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Desmond R. Layne and J.A. Flore

The source-sink ratio of l-year-old, potted `Montmorency' sour cherry (Prunus cerasus) trees was manipulated by partial defoliation (D) or continuous lighting (CL) to investigate the phenomenon of end-product inhibition of photosynthesis. Within 24 hours of D, net CO2 assimilation rate (A) of the most recently expanded source leaves of D plants was significantly higher than nondefoliated (control) plants throughout the diurnal photoperiod. Between 2 and 7 days after D, A was 30% to 50% higher and stomatal conductance rate (g,) was 50% to 100% higher than in controls. Estimated carboxylation efficiency(k) and ribulose-1,5-bisphosphate (RuBP) regeneration rate increased significantly within 2 days and remained consistently higher for up to 9 days after D. Leaf starch concentration and dark respiration rate decreased but sorbitol and sucrose concentration increased after D. The diurnal decline in A in the afternoon after D may have been due to feedback inhibition from accumulation of soluble carbohydrates (sucrose and sorbitol) in the cytosol. This diurnal decline indicated that trees were sink limited. By 9 days after D, photochemical efficiency was significantly higher than in control plants. In the long term, leaf senescence was delayed as indicated by higher A and gs in combination with higher chlorophyll content up to 32 days after D. CL resulted in a significant reduction of A, gs, k, variable chlorophyll fluorescence (Fv), photochemical efficiency, and estimated RuBP regeneration rate of the most recently expanded source leaves within 1 day. During the exposure to CL, A was reduced 2- to 3-fold and k was reduced up to 4-fold. The normal linear relationship between A and gs was uncoupled under CL indicating that A was not primarily limited by gs and since internal CO2 concentration was not significantly affected, the physical limitation to A imposed by the stomata was negligible. The decrease in Fv and photochemical efficiency indicated that leaves were photoinhibited within 1 day. The decrease in instantaneous chlorophyll fluorescence after at least 1 day of CL indicated that there was a reversible regulatory mechanism whereby the damage to photosystem II reaction centers was repaired. Leaf chlorophyll content was not altered by 1,2, or 3 days of exposure to CL, indicating that photooxidation of chlorophytl did not occur. The time to full photosynthetic recovery from CL increased as the duration of exposure increased. CL plants that were photoinhibited accumulated significant starch in the chloroplast in a companion study (Layne and Flore, 1993) and it is possible that an orthophosphate limitation in the chloroplast stroma was occurring. D plants that were continuously illuminated were not photosynthetically inhibited. After 7 days of CL, plants that were then partially defoliated yet remained in CL photosynthetically recovered within 5 days to pre-CL values. Under the conditions of this investigation, end-product inhibition of A occurred in young, potted sour cherry trees but the mechanism of action in D plants was different than in CL plants.

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Sorkel Kadir, Gaganpreet Sidhu, and Kassim Al-Khatib

