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Bingru Huang and Hongwen Gao

Drought is among the most limiting factors for turfgrass growth. Understanding genetic variations and physiological mechanisms in turfgrass drought resistance would facilitate breeding and management programs to improve drought resistance. The experiment was designed to investigate shoot physiological responses of six tall fescue (Festuca arundinacea Schreb.) cultivars representing several generations of turfgrass improvement to drought stress. Grasses were grown in well-watered or drying (nonirrigated) soil for 35 days in the greenhouse. Net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), relative water content (RWC), and photochemical efficiency (Fv/Fm) declined during drought progression in all cultivars, but the time and the severity of reductions varied with cultivar and physiological factors. The values of Pn, RWC, gs, and Tr decreased significantly for `Rebel Jr', `Bonsai', and `Phoenix' when soil water content declined to 20% after 9 days of treatment (DOT) and for `Houndog V', `Kentucky-31', and `Falcon II' when soil water content dropped to 10% at 15 DOT. A significant decrease in Fv/Fm was not observed in drought-stressed plants until 21 DOT for `Rebel Jr', `Bonsai', and `Phoenix' and 28 DOT for `Houndog V', `Kentucky-31', and `Falcon II'. The decline in Pn resulted mainly from internal water deficit and stomatal closure under mild drought-stress conditions. After a prolonged period of drought stress (35 DOT), `Falcon II', `Houndog V', and `Kentucky-31' maintained higher Pn than did `Rebel Jr', `Bonsai', and `Phoenix', which could be attributed to their higher Fv/Fm. This study demonstrated variation in drought resistance among tall fescue cultivars, which was related to their differential responses in photosynthetic capacity and water relations.

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Sung Kyeom Kim, Duk Jun Yu, Ro Na Bae, Hee Jae Lee and Changhoo Chun

Grafted transplants are widely used for watermelon culture in Korea mainly to reduce the yield and quality losses caused by soil-borne diseases. It is normal practice to cure the grafted transplants under high relative humidity (RH) and low photosynthetic photon flux (PPF) conditions for a few days after grafting to prevent the wilting of the transplants. Transpiration rate (TR) and net photosynthetic rate (NPR), however, could be suppressed under those environmental conditions. In the present study, TR and NPR of the grafted watermelon transplants were compared during graft union formation under 18 environmental conditions combining two air temperatures (20 and 28 °C), three RHs (60%, 80%, and 100%), and three PPF s (0, 100, and 200 μmol·m-2·s-1). Percentages of graft union formation and survival were also evaluated. TR and NPR dramatically decreased just after grafting but slowly recovered 2 to 3 days after grafting at 28 °C. The recovery was clearer at higher PPF and lower RH. On the other hand, the recovery of TR and NPR was not observed in 7 days after grafting at 20 °C. Differences in TR and NPR affected by RH were nonsignificant. Percentage of graft union formation was 98% when air temperature, RH, and PPF were 28 °C, 100%, and 100 μmol·m-2·s-1, respectively, which was the highest among all the treatments. Percentage of survival was over 90% when air temperature was 28 °C and RH was higher than 80% (when vapor pressure deficit was lower than 0.76 kPa). In addition, higher PPF enhanced TR and NPR and promoted rooting and subsequent growth of grafted transplants. Results suggest that the acclimation process for grafted watermelon transplants can be omitted by properly manipulating environmental factors during graft union formation.

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Yiwei Jiang and Bingru Huang

Heat and drought are two major factors limiting growth of cool-season grasses during summer. The objective of this study was to compare the effects of heat stress alone (H) or in combination with drought (H+D) on photosynthesis, water relations, and root growth of tall fescue (Festuca arundinacea L.) vs. perennial ryegrass (Lolium perenne L.). Grasses were exposed to H (35 °C day/30 °C night) or H+D (induced by withholding irrigation) in growth chambers for 35 days. Soil water content declined under H+D for both grasses but to a greater extent for fescue than for ryegrass. Declines in canopy net photosynthetic rate (Pn), leaf photochemical efficiency (Fv/Fm), and leaf relative water content (RWC) and the increase in electrolyte leakage (EL) were much more severe and occurred earlier for ryegrass than fescue subjected to both H and H+D and for both species than under H+D then H. Evapotranspiration (ET) rate increased to above the control level within 3 or 6 days of H and H+D for both species, but fescue had a higher ET rate than ryegrass at 3 and 6 days of H and 6 days of H+D. Root dry weight and viability in all soil layers decreased under H and H+D for both species. However, fescue had higher root dry weight and viability than ryegrass in the 20-40 cm layer under H and in both the 0-20 and 20-40 cm layers under H+D. The results indicated that maintenance of higher Pn, Fv/Fm, ET, RWC, and root growth and lower EL would help cool-season turfgrass survive summer stress, and that their characteristics could be used for selecting stress tolerant species or cultivars.

