Further Studies on the Relationship between Growth Regulators and Tuberization of Dahlias

Abscisic acid (ABA) and gibberellin (GA) treatments, applied to budless leaf cuttings of Dahlia under long day conditions, did not increase the total weight of the cuttings but did affect the region where photosynthates accumulate. GA treatment inhibited the growth of tubers and roots, and enhanced thickening at the petiole base. ABA did not affect the weight of roots or petiole, but promoted the growth of tuberous roots. Anatomical examination of the tuberization revealed that it is caused by inter-fascicular cambial activity producing parenchyma cells inward. No anatomical difference could be observed between the GA-induced thickening at the petiole base and the natural or ABA-induced thickening of the roots. In single node cuttings, taken from various sites along the branch, tuberization ability increased with proximity to the base of the branch, where axillary buds are more inhibited. Short-day conditions which promote tuberization, increased the endogenous level of ABA-like inhibitors in intact plants. External conditions and growth substances which inhibit growth also inhibited tuberization. Benzyladenine treatments did not affect tuberization. It is suggested that in addition to their respective indirect effects via the enhancement or suppression of top growth, GA and ABA directly control tuberization. ABA levels which increased under short-day conditions seem to produce a sink in the roots. GA probably inhibits tuberization by interfering with the trans-location of photosynthates to the root area.     

Factors Determining Petal Colour of Baccara Roses II: THE EFFECT OF PIGMENT CONCENTRATION

‘Blueing’ in young and senescing petals was comparedin the red rose cv. Baccara. The ‘blueing’ of senescingflowers is accompanied by a bathochromic shift in the lightreflectance curve, a rise in the pH value and a decrease inthe malic acid concentration of the petal tissue. These factorsindicate that a complex with a co-pigment is produced. Similarchanges were not found in the ‘blueing’ of youngflowers, where a decrease was found in pigment concentrationper unit weight as well as per unit area of petal. A similar‘blueing’ was achieved by diluting a solution ofcrystalline cyanin. The phenomenon of ‘blueing’by dilution is discussed in the light of Bougeur's law.     

Factors determining Petal Colour of Baccara Roses I: THE CONTRIBUTION OF EPIDERMIS AND MESOPHYLL

The partition of light radiated on to the outer epidermis ofa Baccara rose petal or on to an intact petal was examined.Most of the red light was either reflected or transmitted whereasother wavelengths and especially the green range were absorbed.When the total amount of light transmitted (epidermis) or reflected(intact petal) increased, a rise in the blue range was recordedand the colour of the petal, determined objectively by CIE orMunsell's method, became more purple. Examination of the partition of light in the different layersof the petal revealed that light reflected from the outer epidermisis made up of two parts; one part is reflected directly andthe other part is first transmitted through the epidermis, reachesthe mesophyll, is reflected from it and is then transmittedthrough the epidermis. This latter part causes a shift in colourfrom purple to red. Colour differences between different petals on one flower anddifferent parts of the same petal were defined objectively.The change from red to purple colour was connected with vigorousgrowth of either the petal or epidermal cells, respectively. The contribution of the mesophyll in changing the reflectancecurve of petals is explained and it is suggested that althoughthe mesophyll is colourless, it contributes to a great extentto the changes occurring in petal colour.     

Effects of Varying Light Intensities and Temperature Treatments Applied to Whole Plants, or Locally to Leaves or Flower Buds, on Growth and Pigmentation of 'Baccara' Roses

Varying light intensity and temperature treatments were applied to whole plants, or to the leaves, or to the flower buds of ‘Baccara’ roses. The effect of these treatments on flower dimensions and pigmentation of the petals was examined. Cooling only the leaves had no effect; cooling only the buds enhanced both bud weight and pigmentation, but the effect was less marked than when the whole plant was cooled. Reducing plant temperature by misting with desalinated water enhanced both pigmentation and flower size. Darkening of only the leaves, or their removal, resulted in an inhibition of the pigmentation and also in a decrease in bud weight. Darkening of only the flower bud did not affect either pigmentation or bud weight, but caused bud elongation. It is suggested that light intensity and temperature affect flower growth and pigmentation via their effects on the availability of sugars in the flower bud.     

