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Canopy net carbon dioxide exchange rate (NCER) is at the basis of orchard productivity and may be a limiting factor to fruit quality; particularly in over-cropped sweet cherry canopies. NCER predominantly occurs in the leaves and relates directly to the production of carbohydrates used in fruit and vegetative growth.
Characterizing daily and seasonal NCER is the first step towards estimating tree carrying capacity (i.e., how many top-quality fruit a canopy can support) Tree carrying capacity is influenced by the ratio between the number of fruit per tree and the canopy leaf area (fruit:LA); and imbalance in this ratio results in reduced fruit quality or inefficient production.
Whole-canopy analyses are well-suited to studying such fundamental physiological relationships because they integrate the entire leaf area and the tremendous variability within (i.e. sun-exposed and shaded leaves). New sweet cherry rootstocks (e.g., Gisela® 5) induce precocious and abundant fruiting early in the tree’s life and increase the fruit:LA balance compared to other rootstocks (e.g., Mazzard). This often results in high yields of small fruit and poor returns to the grower. Knowledge of the whole-tree carbon cycle, including its daily and seasonal dynamics, is essential to maintaining balanced cropping and maximizing tree productivity and orchard profitability.
The objective of this research was to determine the influence of crop load on fruit quality and the daily and seasonal trends of NCER in Bing/Gisela® 5 sweet cherry canopies
Figures 1 and 2. Left: Typical control (no manipulation) cropload of Bing Gisela® 5 rootstock. Right: Typical balanced cropload of Bing on Gisela® 5 rootstock.
The plant material used were 9 six-year-old, free-standing multiple-leader Bing/Gisela® 5 sweet cherry trees. Three crop load treatments were imposed on March 31, 2000:
Whole-canopy gas exchange (NCER) was determined using a suite of 3 canopy chambers (Figure 3) in conjunction with an automatic monitoring (TPS-1, PP Systems, U.K.) and control system. Switching among chambers occurred at 2-min intervals and was controlled by solenoid valves and a datalogger (CR10x, Campbell Scientific, Logan, UT). Inlet air velocity was monitored continuously by differential pressure transducers in conjunction with the datalogger.
Canopy leaf area was determined non-destructively as the sum of spur and shoot LA. Canopy spur LA was estimated as the product of the number of spurs per tree and the measured mean LA (CID-283, CID Inc., Vancouver, WA) of 20 randomly selected spurs. The relationship between shoot length and shoot LA was established from 10 randomly selected shoots per tree and was used to estimate shoot leaf area per canopy from mean shoot length and total number of shoots per canopy.
Fruit number and weight were recorded for each tree at harvest [June 27, 2000, 76 days after full bloom (DAFB)]. From each tree, 100 randomly sampled fruit were weighed and tested for size and sugars.
Figure 3. A) Suite of three chambers used for measuring whole-canopy gas exchange. B) Top-view of chambers. C) Close-up of chamber base. D) Close-up of whole-tree chambers.
Crop load manipulation produced two distinct ranges of fruit:LA which influenced fruit quality and canopy NCER. Fruit quality is inversely proportional to the fruit:LA.
|Treatment||% > 10-row||g/fruit||° brix||fruit/m3 LA|
Heavy-cropping trees showed higher canopy NCER prior to harvest (Figures 4,5) compared to light- and non-cropping trees. Daily NCER exhibited a hyperbolic trend prior to harvest and a ‘flattened’ response after harvest (Figure 5). Seasonal canopy NCER varied significantly and declined between 66 and 91 DAFB for all treatments (Figures 4,5):
Figure 4. NCER for three cropping level treatments at 41, 66, 91, 127, and 155 DAFB.
Figure 5. Daily NCER at 41, 66, 91, 127, and 155 DAFB.
Sweet cherry canopies are very productive: approx. 250, 290, 325 g of carbon dioxide fixed per tree per day (Figure 6) (seasonal means for No Fruit, Balanced, and Control, respectively)
Figure 6. Net assimilation for three cropping level treatments at 41, 66, 91, 127, and 155 DAFB.
Canopy-enclosing chambers are ideal for integrative estimates of tree performance and particularly well-suited for studying the effects of cultural and management practices of sweet cherries.
Crop load positively affects whole-canopy NCER in sweet cherry trees. The increased NCER in heavy-cropping trees is insufficient to compensate for the high fruit:LA. Alternate sinks exist in non-cropping trees to maintain relatively high NCER, but carbohydrate supply is finite and fruit compete with alternate sinks (including other fruit) for assimilates.
Small fruit size on Bing/Gisela® 5 trees is a result of overcropping (high fruit:LA) and can be overcome by thinning. However, a greater range of fruit:LA needs to be tested in order to determine the optimum fruit:LA of Bing/Gisela® 5 trees. Abundant and productive leaf area is required to maintain balanced cropping. Management practices which improve NCER would be beneficial and potentially increase canopy carrying capacity and fruit quality.
(Available links generally refer to the abstract from the article. Access to full text of the article may require membership and/or subscription.
Whiting, M.D. and G.A. Lang. 2001. Canopy architecture and cuvette flow patterns influence whole-canopy net CO2 exchange and temperature in sweet cherry. HortScience 36:691-698.
Whiting, M.D. and G.A. Lang. 2001. Sweet cherry photosynthesis, crop load, and fruit quality relationships. Fruit Grower News 40(11):42-46.