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C. Yang, D.Y. Jiao, Z.Q. Cai, H.D. Gong and G.Y. Li

Plukenetia volubilis Linneo, a tropical recurrent woody oilseed plant native to South America, was successfully introduced in China. A field experiment was conducted to determine the effect of the dry-season foliar sprays once every 2 weeks with 50 μm water or five different plant growth regulators (PGRs) viz., gibberellic acid (GA3), kinetin (KIN), indole-3-acetic acid (IAA), abscisic acid (ABA), and salicylic acid (SA) on the growth and yield of P. volubilis plants in Xishuangbanna, southwest China. Results showed that PGRs affected the leaf stomatal conductance (g S) and water-use efficiency (WUEi), rather than the net photosynthetic rate (PN). The phenological development of P. volubilis plants, including the time of flowering and maturity, and the dynamic pattern of fruit ripening, was not altered by PGR treatments. ABA and SA resulted in highest fruit set, seed oil content, and total fruit or seed oil yield, whereas GA3, IAA, and KIN were effective in increasing seed size. The nonstructural carbohydrates (NSC) are related to subsequent abscission or retention of the developing fruit, which was indicated by the positive relationship between carbohydrate concentration and fruit set across PGR treatments. The positive influences of PGRs on the total fruit yield (increased 4.3% to 15.2%) and total seed oil yield (increased 4.9% to 24.9%) per unit area throughout a growing season were found when compared with the control, depending to a great extent on the balance between vegetative and reproductive growth during the reproductive stage. This study suggests that PGRs, especially for ABA and SA, can become a valuable tool for promoting the seed oil yield of P. volubilis plants while maintaining high seed quality in the field.

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Lili Zhuang, Mengxian Liu, Xiuyun Yuan, Zhimin Yang and Bingru Huang

Aquaporin (AQP) proteins serve important roles in regulating water movement across cellular membranes and affect plant responses to drought stress. The objective of this study was to characterize and examine functions of an AQP gene FaPIP2;1, isolated from a drought-tolerant perennial grass species tall fescue (Festuca arundinacea), for involvement in leaf dehydration status during water stress by overexpressing the gene in arabidopsis (Arabidopsis thaliana). FaPIP2;1 had characteristic transmembrane domains and Asn–Pro–Ala motifs and was similar to PIP2;1 in rice (Oryza sativa) and maize (Zea mays). Quantitative real-time reverse transcriptase polymerase chain reaction analysis showed that FaPIP2;1 was upregulated during moderate water stress (hydroponic culture, osmotic potential (ΨS) at −0.47 and −0.78 MPa) and the transcript level decreased as ΨS further decreased. Transgenic arabidopsis plants overexpressing FaPIP2;1 showed greater number of leaves per plant and improved survival rate compared with the wild type (WT) during drought stress. Transgenic plants also maintained higher leaf relative water content (RWC), chlorophyll content (Chl), net photosynthetic rate (Pn), and lower leaf electrolyte leakage (EL) than the WT. However, there was no difference in root length between the transgenic and WT plants following drought stress. The results demonstrated that overexpressing FaPIP2;1 could improve plant tolerance to drought stress by enhancing leaf water status, Chl, and photosynthetic rate, as well as maintaining improved cellular membrane stability relative to the WT plants. FaPIP2;1 may be used as a candidate gene for genetic modification of perennial grasses to develop new drought-tolerant germplasm and cultivars.