Effects of Short-Term Heat and Shade Treatments on Petal Colour of 'Baccara' Roses

Growth and pigmentation of ‘Baccara’ rose buds were followed. Petal elongation mainly occurred during a short period of time after the flowering shoot had stopped elongating. Approximately 90% of the flower pigments were synthesized during a short period lasting from the time the bud was 75% to the time it was 100% of its diameter at opening time. Petal growth continued after this period so that the pigments in the petal were “diluted”. Heat stress applied to whole plants or to flower buds only, or a short duration of low light intensities caused “blueing” of the petals when these conditions prevailed during the period of maximum pigmentation. Stress conditions prior to this period or following it, had no effect. We suggest that stress conditions such as high temperature or low light intensities reduce the availability of sugars. When this occurs at the time of maximum pigment production, a fall in the level of pigment occurs, which results in the “blueing” of the petals.     

Factors determining Petal Colour of Baccara Roses III: EFFECT OF THE RATIO BETWEEN CYANIN AND PELARGONIN

The changes in colour and in the pigment concentration of thetwo sides of Baccara rose petals which occur when plants aregrown under various temperature regimes, were examined. Theinner side of the petal is redder and the predominant pigmentis pelargonin whereas the outer petal surface tends to ‘blue’,and, the predominant pigment on this side is cyanin. The cyanin:pelargonin ratio on the outer side of petals increased three-foldunder the influence of low temperatures. The outer surface of petals growing for a long period underlow tempertaures was ‘blue’ when compared with thered petals which had been subjected to low temperatures fora short period. The cyanin: pelargonin ratio of ‘blue’petals was higher than that of red petals. Total pigment contentwas similar in both types of petal. Flowers grown under hightemperatures ‘blued’ without a concomitant fallin the cyaninpel: argonin ratio. Examination of colour solutions in which the ratio between cyaninand pelargonin was varied revealed that the colour of the solutionbecame bluer as this ratio increased. We suggest that the ‘blueing’ of Baccara rose petalsis caused primarily by a dilution of the cyanin content, butwhen the ratio between cyaniri and pelargonin increases sharply,‘blueing’ may also occur in dark flowers in whichthe total pigment content did not diminish.     

The Relationship between Exogenous Growth Inhibitors and Endogenous Levels of Ethylene, and Tuberization of Dahlias

Treatments with abscisic acid (ABA), succinic acid-2,2-dimethylhydrazide (SADH), or 2 chlorethyl phosphonic acid (ethephon) promoted the tuberization of dahlia plants in long-days. This effect was smaller, however, than the effect short days have on tuberization. In contrast, gibberellic acid (GA) treatments inhibited tuberization. SADH and ethephon treatments of budless leaf-cuttings inhibited tuberization whereas ABA treatments slightly enhanced it. Evolution of endogenous ethylene reached a peak between the second and third week after the start of short-day treatments, and then decreased to the low level found in plants growing under long days. The peak in ethylene evolution occurred one week before the onset of tuberization.     

The Relationship Between Rooting of Dahlia Cuttings and the Presence and Type of Bud

Dahlia cuttings with actively growing buds are relatively hard to root as compared with those having non-growing or inhibited buds. In cuttings containing buds which sprouted during the rooting period, an inverse relationship was found between rooting percentage and growth rate of buds. Reproductive buds suppress rooting more than vegetative ones. Removing the growing terminal, parts of the cuttings (vegetative or reproductive) increased rooting percentage of cuttings. It is suggested that growing buds may affect rooting of cuttings in two opposed directions. The first is inhibition of rooting by competing with the roots for metabolites and the second is promotion of rooting by enhancing cambial activity.