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Robert M. Augé, Ann J.W. Stodola, and Brian D. Pennell

The influence of irradiance and drought on osmotic and turgor adjustment was examined in leaves of rose (Rosa hybrida L. `Samantha'). Plants cultured under full ambient light in the greenhouse were placed in shade chambers and, after 2 weeks of acclimation, exposed to drought for 21 days. Treatments consisted of a water stress factor (well-watered and drought-stressed) and an irradiance factor (100%, 70%, and 30% of ambient irradiance). Pressure-volume analyses of leaves indicated that osmotic potentials at full turgor were decreased 0.42, 0.36, and 0.23 MPa by drought in the 100%, 70%, and 30% irradiance treatments, respectively. Plants stressed under 100% and 70% irradiance exhibited similar osmotic adjustments. Plants under 30% irradiance had higher osmotic potentials at full turgor under well-watered conditions than plants in the other two irradiance treatments and showed only 55% as much adjustment to drought. In each irradiance treatment, drought induced an increase in elastic modulus and a decrease in relative water content at zero turgor. Turgor pressures were higher across a range of relative water contents in plants in the two higher irradiance treatments under both soil moisture treatments. Turgor also was higher at any particular water potential at 100% and 70% irradiance than 30% irradiance, within each soil moisture treatment. Heavy, but not mild, shading inhibited osmotic and turgor adjustments in leaves during drought.

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H.C. Wien, A.D. Turner, and S.F. Yang

Abstract

A series of field and greenhouse experiments was conducted with three cultivars of bell pepper (Capsicum annuum L.) to determine the hormonal basis for flower bud and flower abscission as induced by low light intensity (LLI). Imposition of 80% shade for 6 days increased abscission of reproductive structures by 38% and resulted in an increase in bud ethylene production. Concomitantly, bud reducing sugars and sucrose decreased and these were negatively correlated with ethylene levels and those of its precursor, ACC. Infusion of ACC into the pedicel resulted in flower bud abscission within 48 hr. The results indicate that ethylene is the primary causal agent of pepper flower bud abscission. Production of auxin by the bud plays a role in prevention of abscission. The abscission of disbudded pedicels was prevented by infusion of NAA. Although the three cultivars had similar responses to ACC, they differed in the amount of abscission under stress, bud sugar levels, and the time of onset of ACC and ethylene production. Chemical names used: 1-aminocyclopropane-1-carboxylic acid (ACC); α-napthaleneacetic acid (NAA); (2-chloro-ethyl)phosphonic acid (ethephon).

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Christian M. Baldwin, Haibo Liu, Lambert B. McCarty, Hong Luo, and Joe E. Toler

Beard, 1981 ), because tree water transpiration is greatest at night, extending dew duration on turf ( Williams et al., 1996 , 1998 ). Management strategies to combat shade stress include raising the height of mowing ( Bunnell et al., 2005b ; White

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H.C. Wien, A.D. Turner, and R. Nyankanga

High temperatures during flowering frequently limit yields of some bell pepper cultivars in New York fields. Previous research has shown that subjecting the plants to low light at flowering can have similar effects. To determine if cultivar differences in flower abscission and yield could be accentuated by such a shade stress, field plots of six cultivars were subjected to 1 week of low light during flowering. Shade cloth tunnels were erected over the plant rows in two experiments, reducing incident light by 80%. Nondestructive abscission counts were taken at the start, and 7 days after the end of a 7-day shade period. Mature green fruit were harvested periodically. Low light stress resulted in 68% and 86% abscission at the first three fruiting nodes in 1992 and 1994, respectively. Cultivars showed differential abscission in unshaded plots, and after shade, producing a significant cultivar: shade interaction. `Ace' showed least abscission and maintained yields with shading; `Camelot' lost nearly all flowers and buds with low light stress, and was reduced by 75% and 91% in marketable yield in 1992 and 1994, respectively. Results indicate that shade stress accentuates abscission susceptibility in bell pepper cultivars. Pepper lines selected for low light tolerance may show promise in resisting flower abscission at high temperature.

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A.D. Turner and H.C. Wien

Cultivars of bell pepper differ in susceptibility to bud/flower abscission. Reduction in the level of assimilate, and alterations in assimilate partitioning may be involved in the processes leading to bud/flower abscission. Four growth analysis experiments were conducted to determine whether two pepper cultivars differing in susceptibility to stress-induced abscission showed corresponding differences in growth and rates and dry matter partitioning when subjected to shade stress. The reduction in RGR and NAR with shading was significantly greater for the abscission-susceptible `Shamrock' than the more tolerant `Ace'. Partitioning of dry matter to reproductive structures was reduced by shading. There were no cultivar differences in the proportion of dry matter partitioned to young developing leaves. Fully expanded leaves comprised a larger proportion of total dry matter in `Shamrock'. The lower NAR of `Shamrock' under stress may have led to greater bud/flower abscission than `Ace' under shade stress. If preferential partitioning of dry matter to competing structures (developing leaves) is also involved, it was not detected using this technique.

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Kenton W. Peterson, Jack D. Fry, and Dale J. Bremer

Shade stress is a problem affecting the quality of an estimated 20% to 25% of all turfs ( Beard, 1973 ; Dudeck and Peacock, 1992 ). Shading reduces incident solar radiation and alters the microclimate in which the turf grows ( Beard, 1997

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Ze Li, Kai Shi, Fanhang Zhang, Lin Zhang, Hongxu Long, Yanling Zeng, Zhiming Liu, Genhua Niu, and Xiaofeng Tan

). Light is a major environmental factor that affects leaf photosynthesis, traits, and plant growth, and determines the geographic distribution of plants ( Kim et al., 2011 ). Plants experiencing shade stress often exhibit serious dysfunctions in terms of

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Edward J. Nangle, David S. Gardner, James D. Metzger, John R. Street, and T. Karl Danneberger

irrigated in at a rate of 2.5 g·m −2 during plot establishment to improve bentgrass growth and vigor before shade stress initiation in 2006. Other nutrients [phosphorus (P) and potassium (K)] were applied to prevent any deficiencies in both years based on

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Ben Wherley, Ambika Chandra, Anthony Genovesi, Mason Kearns, Tim Pepper, and Jim Thomas

determination of carbohydrates with Dreywood’s anthrone reagent Science 107 254 255 Richardson, M.D. Karcher, D.E. Purcell, L.C. 2001 Quantifying turfgrass cover using digital image analysis Crop Sci. 41 1884 1888 Stier, J.C. Gardner, D.S. 2008 Shade stress and

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B.G. Wherley, P. Skulkaew, A. Chandra, A.D. Genovesi, and M.C. Engelke

tolerance and non-structural carbohydrates of shaded supina bluegrass Acta Hort. 661 207 215 Stier, J.C. Gardner, D.S. 2008 Shade stress and management 463 Pessarakli M. Turfgrass management and physiology CRC Press Boca Raton, FL Tan, Z.G. Qian, Y.L. 2003