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John Pote, Zhaolong Wang and Bingru Huang

Knowledge of the level of soil temperatures that is detrimental for shoot and root growth for cool-season grasses may help develop heat-tolerant plants and effective management practices to improve summer performance. The objectives of this study were to determine the level and duration of high temperatures in the root zone that will induce decline for various growth and physiological parameters and to compare the responses of different physiological parameters and cultivars to high root-zone temperatures. Nine creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw.] cultivars were subjected to eight root-zone temperatures (20, 21, 22, 23, 25, 27, 31, 35 °C) in water baths while exposed to a constant air temperature of 20 °C for 54 days. Root number, dry weight, and depth, active root biomass, turf quality, leaf cytokinin content, and canopy net photosynthetic rate (Pn), decreased in all nine cultivars as root-zone temperature increased from 20 to 35 °C, but the time and temperature at which the decline occurred varied for each parameter measured. Pn, cytokinin content, root number, and turf quality declined at 23, 27, 27, and 35 °C, respectively, after 28 days of exposure. Active root biomass, root number, root dry weight, turf quality, and rooting depth declined at 23, 25, 25, 25, and 35 °C, respectively, at 54 days. At a 31 °C root-zone temperature the decline in root number, cytokinin content, and turf quality occurred at 19, 37, and 47 days, respectively. The results suggest that root-zone temperatures of 23 °C or above this level were detrimental to root activities, Pn, and overall turf growth. Root and Pn decline at lower temperatures and earlier in the study than turf quality suggest that the disturbance of physiological activities of roots and leaves could lead to turfgrass quality decline at high root-zone temperatures.

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Cheryl R. Hampson, Anita N. Azarenko and John R. Potter

In hazelnut (Corylus avellana L.), vigorous vegetative growth and traditional orchard practices that include little or no pruning combine to produce a dense, shady canopy. A study designed to quantify the effect of shade on reproduction and photosynthetic rate in this shade-tolerant species was undertaken to assess whether some degree of pruning might improve productivity. Shade cloth was used to exclude 30%, 47%, 63%, 73%, or 92% of ambient sunlight from whole `Ennis' and `Barcelona' trees from mid-May until harvest. Photosynthetic light response curves were obtained for leaves that had developed in full sunlight, deep inside the canopy of unshaded trees, or in 92% shade. Light-saturated net photosynthetic rates were 12.0, 6.1, and 9.3 μmol·m-2·s-1 of CO2 and dark respiration rates were 2.0, 1.1, and 0.7 μmol·m-2·s-1 of CO2, respectively, for the three light regimes. Light-saturated photosynthetic rates of leaves from 30% or 63% shade differed little from the control (0% shade). Area per leaf increased by 49% and chlorophyll concentration (dry weight basis) by 157% as shading increased from 0% to 92%. Shading to 92% reduced specific leaf weight (68%), stomatal density (30%), light compensation point (69%), and dark respiration rate (63%) compared to controls. Female inflorescence density declined by about one-third and male inflorescence density by 64% to 74% in the most heavily shaded trees of both cultivars compared to controls. Shade was more detrimental to yield than flowering: yield per tree dropped by >80%, from 2.9 to 3.4 kg in full sun to 0.6 to 0.9 kg in 92% shade. Shade reduced yield primarily by decreasing nut number and secondarily by decreasing nut size. The incidence of several kernel defects increased as shade increased. Therefore, hazelnut leaves showed considerable capacity to adapt structurally and functionally to shade, but improving light penetration into the canopy would probably increase orchard productivity.

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Chieri Kubota, Makiko Ezawa, Toyoki Kozai and Sandra B. Wilson

The effects of initial sucrose (suc) concentrations in the medium (S0) on the carbon balance and growth of sweetpotato [Ipomoea batatas (L.) Lam. `Beniazuma'] and tomato (Lycopersicon esculentum Mill. `HanaQueen') plantlets were studied under controlled environmental conditions. Plantlets were cultured with 0, 7.5, 15, or 30 g·L-1 of S0 under high photosynthetic photon flux (160 to 200 μmol·m-2·s-1) and CO2 enriched (1400 to 2050 μmol·mol-1) conditions. Net photosynthetic rate per leaf area (Pl) decreased and dry weight per plantlet (Wd) increased with increasing S0, but did not differ significantly between S0 of 7.5 to 30 g·L-1 for sweetpotato or 15 to 30 g·L-1 for tomato. Carbon influxes and effluxes of the plantlets by metabolism of medium suc and/or photosynthesis, and respiration were estimated based on measurements of in situ and steady state CO2 exchange rates and sugar uptake during culture. At S0 from 7.5 to 30 g·L-1, photosynthesis was responsible for 82% to 92% and 60% to 67% of carbohydrate assimilation for sweetpotato and tomato, respectively. Estimated carbon balances of plantlets based on the estimated and actual increases of moles of carbon in plant tissue demonstrated that in situ estimation of carbon balance was reasonably accurate for sweetpotato at S0 of 0 to 15 g·L-1 and for tomato at S0 of 0 g·L-1 and that the actual contribution of photosynthesis for tomato at high S0 might be lower than the values estimated in the present experiment. Results showed that initial suc concentration affected the relative contribution of photosynthesis on their carbon balances and that the responses were species specific. The failure of validation at S0 in a range specific to each species suggested the need for further study on carbon metabolism of in vitro plantlets cultured with sugar in the medium.

