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  • Author or Editor: Nihal C. Rajapakse x
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The interest in using nonchemical alternatives for growth control of horticultural crops has recently increased due to public concerns for food safety and environmental pollution. Several research teams around the world are investigating alternative growth control measures, such as genetic manipulation, temperature, water and nutrient management, mechanical conditioning, and light quality manipulation. This review discusses the recent developments in light quality manipulation as a nonchemical alternative for greenhouse plant height control.

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Growth chambers constructed of photoselective plastic films were used to investigate light quality effects on flowering and stem elongation of six flowering plant species under strongly inductive and weakly inductive photoperiods. Three films were used: a clear control film, a far red (FR) light absorbing (AFR) film and a red (R) light absorbing (AR) film. The AFR and AR films intercepted FR (700 to 800 nm) and R (600 to 700 nm) wavelengths with maximum interception at 730 and 690 nm, respectively. The phytochrome photoequilibrium estimates of transmitted light for the control, AFR, and AR films were 0.71, 0.77, and 0.67. The broad band R:FR ratios were 1.05, 1.51, and 0.77, respectively. The photosynthetic photon flux was adjusted with neutral density filters to provide similar light transmission among chambers. Zinnia elegans Jacq., Dendranthema×grandiflorum Kitam. (chrysanthemum), Cosmos bipinnatus Cav., and Petunia×hybrida Vilm.-Andr. plants grown under the AFR film were shorter than control plants. The AFR film had no effect on height of Antirrhinum majus L. (snapdragon) or Rosa×hybrida (miniature rose). Anthesis of zinnia, chrysanthemum, cosmos (short-day plants), and miniature rose (day-neutral plant) was not influenced by the AFR films. Anthesis of petunia and snapdragon (long-day plants) was delayed up to 13 days by AFR films under weakly inductive photoperiods. In petunia, initiation and development of floral structures were not affected by the AFR films during strongly inductive photoperiods. However, during weakly inductive photoperiods, initiation of the floral primordia was significantly delayed and overall development of the floral meristem was slower than control plants indicating that the AFR films could increase the production time if long-day plants were produced off-season. Daylength extension with electric light sources could overcome this delay in anthesis yet achieve the benefit of AFR films for height reduction without the use of chemical growth regulators.

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Steady-state oxygen diffusion in flesh of apples (Malus domestics Borkh. cvs. Braeburn and Cox's Orange Pippin), Asian pears (Pyrus serotina Rehder. cvs. Hosui and Kosui), and nectarines [Prunus persica (L.) Batsch. cvs. Red Gold and Sunglo] was studied using a nondestructive method at 20C. Fruit flesh was found to exert a significant resistance to O2 diffusion resulting in measurable O2 gradients between tissues immediately beneath the skin and those at the fruit center for all these fruits. The magnitude of these O2 gradients varied between crops and cultivars and depended on the respiration rate and on effective O2 diffusivity in fruit flesh (De). Values of Dc varied with the cultivar and were broadly consistent with intercellular space volume. The range of De values obtained suggested that 02 diffusion in fruit flesh takes place in a combination of series and parallel modes in the intercellular space and fluid/solid matrix of the flesh. The results imply that O2 diffusivity in flesh tissues must be taken into consideration in the determination of critical external O2 level in controlled/modified atmosphere (CA/MA) storage.

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Responses to selected chemical growth retardants (daminozide, paclobutrazol, and prohexadione-Ca) and GA1 and GA3 under photoselective greenhouse covers with various phytochrome photoequilibrium estimates (φe) were evaluated using `Bright Golden Anne' chrysanthemum [Dendranthema ×grandiflora Kitam. (syn. Chrysanthemum morifolium Ramat.)] as the model plant to better understand the height control mechanism by far red (FR) light depleted environments. Plant height linearly decreased as φe increased from 0.72 to 0.83. The rate of height decrease of daminozide treated plants was less than that of water (control) or GA3-treated plants. The rate of height reduction was not different between control and GA3-treated plants among chambers with various φe. Both paclobutrazol and prohexadione-Ca reduced plant height regardless of φe, but the height reduction by paclobutrazol was more than that by prohexadioneCa. The combination of paclobutrazol and prohexadione-Ca reduced plant height more than either alone. GA1 reversed the height reduction caused by paclobutrazol and prohexadione-Ca regardless of φe, but the height increase by GA1 was more when it was applied with prohexadione-Ca than when applied alone. Results show that photoselective covers with high φe were effective in controlling height of chrysanthemums without chemical growth retardants. The linear relationship between plant height and φe suggests that effectiveness of photoselective covers increased as φe increased. The photosynthetic photon flux (PPF) transmission of photoselective covers decreased as the φe increased because of the increasing dye concentration. Identifying photoselective covers that effectively filter out FR light from sunlight and reduce plant height while minimizing the PPF reduction is critical for commercial success of photoselective covers. Gibberellins are, at least partially, involved in height control by photoselective covers. Photoselective greenhouse covers did not reduce responsiveness to gibberellins, and it appears that the mechanism may be to suppress gibberellin biosynthesis. Results also suggest that increased metabolism of GA1 to GA8 was not the mechanism of height control by photoselective covers. Chemical names used: butanedioic acid mono (2,2-dimethylhydrazide) [daminozide]; (±)-(R*,R*)-b-((4-chlorophenyl)methyl)-a-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol [paclobutrazol]; 3,5-dioxo-4-(1-oxopropyl)cyclohexanecarboxylic acid [prohexadione-Ca]; gibberellic acid [GA].

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