Large Nerine bowdenii bulbs (>14 cm in circumference) were exposed to low ligbt intensities for different periods during two successive growing seasons. The flowering percentage and number of florets in the current season's inflorescence were recorded at anthesis. Small and large bulbs were subjected to continual defoliation starting at different times during the growing season. Bulbs were dissected at planting (26 Sept. 1992) and on 12 Jan. 1993 (nondefoliated control bulbs) to determine growth and developmental stage. At anthesis, inflorescences were harvested and the florets per inflorescence were counted. After anthesis in the fall, all bulbs were dissected and the following variables recorded: 1) percentage flowering, quiescence, or abortion of the current season's inflorescence; 2) developmental stage of quiescent inflorescences; 3) number of florets in the outermost inflorescence; 4) developmental stage of the innermost inflorescence; 5) number of leaves or leaf bases in each growth unit; 6) number of daughter bulbs; and 7) dry weight of new leaf bases. There were three reasons for nonflowering of the bulbs, viz., failure to initiate an inflorescence, inflorescences remaining quiescent, and inflorescence abortion. Individual florets that had not reached stage “Late G” (gynoecium elongated, carpels fused) at the start of rapid inflorescence elongation aborted. The more florets that aborted, the greater the probability that the entire inflorescence aborted. The inflorescence was more vulnerable to stress during the first half of the growing season due to its relatively weak position in the hierarchy of sinks within the bulb.
K.I. Theron and G. Jacobs
Nerine bowdenii bulbs were sampled from a commercial planting and dissected at 2-week intervals from 9 Oct. 1990 to 12 Nov. 1991. The following variables were recorded: number of florets per inflorescence, differentiation stage of the oldest floret, exterior dimensions of the inflorescence, and the number of leaf primordia between the two innermost inflorescences and between the innermost inflorescence and the vegetative apex. Scanning electron micrographs were made of the inflorescences at different stages of development. The development of the inflorescences of the growth units appeared to be synchronized. When the florets of the outermost inflorescence reached stage Late G (gynoecium elongated, carpels fused), the spathe started to elongate. When florets in the second inflorescence reached stage Mid G (three carpels elongated, not fused), the innermost inflorescence was initiated. Inflorescence development passed through three phases: 1) a floret initiation phase of ≈1 year, when nine to ten florets were initiated, 2) a differentiation phase, also ≈1 year long, when individual florets developed up to stage Late G, and 3) an inflorescence enlargement phase, which lasted ≈4 months and culminated in anthesis. Leaf primordia were initiated at the same rate as florets (one per month) and the vegetative phase for a growth unit was completed within 1 year.
Daniel G. Malan and Gerard Jacobs
Incandescent light night break (NB) and day continuation (DC) prevented flower formation in Leucospermum R.Br. cv. Red Sunset. Natural short days (NSD) during winter were inductive for flowering of intact shoots until 28 Aug. (Southern Hemisphere), but only until 24 July for decapitated shoots. Vegetative axillary buds released from correlative inhibition by shoot decapitation were less responsive to inductive short days (SD) than distal axillary buds on intact shoots. At least 42 inductive SD cycles were required for normal flowering after cessation of shoot growth. The effective length of the NB depended on the length of the NSD of winter. A 2-hr NB prevented flowering in vegetative buds released from correlative inhibition by shoot decapitation on 3 Mar., but was inadequate for axillary buds on shoots decapitated on 1 May. When the NB was begun during winter and discontinued before natural day (ND) lengths became too long in spring, the flowering time was delayed.
K.I. Theron and G. Jacobs
Flowering-size Nerine bowdenii bulbs were sampled from a commercial planting at 2-week intervals from 13 Aug. 1991 to 14 June 1992. They were dissected, the dry weight of foliage and leaf bases was recorded, and carbohydrate analysis was performed on the foliage leaves, leaf bases, and roots. Starch was the dominant storage carbohydrate, and leaf bases were the principal bulb structures where it was stored. Changes in starch content closely followed dry weight changes in the bulb. When exposed to low temperatures, starch was converted to sugars. Except for these high levels in the leaf bases, sugars, expressed both as concentration and total content, were low in bulb components, indicating continued export and conversion to starch. Low sugar levels during the period that florets in the current season's inflorescence develop to stage Late G (gynoecium elongated, carpels fused) is implicated in the abortion of the inflorescence.
K.A. Jacobs and G.R. Johnson
Seedlings of eight Prunus taxa were evaluated for variation in susceptibility to a single, 4- or 5-day flooding period and root rot caused by Phytophthora cryptogea Pethybr. & Lafferty. Survival, plant defoliation, disease severity index, root necrosis, and net photosynthesis indicated that the combination of flooding and pathogen was significantly more severe to all taxa than either individual treatment. Most response variables reflected early plant dysfunction but were not correlated with long-term survival. Long-term survival was 70% in the combination treatment compared to 99% in the control group. Flooding injured seedlings more than the pathogen in most taxa. Taxa differed only slightly in tolerance to the treatments, as measured by survival rate. Prunus takesimensis Nakai had the highest survival rate of 100% and along with P. mahaleb L. and P. yedoensis Matsum. showed some tolerance to flooding and the pathogen. Prunus sargentii Rehd. had the lowest survival rate of 81% and appeared to be least tolerant to the pathogen.