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Rui Zhou and Bruno Quebedeaux

Photosynthesis and carbohydrate metabolism in apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] source leaves were monitored during a 7-day period after source-sink manipulations by girdling or partial defoliation treatments. In the girdling treatment, sorbitol, sucrose, glucose, and starch accumulated in leaves, and net photosynthetic rates (Pn) at 350 μL·L-1 CO2 decreased during a 7-day period. Pn measured at 1000 μL·L-1 [CO2] was also decreased but the changes were less. Stomatal conductance and intracellular CO2 concentration decreased markedly in leaves of girdled shoots. When shoots were partially defoliated, starch and glucose concentrations in remaining source leaves declined steadily during the 7-day study period. Sorbitol and sucrose concentrations decreased during the first 2 days after defoliation, then increased the following 5 days. Pn of the remaining leaves measured at ambient and elevated CO2 levels were enhanced markedly. Aldose-6-phosphate reductase activity in source leaves increased markedly from 27.5 to 39.2 μmol·h-1·g-1 fresh weight (FW) after partial defoliation but remained unchanged in leaves after girdling. Selective and maximum sucrose phosphate synthase (SPS) activities increased following partial defoliation and decreased following girdling. ADP-glucose pyrophosphorylase activity remained relatively unchanged in the partial defoliation treatments but increased markedly in the girdled-shoot leaves. These results suggested that girdling-induced photosynthetic inhibition is mainly due to stomatal limitation, however, the photosynthesis enhancement by partial defoliation may be due primarily to acceleration of photosynthetic capacity per se. These studies showed that the metabolism of sorbitol, sucrose and starch, three photosynthetic end products in mature apple leaves, was coordinately regulated in source leaves in response to source-sink manipulations.

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Xuan-Chun Piao, Debasis Chakrabarty, Eun-Joo Hahn and Kee-Yoeup Paek

In vitro nodal cuttings of potato (Solanum tuberosum L.) `Atlantic' and `Russet Burbank' from bioreactor culture were hydroponically cultured for 28 days using a deep flow technique (DFT) system. The response of plant growth and photosynthesis to different levels of solution electrical conductivity (EC; 0.08, 0.15, 0.22 and 0.36 S·cm-1) and pH (3, 4, 5, 6 and 7) were studied. The best growth, characters of shoot length, total shoot and root fresh and dry weight, were obtained in nutrient solution of pH 6.0 and EC 0.15 S·cm-1 for `Atlantic', while pH 7.0 and EC 0.15 S·cm-1 were found to be best for `Russet Burbank'. Plantlet growth was reduced by low solution pH (3.0) and high EC level (0.36 S·cm-1). Photosynthetic rate, stomatal conductance, and transpiration rate were also found to be affected by EC levels. Down regulation of photosynthesis, as indicated by chlorophyll fluorescence results, were observed when potato plantlets were cultured under nutrient solution of higher EC level. Plantlet growth and photosynthetic rate increased as photosynthetic photon flux (PPF) levels increased from 50 to 250 μmol·m-2·s-1. Particularly, increasing PPF level had a more distinctive effect on plantlet growth than CO2 enrichment condition. It was apparent from this study that nutrient solution of pH 6.0 and 0.15 S·cm-1 EC in combination with high PPF level (250 μmol·m-2·s-1) were suitable for hydroponic culture of potato plantlets as it would maximize net photosynthetic rate, and achieve the highest growth rates.