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Qingzhang Xu, Bingru Huang and Zhaolong Wang

Heat injury in creeping bentgrass (Agrostis stolonifera var. palustris Huds) has been associated with decreases in carbohydrate availability. Extending light duration may increase carbohydrate availability and thus improve growth of creeping bentgrass under heat stress. The objective of this study was to investigate whether turf performance and carbohydrate status could be improved by extending daily light duration for creeping bentgrass exposed to supraoptimal temperature conditions. `Penncross' plants were initially grown in growth chambers set at a day/night temperature of 20/15 °C and 14-hour photoperiod and then exposed to a day/night temperature of 33/28 °C (heat stress) and three different light durations: 14 (control), 18, and 22 hours (extended light duration) for 30 days. Turf quality and tiller density decreased with the duration of heat stress, as compared to the initial level at 20 °C, regardless of the light duration. However, both parameters increased with extended light duration from 14 to 18 or 22 hours. Extended light duration, particularly to 22 hours, also improved canopy net photosynthetic rate from -1.26 to 0.39 μmol·m-2·s-1 and daily total amount of carbon assimilation from -6.4 to 31.0 mmol·m-2·d-1, but reduced daily total amount of carbon loss or consumption to 50% through dark respiration compared to 14 hours treatment by the end of experiment. In addition, extending light duration from 14 to 22 hours increased water-soluble carbohydrate content in leaves both at the end of light duration and the dark period. These results demonstrated that extending light duration improved turf performance of creeping bentgrass under heat stress, as manifested by the increased tiller density and turf quality. This could be related to the increased carbohydrate production and accumulation. Supplemental lighting could be used to improve performance if creeping bentgrass is suffering from heat stress.

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F. Christine Pettipas, Rajasekaran R. Lada, Robert Gordon and Tess Astatkie

Increasing temperature as a result of global climate change is expected to exert a great influence on agricultural crops, possibly through effects on photosynthesis. Response to temperature of leaf gas exchange parameters of carrot (Daucus carota L. var. sativus) cultivars Cascade, Carson, Oranza, and Red Core Chantenay (RCC) were examined in a controlled growth room experiment. Leaf net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) were measured at temperatures ranging from 15 to 35 °C at 370 μmol·mol-1 (CO2) and 450±20 μmol·m-2·s-1 PAR. The cultivars responded similarly to increasing temperature and did not differ in most photosynthetic parameters except gs. The PN increased between 20 and 30 °C, thereafter increasing only slightly to 35 °C. On average, increasing temperature from 20 to 30 °C increased PN by 69%. Carboxylation efficiencies (Ca/Ci ratio) ranged from 1.12–2.33 mmol·mol-1 while maximum PN were 3.25, 3.90, 5.49, 4.19 μmol·m-2·s-1 for Carson, RCC, Cascade, and Oranza, respectively. The E did not reach maximum at 35 °C while gs peaked at 30 °C and then decreased by 93% at 35 °C. The water use efficiency (WUE) decreased with an increase in temperature due to increases in both PN and E. The results indicate that increasing temperatures above the seasonal average (<20 °C) increases both PN and E up to 30–35 °C. An increase in photosynthesis due to an increase in temperature is expected to hasten growth. Carrots may be able to withstand a moderate increase in temperature.

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Paolo Sabbatini and James Flore

Crop load (CL) is a critical regulator of production and quality on apple. It affects leaf photosynthetic rate and usually an increase is detected in leaves close to developing sinks. The objective of this work was to test if 13C discrimination during photosynthesis could be an indicator of carbon sink limitation. The natural plant carbon isotope composition (13C/12C ratio: d13C) is an indicator of water use efficiency and it is an effective tool to study environmental stresses in plants. Seven-year-old trees of Imperial Gala/Bud 9 (n=30), field-grown at the Clarksville Horticultural Research Station in Michigan, were hand-thinned to five levels of CL manipulating the leaf to fruit ratio (LFR: 4, 8, 16, 32, 64) after June drop. Net photosynthetic rate (A) of leaves was monitored daily during the season and elevated rates were observed in low LFR. The A was inhibited in low CL trees (LFR 32 and 64) more in the afternoon (from 20% to 42% in relation to normal CL: LFR 16) than in the morning (from 5% to 20%), and stomatal conductance declined over the afternoon. Shoot and fruit growth were affected (fruit size –11/+11%, shoot length –13/+18% from normal CL; LFR 16). Variations of the stable carbon isotope composition of leaves show a significant reduction of 13C discrimination in low CL trees (–3.2%: d13C –25.82) and an increase of 13C discrimination (+2.7%: d13C –27.38) in relation to normal CL trees (LFR 15.63). The results were similar to those reported in 2004, which imply isotopic discrimination in relation to source limitation. This is the opposite of what you would expect under water stress conditions. Although trees were well-watered during the season, the effect of water stress on apple trees and its interaction with source limitation will be discussed.

