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Mahalaxmi Veerasamy, Yali He, and Bingru Huang

Temperatures greater than the optimal growth temperature negatively affect plant growth and induce various physiological and metabolic changes, including premature leaf senescence. Leaf senescence is characterized by loss of chlorophyll and

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Christopher J. Currey, Roberto G. Lopez, Vijay K. Rapaka, James E. Faust, and Erik S. Runkle

increased respiration, reduced carbohydrate concentration, and increased ethylene generation in geranium cuttings, which caused lower-leaf senescence during propagation ( Mutui et al., 2005 ; Rapaka et al., 2008 ). Loss of lower leaves during propagation

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Yan Xu and Bingru Huang

High temperature is a major factor limiting growth of cool-season plant species. One of the typical symptoms of heat injury for many plant species is leaf senescence ( Thomas and Stoddart, 1980 ). Leaf senescence is characterized by loss of

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Jinpeng Xing, Yan Xu, Jiang Tian, Thomas Gianfagna, and Bingru Huang

Naturally occurring and environmentally induced leaf senescence limits whole-plant photosynthetic capacity and plant productivity, as well as the aesthetic value of horticultural plants. Leaf senescence is characterized by yellowing or chlorosis

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Yan Zhang, Cuiyue Liang, Yan Xu, Thomas Gianfagna, and Bingru Huang

Leaf senescence is a natural process of leaf maturation, but it can be induced prematurely by environmental stresses. Cytokinins (CKs) have been well known for their function in delaying leaf senescence, and even in reversing the process of leaf

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Rosanne E. Franco and Susan S. Han

Senescence of lower leaves of Easter lilies (Lilium longiflorum Thunb.) was previously shown to be delayed with application of the growth regulators, gibberellic acid (GA3) and benzyladenine (BA). This study was done to determine the physiological effects of GA3 and BA in relation to the delay of leaf senescence. Foliar application with 500 ppm BA or GA3 delayed chlorosis and lowered respiration rate in Easter lily leaves. A combination of 500 ppm BA and 500 ppm GA3 was more effective than the individual application of each. Gibberellic acid, BA, or their combination before cold storage resulted in delayed chlorosis and lowered respiration following removal from cold storage. Treatment with growth regulators after cold storage was less effective. Senescence of leaves was not associated with ethylene since ethylene production by leaves was undetectable by gas chromatograph. In addition, pulsing or continuous treatment with silver thiosulphate (STS), an inhibitor of ethylene synthesis, did not delay foliar chlorosis. Analysis of carbohydrate levels in Easter lily leaves treated with GA3, BA, or their combination may contribute to the understanding of the physiological effects of these two growth regulators.

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Emily B. Merewitz, Thomas Gianfagna, and Bingru Huang

Drought is a detrimental abiotic stress for plant growth, including perennial turfgrass species. A typical drought stress symptom in turfgrass is a decline in turf quality (TQ) resulting from leaf senescence, slow shoot and root growth, and leaf

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David G. Clark, Chris Dervinis, Francine Cuquel, Harry Klee, Jim Barrett, and Terril Nell

In an effort to modify and study leaf senescence, we have produced several different transgenic petunia lines with altered leaf senescence phenotypes. Using two promoters from senescence-associated genes (sag12 & sag13) fused to the isopentenyl transferase (IPT) gene, which catalyzes the rate-limiting step of cytokinin production, we have produced transgenic petunia plants with delayed lower leaf senescence. We have observed that apparent “leaky” expression of IPT gives rise to plants with other morphological alterations such as increased branching habit and decreased root formation. Plants with delayed leaf senescence phenotypes were selected and bred to produce progeny that were evaluated in greenhouse experiments. Breeding characteristics, horticultural performance and reproduction of these plants will be discussed in terms of potential commercial benefits and limitations. Using the sag12 promoter to drive expression of the knotted (KN1) gene, we have also been able to engineer petunia plants with delayed lower leaf senescence. Initial progeny evaluations of sag12-KN1 petunias will also be discussed.

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David G. Clark, Chris Dervinis, James E. Barrett, Harry Klee, and Michelle Jones

Cytokinins have been shown to delay the onset of leaf senescence. The focus of this project was to produce transgenic petunia (Petunia ×hybrida) plants that over-produced endogenous cytokinins in a senescence specific manner. This was achieved by transforming plants with the IPT (isopentenyl transferase) gene driven by the senescence-associated transcriptional promoter, PSAG12. Two independent transgenic events produced T1 and T2 generation seedling lines that demonstrated the desired nonsenescent phenotype in progeny trials. These lines were used to evaluate the horticultural performance of PSAG12-IPT petunia plants in terms of delayed senescence, rooting of vegetative cuttings, lateral branch growth, flower number, floral timing, and fruit set. Although both lines displayed a delayed senescence phenotype the two PSAG12-IPT transgenic lines differed from each other in regard to other horticultural traits. In addition to delayed leaf senescence, line I-1-7 also demonstrated a decrease in adventitious rooting and an increased number of branches during plant production. Line I-3-18 also demonstrated a delayed leaf senescence phenotype; however, plants of this line were not greatly altered in any other horticultural performance traits in comparison to wild-type `V26'. IPT transcript was detected in young fully expanded leaves of both lines, although mRNA levels were higher in I-1-7 plants. A greater than 50-fold increase in IPT transcript abundance was detected in leaves of transgenic plants following drought stress. These results demonstrate that it is possible to use PSAG12-IPT to produce transgenic plants with delayed leaf senescence but differences in plant morphology between transgenic lines exist, which may alter horticultural performance characteristics.

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Mustafa Özgen, Sookhee Park, and Jiwan P. Palta

Mitigation of ethylene promoted leaf senescence by lysophosphatidylethanolamine (LPE) was studied. Micropropagated `Russet Burbank' potato (Solanum tuberosum L.,) plantlets were grown on MS media in sterile culture tubes. After 2 weeks of growth, tubes were sealed and ethylene gas was applied to obtain 5 nL·L–1 final concentration in the culture tubes. Observations and measurements were taken two weeks after ethylene injection. Potato plantlets treated with ethylene showed severe leaf senescence symptoms such as epinasty, lack of growth, yellowing and axillary shoot formation. These observations indicate that apical dominance has been lost with ethylene treatment. The same experiment was repeated with different concentrations of LPE in the MS medium. Inclusion of 50 or 100 mg·L–1 of LPE in the medium mitigated the damage normally caused by applied ethylene. Leaves of plantlets exposed simultaneously to LPE and ethylene had significantly higher chlorophyll content and more healthy leaves compared to plantlets grown on medium lacking LPE. Results of this study suggest that LPE may have the potential to retard ethylene-promoted leaf senescence and may mitigate ethylene induced loss in apical dominance of micropropagated potato plantlets.