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Jens N. Wünsche and John W. Palmer

We thank J. Julian for his valuable contribution by designing and constructing the canopy cuvettes and P. Alspach for writing the SAS analysis program for calculating whole-canopy gas exchange. The cost of publishing this paper was defrayed in

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Kuo-Tan Li and Alan N. Lakso

Summer pruning is primarily used in apples to increase the light penetration into inner canopy to improve fruit color. However, summer pruning may reduce fruit size. We hypothesize that removing healthy exterior shoots reduces the whole-tree carbon supply in relation to pruning severity. If the crop load (i.e., demand) is high, fruit size and quality will be reduced. The effects of summer pruning on photosynthetic activity and recovery of shaded leaves after re-exposure were monitored on a range of exposures in canopies of `Empire' apple trees. The photosynthetic ability of leaves was positively related to its prepruning exposure. There was little recovery of photosynthetic activity of shade leaves until late growing season, indicating the re-exposure of shade leaves after summer pruning cannot replace the role of exterior leaves removed by pruning. Whole canopy net CO2 exchange (NCER) was measured on `Empire'/M9 trees with a commercial range of pruning severity. Reductions in NCER were approximately proportional to pruning severity and % leaf area removed and were as great as 60% in the most severe pruning. Canopy light interception decreased slightly. The effects on canopy NCER thus appeared to be primarily related to reduced photosynthetic efficiency and secondarily to reduced light interception.

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G.W. Stutte and N.C. Yorio

The relationship between whole canopy and single leaf measurements of gas exchange has not been well documented. Two experiments were conducted in the Biomass Production Chamber at Kennedy Space Center (20-m2 growing area) to compare whole canopy versus single leaf net carbon assimilation rate (Anet) measurement of a stand of tomato (Lycopersicon esculentum Mill. cv. Reimann Philipp) and soybean [Glycine max (L.) Merr. cv. Hoyt]. Both crops were grown under a 12/12 hour photoperiod under HPS lamps at PPF of 800 (mol·m–2·s–1, at 26/22°C (light/dark), and constant 65% RH for 90 days. CO2 concentration was controlled to 1200 (mol·mol–1 during the light cycle. Midday measurements of Anet of single leaves were obtained weekly from upper canopy leaves using a portable photosynthesis system. Whole canopy measurements of Anet were calculated daily from CO2 addition data obtained at 5-minute intervals by the BPC monitoring and control system. Single leaf rates exceeded whole canopy rates prior to full canopy coverage then averaged 0.63 of whole canopy for both species during the period of full canopy coverage. Results suggest that reliable estimates of canopy gas exchange can be obtained from single leaf measurements under relatively constant environment conditions.

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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.

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D. Michael Glenn

and without supplemental irrigation, over a range of LAI, seasonal evapotranspiration, and VPD to determine the mechanisms of action affecting WUE in apple. Short-term whole canopy gas exchange studies and isotope discrimination analysis were used to

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Julie M. Tarara and Jorge E. Perez Peña

moderate water stress for the entire period of fruit set to harvest. The daily maximum Tr V,LA values presented here are comparable with others in the literature that evaluated water-stressed vines using whole-canopy gas exchange [e.g., ≈5 mmol·m −2 ·s −1

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A.H.D. Francesconi, A.N. Lakso, J.P. Nyrop, J. Barnard, and S.S. Denning

The hypothesis that carbon balance is the basis for differences in responses by lightly and normally cropped apple trees to European red mite (ERM) [Panonychus ulmi (Koch)] damage was tested. Mature `Starkrimson Delicious' (Malus domestica Borkh.)/M.26 apple trees were hand-thinned to light (125 fruit/tree, about 20 t/ha) or normal (300 fruit/tree, about 40 t/ha) target crop levels and infested with low [<100 cumulative mite-days (CMD)], medium (400 to 1000 CMD) or high (>1000 CMD) target levels of ERM. A range of crop loads and CMD was obtained. Mite population density, fruit growth, leaf and whole-canopy net CO2 exchange rates (NCER) were measured throughout the growing season of 1994. Leaf area and vegetative growth per tree were also measured. Yield and final mean fruit size were determined at harvest. Return bloom and fruiting were determined the following year. Total shoot length per tree was not affected by crop load or mite damage. ERM reduced leaf and whole-canopy NCER. Normally cropped trees showed fruit weight reduction earlier and more severely than lightly cropped trees with high mite injury. Variation in final fruit weight, return bloom and return fruiting was much better related to whole-canopy NCER per fruit than to CMD.

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S. Poni, E. Magnanini, and B. Rebucci

The reported system interfaces a commercially available portable infrared gas analyzer with a measurement and control module for continuous and automated measurements of whole-canopy gas exchange. Readings were taken for several days, under mostly sunny or partly cloudy conditions, on two potted vines (total leaf area per vine of ≈1.3 m2) enclosed in inflated polyethylene chambers. The air flow rate through the chambers was provided by a centrifugal blower and set at 5 L·s-1 by a butterfly valve. It prevented ΔCO2 from dropping below –40 mL·L-1. Switching of the two CO2 analysis channels to the infrared gas analyzer (operated in a differential mode) was achieved by solenoid valves, whereas wet and dry-bulb temperatures at chambers' inlet and outlet were measured by low-cost, custom-made thermocouple psychrometers. Whole-vine assimilation rate (WVA) and whole-vine transpiration rate were calculated from the inlet—outlet differences in CO2 and absolute humidity. When compared to assimilation measured on single leaves (SLA) under saturating light at equivalent times, the WVA reduction (area basis) was ≈50%, suggesting that whole-canopy photosynthetic efficiency based on SLA readings can be greatly overestimated.

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Luca Corelli-Grappadelli, Gianfranco Ravaglia, and Eugenio Magnanini

Training system efficiency may be defined as the ratio of fruit produced to the amount of light intercepted by the canopy. In apple, a positive, linear relationship between yield and light intercepted is generally found, but in peach similar data are hard to come by. This paper reports data from an ongoing training systems trial now in the 7th year, with trees trained as Y, palmette, and delayed vase. During the life of the orchard, light interception has been measured for the different tree shapes, the yields have been recorded, and, in some years, whole-canopy gas exchanges of cropping trees have been measured. In general, the trees have been intercepting light in amounts proportional to canopy shape and tree density, with the Y (planted at higher density) intercepting more light than the other two systems, which appear more comparable to each other, despite the fact that they intercept light during the day in different ways, with the delayed vase exposing more or less the same leaves to incoming light during most of the day. Cropping has followed the amounts of light intercepted, with higher yields for the Y, without appreciable differences in fruit quality traits. The data accumulated so far indicate furthermore that the palmette and the delayed vase, despite slightly different light interception potentials (lower for the palmette), have similar yields. This might depend in part on the fact that these two systems intercept light according to different patterns during the day, with the palmette—which distributes the light intercepted in a more even fashion between the two sides—perhaps at an advantage over the vase in terms of managing the stress of excessive light (heat) loads during the central hours of the day. Whole canopy Carbon exchange data have been found to be in agreement with the patterns of light interception.

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Guohai Xia, Lailiang Cheng, Alan Lakso, and Martin Goffinet

shaded. Thus, whole canopy gas exchange measurement is more representative of the whole tree carbon assimilation than single leaf gas exchange measurement. Indeed, the increase in whole canopy gas exchange matches well with the increase in net dry matter