C. Selby

The use of growth retardants to improve microtuber formation by potato (Solanum tuberosum)

The growth retardant chlormequat stimulated microtuber formation by a recalcitrant cultivar of potato (Solanum tuberosum), but reduced microtuber fresh weight in a cultivar that tuberised readily in its absence. Inhibition of microtuber growth by high concentrations of chlormequat was confirmed using a different in vitro system where all cultivars tuberised in the absence of growth retardants.Alternative growth retardants were tested. Daminozide also had a detrimental effect on microtuber fresh weight, but ancymidol and paclobutrazol did not inhibit microtuber growth at the concentrations required for stimulation of tuberisation by recalcitrant cultivars. In addition, 10-5 M ancymidol and paclobutrazol inhibited premature sprouting of microtubers in vitro.

Narcissus bulblet formation in vitro: effects of carbohydrate type and osmolarity of the culture medium

Shoot clump cultures of Narcissus cultivars St. Keverne and Hawera were used to investigate the effects of culture medium carbon supply, type of carbohydrate and osmolarity on in vitro bulblet development. Increasing the medium osmolarity using mannitol or sorbitol, which did not act as substrates for growth, failed to stimulate bulblet formation with either cultivar. An exception to this was a relatively small increase in total bulblet dry weight per culture, in the cultivar Hawera only, caused by adding 30 g l −1 sorbitol in combination with 30 g l−1 sucrose. Simultaneously increasing the medium osmolarity and carbon supply using the metabolisable carbohydrate sources, sucrose, glucose, fructose or an equimolar mixture of glucose and fructose stimulated bulblet production, total dry matter accumulation and partitioning into bulblets. At comparable levels of carbon supply up to 19.0 g l−1, bulblet development of both cultivars was similar with monosaccharide and sucrose media. This indicates that substrate supply is more important for bulblet development than osmolarity of the culture medium. The cultivar Hawera also showed similar responses to monosaccharide and sucrose media supplying 37.9 g C l−1, despite the high osmolarity of monosaccharide media (c. 650 m Osm kg−1, equivalent to −1.6 MPa, compared to 380 m Osm kg−1 for sucrose medium). However in St. Keverne total dry matter accumulation and dry weight per bulblet were further stimulated only by increasing the sucrose supply from 19.0 to 37.9 g C l−1, not by increasing the monosaccharide supply. Implications of the findings for Narcissus micropropagation are discussed.

A simple method for maintaining high multiplication of Narcissus shoot cultures in vitro

AbstractA simple method for stimulating and maintaining high in vitro multiplication of Narcissus shoot clump cultures was developed. Shoot clumps were subjected either to ‘normal cutting’ where leaves were trimmed to 20 mm in length at the beginning of each culture passage or to ‘severe cutting’ where shoot clumps were cut down to the basal plate region removing all green tissue. ‘Severe cutting’ at the beginning of each culture passage initially doubled the leaf multiplication, compared to ‘normal cutting’, but the difference between cutting treatments declined in successive passages. The improvement in leaf multiplication was maintained when shoot clumps were subjected to ‘severe cutting’ only at every other culture passage, with no cutting in the alternate ‘recovery’ passages. In vitro multiplication was increased by ‘severe cutting’ in all seven Narcissus cultivars which were tested.

Stimulation of in vitro root and shoot growth of potato by increasing sucrose concentration in the presence of fluridone, an inhibitor of abscisic acid synthesis

Fluridone, an inhibitor of abscisic acid (ABA) biosynthesis, strongly stimulated rooting of nodal stem segments of potato (Solanum tuberosum L.) cultivar Arran Banner cultured in darkness on tuberisation medium. Inclusion of 10-6 M ABA in the culture medium prevented this rooting response, indicating that root proliferation in the presence of fluridone could be due to inhibition of ABA synthesis. The rooting response to fluridone (increased total root number and root fresh weight) was obtained only at high sucrose concentrations (0.175 and 0.234 M) and was demonstrated with two potato cultivars and two culture media; one which favoured tuberisation and one which did not. Shoot numbers were also increased, but to a lesser extent than root numbers, and total fresh weight of plant material per culture was greatly increased by inclusion of both fluridone (10-6 or 10-5 M) and 0.234 M sucrose in the culture medium. The role of sucrose was not simply osmotic because when the osmolarity of fluridone medium was increased using mixtures of mannitol and sucrose, no root proliferation occurred unless sucrose predominated in the mixture.

Soluble iron is lost from MS medium pre-exposed to light but growth of potato plantlets is not inhibited

Plant tissue culture medium which contained FeEDTA as sole iron source was incubated aseptically in light (16-h photoperiod, 100 μmol m-2 s-1 PAR) at 20°C without plant tissue. Soluble iron dropped from an initial concentration of 4 mg 1-1 to less than 0.1 mg 1-1 in 4 weeks. This occurred in both glass and plastic culture vessels. No loss occurred when medium was incubated at 20°C in darkness. A further experiment showed that soluble iron concentration fell to <0.2 mg 1-1 in only 4 days but the loss was slower at lower irradiances.Effects of the loss of soluble iron on plantlet growth were assessed by culturing single node stem segments of in vitro potato (Solanum tuberosum L. cv. Arran Banner) plantlets on medium previously exposed to light. Pre-exposure sufficient to reduce soluble iron concentration to <0.1 mg 1-1 had no inhibitory effect on plantlet development in solidified medium or in liquid medium, except when the liquid medium had been centrifuged before inoculation to remove iron precipitated during pre-exposure to light. The plantlets then became chlorotic.

Micropropagation of Narcissus

Natural vegetative propagation of Narcissus is so slow [5, 6] that building up a commercial stock of a new cultivar takes fifteen to twenty-five years [9]. Micropropagation of Narcissus is also slow, e.g. Squires and Langton [7] obtained multiplication rates ranging from 1.44 to 2.30 for five cultivars over a series of 4–6 wk culture passages.

An improved method for callus proliferation and regeneration of Fuchsia hybrida.

Best callus initiation was obtained when single-node explants of Fuchsia hybrida were incubated in the light on Gamborg B5 medium containing 5×10-6 M indoleacetic acid and benzylaminopurine at 5×10-7 M or 10-6 M. Healthy callus proliferation was maintained in darkness on full-strength B5 medium supplemented with 5×10-6 M IAA and 5×10-7 M BAP. Regeneration from callus was obtained in 3 to 6 weeks, using half-strength hormone-free Campbell & Durzan medium.

Stimulation of rooting in vitro: effects of inhibitors of abscisic acid synthesis

The herbicides fluridone [1-methyl-3-phenyl-5-(α, α, α,-trifluoro-m-tolyl)-4-pyridone and norflurazon] [4-chloro-5-methylamino-2-(α, α, α -trifluoro-m-tolyl)-pyridazin-3(2H)-one] inhibit phytoene desaturase and thus block caro-tenoid biosynthesis [6]. Consequent photobeaching of chlorophyll accounts for phytotoxicity of the herbicides [1]. Plants treated with fluridone or norflurazon and grown in darkness remain healthy [1, 8] but have very low endogenous concentrations of abscisic acid [3], abscisic acid being synthesized from carotenoid precursors [7]. Dramatic root proliferation was observed when these inhibitors were used to investigate the role of abscisic acid in tuberisation of potato in vitro.