Thermotolerance of photosynthesis and productivity in `Chandler' and `Sweet Charlie' strawberry plants (Fragaria ×ananassa Duch.) exposed to three temperature regimes was studied. Net CO2 assimilation rate (A), variable chlorophyll fluorescence (Fv), efficiency of photosystem II (Fv/Fm), relative chlorophyll content, plant growth, and fruit yield and quality were measured. High temperature (40 °C day/35 °C night) was more detrimental to photosynthesis and productivity than the moderate or low temperature (30/25 or 20/15 °C). Net CO2 assimilation rate in both cultivars was markedly reduced by 40/35 °C, although there was slight decline in `Sweet Charlie' at 30/25 °C. `Chandler' maintained significantly higher A rates than `Sweet Charlie' for at least three weeks of heat stress, indicating that `Chandler' might tolerate longer exposure to high temperature. In parallel to the decrease in A rate, intercellular CO2 concentration (Ci) and instantaneous water use efficiency (WUE) were significantly decreased at high temperature. `Chandler' leaves were cooler and transpired more than `Sweet Charlie' leaves, suggesting that each cultivar adopted different heat resistance mechanisms at 40/35 °C. There were changes in Fv and Fv/Fm with increasing temperature, indicating irreversible damage to photosystem II at 40/35 °C might have occurred. The trend of reduction in stomatal conductance (g S) in both cultivars at high temperature did not coincide with the reduction in A rates. Decline in A rates at high temperature was more related to changes in Fv/Fm than to g S activity. The optimal temperature for vegetative growth was 30/25 °C. Reduction in A rate at high temperature resulted in reduction in total leaf area (LA), shoot, root, and leaf biomasses. Strawberry roots were more responsive than shoot growth to temperatures above 20/15 °C. Fruit yield for `Chandler' was higher at 20/15 °C than at 30/25 °C, suggesting that `Chandler' might have a higher source-to-sink relationship at 20/15 °C than at 30/25 °C. Fruit skin color was temperature dependent only for `Chandler'. A quadratic relationship between flower development and duration of exposure to 30/25 °C for both cultivars was observed; more than two weeks of 30/25 °C can be detrimental to flower development. Regardless of the cultivar and duration of exposure, 40/35 °C was the temperature regime most detrimental to fruit set.

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Marlene Ayala and Gregory Lang

( Tables 4 and 5 ). Conversely, Kappel (1991) reported that, with ‘Lambert’ sweet cherry on vigorous P. avium seedling rootstocks, ES growth had a greater sink strength for photosynthates than fruit. Source–sink relationships and relative C

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Rebecca L. Darnell, Nicacio Cruz-Huerta, and Jeffrey G. Williamson

effects on overall growth, cell number, and cell size in bell pepper ovaries Ann. Bot. (Lond.) 110 987 994 10.1093/aob/mcs173 Hall, A.J. Milthorpe, F.L. 1978 Assimilate source-sink relationships in Capsicum annuum L. 3. Effects of fruit excision on

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Jieshan Cheng, Peige Fan, Zhenchang Liang, Yanqiu Wang, Ning Niu, Weidong Li, and Shaohua Li

Crop yield and fruit quality in fruit trees are highly dependent on efficient capture of solar energy and subsequent allocation of photoassimilate. Source-sink relationships are important factors influencing these allocation patterns. Fruit

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A. Maaike Wubs, Ep Heuvelink, Leo F.M. Marcelis, Robert C.O. Okello, Alisa Shlyuykova, Gerhard H. Buck-Sorlin, and Jan Vos

(Expts. 1 and 3) was calculated per plant and average per plot. The relationship between source:sink ratio and the average number of outgrowing buds per plot was tested by Spearman’s rank correlation coefficient. Timing of budbreak (Expts. 1 and 2) was

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Horacio E. Alvarado-Raya, Rebecca L. Darnell, and Jeffrey G. Williamson

al., 2006 ) may be the result of an overall decrease in root carbohydrate reserves in the annual system, and not differences in source–sink relationships between the two production systems. Further work on the effects of root pruning on raspberry

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Yue Wen, Shu-chai Su, Ting-ting Jia, and Xiang-nan Wang

photosynthesis rate ( Proietti et al., 2000 ) and chlorophyll fluorescence parameters ( Wen et al., 2018a ). Furthermore, various leaves have different source–sink relationships, resulting in differences in assimilate partitioning and carbon contributions to

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Martin M. Williams II

Yield of all types of corn, including sweet corn, is the result of processes that accumulate plant biomass (the “source”) and processes that allocate plant biomass to the ear (the reproductive “sink”). Yield can be limited by the source or sink

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Rebecca L. Darnell, Horacio E. Alvarado-Raya, and Jeffrey G. Williamson

HortTechnology 16 1 6 Fernandez, G.E. Pritts, M.P. 1993 Growth and source-sink relationships in ‘Titan’ red raspberry Acta Hort. 352 151 157 Fernandez, G.E. Pritts, M.P. 1994 Growth