S.A. Oosthuyse, G. Jacobs, and D.K. Strydom
Upright l-year-old apple (Malus domestica Borkh. `Granny Smith') branches were headed at 14-day intervals (branches headed once each) during late winter and in spring [70 days before full-bloom (DBFB) until 28 days after full-bloom (DAFB)] and budbreak and new shoot growth quantified on the remaining branch section after cessation of these events. When heading was performed 70, 56, or 42 DBFB, four to five buds broke on average. When branches were headed subsequently, the average number of buds breaking increased progressively, then decreased with heading date, the maximum number breaking (13) on branches headed 14 DAFB. An average of 10 or 11 buds broke per branch section when heading was performed 28 DAFB. In late summer, the total length of new shoots per branch section for the branches headed before full bloom averaged 113 cm, whereas that on the branches headed at or after full-bloom averaged 76 cm.
J. Ben-Jaacov, A. Ackerman, E. Tal, and G. Jacobs
Jacob George, Harsh Pal Bais, G.A. Ravishankar, and P. Manilal
Response surface methodology was utilized in statistical optimization of three quality factors (the number of multiple shoots, shoot length, and number of leaves) pertaining to regeneration of plantlets from leaf calli of Decalepis hamiltonii Wight. & Arn. (swallow root). The variables evaluated were the levels of sucrose, BA, and NAA each at two different concentrations. Response surfaces for shoot length and multiple shoot number were useful in achieving optimal levels of media constituents and in understanding their interactions, but response surfaces for number of leaves were not. The data indicate that sucrose, BA, and NAA levels may be manipulated to increase or decrease quality factors chosen. This approach may be useful in developing a micropropagation protocol for D. hamiltonii. Chemical names used: benzyladenine (BA); napthaleneacetic acid (NAA).
L.J. von Mollendorff, G. Jacobs, and O.T. de Villiers
`Flavortop' nectarines [Prunus persica (L.) Batsch] were stored at -0.5C or 3C for 0, 1, 2, 3, or 4 weeks, after which the fruit was ripened at 15C. After ripening, fruit samples were tested daily or every second day for extractable juice, internal conductivity, and woolliness. The change in the percentage of extractable juice during ripening differed very little among the five storage periods. A rapid increase in internal conductivity occurred during ripening in fruit with or without cold storage, but the onset of the increase was advanced with longer cold-storage periods. No woolliness developed in fruit not placed in cold storage or in fruit cold-stored for 1 or 2 weeks at - 0.5C or 3C. Woolliness only developed during ripening of fruit cold-stored for 3 or 4 weeks at -0.5C or 3C. Incidence of woolliness increased to high levels during ripening and decreased thereafter to no woolly fruit by the 11th day. The lowest values for extractable juice coincided with the highest incidence of woolly fruit. Fruit stored for 4 weeks took longer to pass the woolliness stage. At the end of the ripening. period, cold-stored fruit were similar in appearance and juiciness to those ripened without cold storage. Nectarines stored at 3C generally developed woolliness earlier, had a lower incidence of woolliness, and took longer to overcome the problem than fruit stored at -0.5C. Incidence of browning of the mesocarp tissue was greater at 3C than at -0.5C.
Jacob G. Ricker, Jessica D. Lubell, and Mark H. Brand
Interest in native landscape plants to support pollinators has increased. Most native plants sold by nurseries are cultivars, and some consumer and conservation groups question the suitability of native cultivars to support pollinators. In 2017 and 2018, insect pollinator visitation was quantified for six native shrub species and one or more cultivars of each species (Aronia melanocarpa, A. melanocarpa ‘UCONNAM012’ Ground Hog®, A. melanocarpa ‘UCONNAM165’ Low Scape Mound®, Clethra alnifolia, C. alnifolia ‘Hummingbird’, C. alnifolia ‘Ruby Spice’, Dasiphora fruticosa, D. fruticosa ‘Goldfinger’, D. fruticosa ‘Pink Beauty’, Hydrangea arborescens, H. arborescens ‘Annabelle’, Kalmia latifolia, K. latifolia ‘Sarah’, Physocarpus opulifolius, and P. opulifolius ‘Monlo’ Diabolo®). Insects were identified into 12 categories (Apis mellifera, Bombus spp., Andrenidae, Halictidae, Megachilidae, other bees, Lepidoptera, Syrphidae, other flies, wasps, Coleoptera, and other insects). The number of inflorescences and insect visitation was similar for C. alnifolia and its cultivars, and the compact cultivar Hummingbird had the greatest floral density. A. melanocarpa had more total visitors of Andrenidae than both of its compact cultivars because it was larger and produced more inflorescences. Compact Aronia cultivars and the straight species were mostly similar for Andrenidae visitation when compared on a per-inflorescence basis. D. fruticosa had more visitors of Bombus spp. and Megachilidae than both of its cultivars. These insects may have been less attracted to ‘Pink Beauty’ because of its pink flower color and ‘Goldfinger’ because of its wider flowers, which result from it being a tetraploid. H. arborescens ‘Annabelle’ had one-third the number of Bombus spp. visitors as H. arborescens because ‘Annabelle’ produces >50% fewer fertile florets. P. opulifolius ‘Monlo’ attracted more syrphids than P. opulifolius possibly because flowers contrasted more strongly with the reddish purple foliage of ‘Monlo’ than with the green foliage of the straight species. Insect visitation was similar for K. latifolia and K. latifolia ‘Sarah’. Based on this work, we determined that native shrub cultivars are not universally less or more attractive to pollinators and must be evaluated on a case-by-case basis.