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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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Shasha Wu, Youping Sun and Genhua Niu

To provide more species for landscapes where poor-quality irrigation water is used, salt tolerance of commonly used landscape plants should be characterized. Nine ornamental species, including six herbaceous and three woody, were irrigated with nutrient solution at electrical conductivity (EC) of 1.2 dS·m−1 (control) or saline solution at EC of 5.0 or 10.0 dS·m−1 (EC 5 or EC 10) for 8 weeks and their growth and physiological responses were determined. Although growth was reduced in orange peel jessamine (Cestrum ‘Orange Peel’) and mexican hummingbird bush (Dicliptera suberecta) as salinity increased, no obvious signs of stress or injury were observed, indicating that orange peel jessamine and mexican hummingbird bush were the most salt tolerant. Flame acanthus (Anisacanthus quadrifidus var. wrightii), rock rose (Pavonia lasiopetala), and ‘Dark knight’ bluebeard (Caryopteris ×clandonensis ‘Dark Knight’) had more growth reduction than that of orange peel jessamine and mexican hummingbird bush with minimal or no foliar damage in EC 5 and slight foliar damage in EC 10. Cardinal flower (Lobelia cardinalis) and mexican false heather (Cuphea hyssopifolia) exhibited mortality rates of 30% and 20%, severe foliar damage, and greater than 70% reduction in leaf area and dry weight in EC 10 compared with their respective controls. Although the growth reductions in butterfly blue (Scabiosa columbaria) were not as great as cardinal flower and mexican false heather, 40% of butterfly blue plants were dead with moderate foliar damage in EC 10. Therefore, cardinal flower, mexican false heather, and butterfly blue plants were considered as moderately salt sensitive. Eastern red columbine (Aquilegia canadensis) was the most salt sensitive among the species investigated with moderate foliar damage in EC 5 and all plants died in EC 10. Four out of the nine species tested had significant differences in net photosynthetic rate (Pn), stomatal conductance (g s), and/or relative chlorophyll content between the control and EC 10, and the difference varied with species. Shoot ion concentrations of the nine ornamentals were also affected by salinity levels and varied among species.

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J. Ryan Stewart, Reid D. Landes, Andrew K. Koeser and Andrea L. Pettay

drought treatment was 3.40 μmol·m −2 ·s −1 [95% confidence interval (CI): 2.07 to 4.72] less than that of plants in the control treatment ( P < 0.0001) ( Table 1 ). No differences in net photosynthetic rate were found, however, between the control and

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Min Wu and Chieri Kubota

. (2002) , the EC was enhanced by increasing the overall strength of nutrient solutions. Romero-Aranda et al. (2001) showed that the leaf net photosynthetic rate of tomato plants was reduced proportionally as NaCl concentration increased in the nutrient

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Lingyan Chen, Jinli Lai, Tianyou He, Jundong Rong, Muhammad Waqqas Khan Tarin and Yushan Zheng

chlorophyll is an important site for photosystem II (PSII) reactions. Insufficient photosynthetic membrane proteins lead to low net photosynthetic rates ( Bertamini and Nedunchezhian, 2003 ; Snider et al., 2015 ). The formation of the thylakoid system depends

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Wei-Ling Chen, Cheng-Hung Hsiao and Hsueh-Shih Lin

unaffected by high temperature. The plant net photosynthetic rates (Pn) under 25/20 °C and 35/30 °C were also not significantly different. In addition, the susceptibility of black rot disease, a major bacterial disease of radish caused by the Xanthomonas

Open access

Noriko Ohtake, Masaharu Ishikura, Hiroshi Suzuki, Wataru Yamori and Eiji Goto

( Yamori et al., 2012 ; Zhang et al., 2015 ). After 30 min of illumination to obtain steady-state photosynthesis, the net photosynthetic rate in the most newly expanded leaves of 31- to 35-d-old plants grown under W24 was measured under growth light