water content (θ) were measured before and after plants were overwatered. Turgor (P) and osmotic π potentials were estimated from the pressure-volume method. Before plants were overwatered, ψ1 was significantly lower in the plants with high EC and low SWPS treatments and also lower in variable EC-treated plants, but P maintained close to the control value. After plants were overwatered, ψ1 recovered close to the control level and P became higher because of the lower π in the treatments of high EC, variable EC, and/or low SWPS. At a given ψ1 the plants with high EC, variable EC, and/or low SWPS maintained higher θ. The analysis of the pressure-volume curve showed that the leaves treated with high EC, variable EC, and/or low SWPS had higher turgid water content, higher symplasmic (osmotically active) water content, lower apoplasmic (osmotically inactive) water content, and lower θ point of zero turgor (incipient plasmolysis). Maintenance of P after overwatering was directly proportional to photosynthetic capacity. We suggest that osmotic adjustment occurs in response to high EC, low SWPS, or both and that overwatering substrates and varying EC can not only avoid salinity stress, but also improve turgor maintenance.

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N.S. Lang, R. Smithyman, L. Mills, R.L. Wample, J. Silbernagel, and E.M. Perry

Blackleaf (a.k.a. chocolate leaf) is of worldwide concern in Vitis due to its negative impact on fruit ripening, yield reduction and overall stress on grapevines. Research suggests blackleaf is induced by high levels of UV radiation and overall light intensity, which induce color changes (purple-brown-black) in exposed leaves, resulting in >50% reduction in photosynthesis. The ability to detect blackleaf symptoms before expression can provide insight into metabolic stresses and the possibility of the use and/or timing of management practices to reduce its impact. Remotely sensed imagery and spatial analysis may elucidate reflectance-related processes and symptoms not apparent to the un-aided eye. In this research we mapped canopy growth (leaves/shoot and shoots/vine), metabolic triggers (photosynthesis, leaf water potential, soil moisture), and percent blackleaf expression within vineyards using global positioning system (GPS), infrared gas analyzer, and digital remotely-sensed images. Each image and data record was stored as an attribute associated with specific vine location within a geographical information system (GIS). Spatial maps were created from the GIS coverages to graphically present the progression of blackleaf across vineyards throughout the season. Analysis included summary statistics such as minimum, maximum, and variation of green reflectance, within a vineyard by image capture date. Additionally, geostatistics were used to model the degree of similarity between blackleaf values as a function of their spatial location. Continuing research will be aimed at identifying spectral characteristics of early season stresses due to UV light, water stress, and reduced photosynthetic capacity. Spatial relationships between early season stress and later blackleaf expression will be assessed using joint spatial dependence measures. Overall, information obtained through digital image and spatial analysis will supplement site level information for growers.

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Jyotsna Sharma* and William R. Graves

Tolerance of shade, flooding, drought, and nutrient-poor substrate is desirable among ornamental plants installed in managed landscapes. Many attractive native taxa have not been evaluated for their resistance to environmental stressors. We assessed Florida corkwood (Leitneria floridana Chapman) in its natural habitat in four disjunct populations in the United States and tested the physical and chemical properties of the soil at the study sites. Measures at all sites were made within two weeks in late June, 2003. Leaf area, plant height, length of new shoots, and the rate of photosynthesis were higher among plants receiving more than 600 μmol·m-2·s-1 of photosynthetically active radiation (PAR) compared to plants that occurred where maximum PAR was lower. Soil texture ranged from clay loam to fine sand, and soil pH across sites was 4.5 to 6.6. Concentration of nitrate-nitrogen, phosphorus, and potassium ranged from 3 to 75 mg·L-1, 7 to 11 mg·L-1, and 3 to 64 mg·L-1, respectively. Bases of plants in Florida were submersed in water, while soil moisture percentages in Missouri and Texas were 6 to 30. The apparent tolerance of L. floridana to shade, low and high soil moisture, and nutrient-poor soil in native habitats indicates that it could be used in a wide range of managed landscapes. Its capacity to adapt to shade may permit the use of L. floridana as an understory species in managed landscapes, but stewards of natural areas may need to maintain open sites within its native habitat to allow expansion of populations. Because this assessment of L. floridana included native populations across the natural range of the species, our results are uniquely suited for both horticultural and ecological interpretation and application.

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Matthew Fidelibus* and David Smart

In response to diminishing returns, the California raisin industry is rapidly adopting mechanical raisin harvesting practices to reduce cost. Whether the fruit will dry on the vine, or be laid on continuous trays to dry, the first step of mechanical raisin harvest generally involves severing the canes of vines with ripe fruit, a practice known as harvest pruning (HP). The potential physiological implications of HP are uncertain, so an experiment was established to assess the effects of HP on 40-year-old `Thompson Seedless' grapevines (Vitis vinifera L.) that were on their own roots, head-trained and cane pruned, and supported by a single wire trellis. Fruit achieved 20 °Brix by 2 Sept., at which time vines had a leaf area of about 21.6 m2. About 60% of the canopy leaf area was from canes, and thus removed by HP. The net CO2 assimilation rate (A) of mature leaves on renewal shoots began to decline after about 8 Aug., but they maintained a positive A until at least 31 Oct. Reduced A was due, in part, to chlorophyll degradation as evidenced by a decline in SPAD units occurring over the period that A declined. Harvest pruning generally did not affect A of mature leaves retained on renewal shoots, but those leaves maintained a positive A for at least 60 d after HP indicating that HP reduced the vines' photosynthetic capacity. Soil respiration also declined between summer and winter, probably in response to decreasing soil temperatures. Soil respiration was similar among HP and non-HP vines, except about 30 d after HP, when HP vines had about 30% lower soil respiration values than non-HP vines. Root growth was observed in summer and fall regardless of whether vines were subjected to HP.

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Peter R. Hicklenton, Julia Y. Reekie, Robert J. Gordon, and David C. Percival

Seasonal patterns of CO2 assimilation (ACO2), leaf water potential (ψ1) and stomatal conductance (g1) were studied in three clones (`Augusta', `Brunswick', and `Chignecto') of lowbush blueberry (Vaccinium angustifolium Ait.) over two growing seasons. Plants were managed in a 2-year cycle of fruiting (year 1) and burn-prune (year 2). In the fruiting year, ACO2 was lowest in mid-June and early September. Rates peaked between 10 and 31 July and declined after fruit removal in late August. Compared with the fruiting year, ACO2 in the prune year was between 50% and 130% higher in the early season, and between 80% and 300% higher in mid-September. In both years, however, mid-season maximum ACO2 for each clone was between 9 and 10 μmol·m–2·s–1CO2. Assimilation of CO2 increased with increasing photosynthetic photon flux (PPF) to between 500 and 600 μmol·s–1·m–2 in `Augusta' and `Brunswick', and to between 700 and 800 μmol·s–1·m–2 in `Chignecto'. Midday ψ1 was generally lower in the prune year than in the fruiting year, reflecting year-to-year differences in soil water content. Stomatal conductance (g1), however, was generally higher in the prune year than in the fruiting year over similar vapor pressure deficit (VPD) ranges, especially in June and September when prune year g1 was often twice that observed in the fruiting year. In the fruiting year, g1 declined through the day in response to increasing VPD in June, but was quite constant in mid-season. It tended to be higher in `Augusta' than in the other two clones. Stomatal closure imposes limitations on ACO2 in lowbush blueberries, but not all seasonal change in C-assimilative capacity can be explained by changes in g1.

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A. James Downer, Donald R. Hodel, and Maren J. Mochizuki

from a saw blade flamed after cutting infected palm petioles. Conclusions Reduction of photosynthetic area reduces the ability of some palms to grow and produce; even slight decreases in green leaf number have been associated with decreased yield in oil

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Jens N. Wünsche, John W. Palmer, and Dennis H. Greer

Effect of crop load on tree growth, leaf characteristics, photosynthesis, and fruit quality of 5-year-old `Braeburn' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] trees on Malling 26 (M.26) rootstock was examined during the 1994-95 growing season. Crop loads ranged from 0 to 57 kg/tree [0 to 1.6 kg fruit/cm2 trunk cross sectional area (TCA) or 0 to 8.7 fruit/cm2 TCA]. Fruit maturity as indicated by background color, starch/iodine score, and soluble solids was advanced significantly on low-cropping trees compared to high-cropping trees. Whole-canopy leaf area and percentage tree light interception increased linearly with a significant trend as crop load decreased. From midseason until fruit harvest, leaf photosynthesis decreased significantly on lighter cropping trees and similarly, a positive linear trend was found between whole-canopy gas exchange per unit area of leaf and crop load. Leaf starch concentration in midseason increased linearly as crop load decreased, providing some explanation for the increased down-regulation of photosynthesis on trees with lower crop loads. After fruit harvest, the previous crop loads had no effect on leaf photosynthesis and preharvest differences in whole-canopy gas exchange per unit area of leaf were less pronounced. At each measurement date, daily whole-canopy net carbon exchange and transpiration closely followed the diurnal pattern of incident photosynthetic photon flux. The photochemical yield and electron transport capacity depended on crop load. This was due mostly to reaction center closure before harvest and an increased nonphotochemical quenching after harvest.