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Item The physiology and control of crown bud formation and development in gentians : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University, Palmerston North, New Zealand(Massey University, 2012) Samarakoon, Uttara ChandaniThe spread in time to flower harvest maturity and an inability to produce an adequate number of crown buds during crop establishment, were perceived as problems by commercial growers of gentians for cut flowers. By identifying and investigating plant and environment related factors that influence growth and development, the current thesis aimed to investigate both the causes of these problems and identify potential solutions. Crown buds which appeared during the previous growth cycle were the source of floral shoot production and, in both true seedlings and clonally propagated plants, these buds typically originated on the transition zone between shoots and roots. When formed on the transition zone the first and, as a consequence, apical bud of the crown bud cluster, originated adventitiously. The individual crown buds which subsequently initiated within the cluster developed as axillary buds from that apical bud. As evident from the appearance of two to five times more crown buds and shoots, and crown bud clusters, the naturally occurring short photoperiodic regime promoted the crown bud formation process, compared to a long photoperiodic regime. With the application of Ethephon, or Ethephon followed by either Thidiazuron or Paclobutrazol, it was possible to achieve a similar increase in the number of crown buds under the long photoperiodic regime. In contrast to the strong photoperiodic response, exposure to the cooler temperatures experienced outside did not influence the crown bud formation process. Once formed, crown buds were under different types and degrees of dormancy (i.e. endo, para or eco). Dependent on both the developmental stage of plants and the hierarchical position of buds within a crown bud cluster, shoot emergence and subsequent development to flowering varied in the quantity, quality of shoots, and timing in response to dormancy breaking treatments (chilling, gibberellic acid (GA3), or clipping), Hence when applied prior to shoot emergence, application of chilling to plants of the cultivar ‘Diva’ resulted in an eight-fold increase in the number of floral shoots, along with a 83 day reduction in the spread in time to harvest maturity. The requirement of chilling could be substituted by GA3. In contrast, when applied following the start of shoot emergence, only application of GA3 was effective in increasing shoot emergence and narrowing the spread in time to harvest maturity.Within a bud cluster, a positive relationship between duration to harvest maturity and a negative relationship with floral shoot length was evident with hierarchical position of the crown bud from proximal to the distal end. This influence of hierarchical position was most evident following application of chilling or GA3, potentially explaining up to 28 days spread in duration to harvest maturity in ‘Diva’. While the aforementioned factors related to shoot emergence, development and hierarchical position contribute to the sources of variability within a plant or an individual cluster, both the sources and extent of variation in time to harvest maturity, varied within cultivars, with wider spread in time to harvest maturity in ‘Starlet’ (41 days) than ‘Diva’ (35 days) and ‘Spotlight’ (29 days). The present study has increased the understanding of factors contributing to growth and development of gentians as an ornamental crop. Treatments have been developed to allow manipulation of the formation of crown buds, subsequent emergence and compactness of flowering duration.Item Dynamics of plant processes and populations in semi-arid Australia and the influences of drought, grazing and fire : a thesis submitted for the degree of Doctor of Science of Massey University(Massey University, 2010) Hodgkinson, K. C. (Ken C.)The experimental, analytical and integrative research achievements of the author, relating to the study of plant functioning and dynamics of plant populations and communities in semi-arid landscapes of eastern Australia and the responses to the stresses of drought, fire and grazing, are described. These achievements occurred during a period when scientific knowledge on the functioning and dysfunctioning of ecological communities in arid and semi-arid pastoral lands were required for the development of new managements that sustained natural resources in these water-limited environments. The research described addresses the principle goals of the author, namely to (i) elucidate physiological and demographic responses to climate variability and interactions with the stresses of grazing and fire, and (ii) to use the knowledge to develop new grazing and fire managements for sustaining pastoral businesses and the natural resources on which they depend, in semi-arid pastoral Australia. In this thesis, the research achievements of the author, the consequent publications, and the recognition of this research, are summarised in the Preface (Section 1). Following the author's curriculum vitae (Section 2), and full list of publications (Section 3 ), a detailed description of the author's research is given in Section 4, which comprises 40 selected publications in refereed journals and books, totalling some 482 pages. These selected publications address the research theme defined by the thesis title Dynamics of plant processes and populations in semi-arid Australia and the influences of drought, grazing and fire, and cover the period 1970-2002, during the candidates' research programs with the Commonwealth Scientific and Industrial Research Organisation at Deniliquin and Canberra, Australia.Item Auxin action and cell elongation : a rational approach : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University(Massey University, 1970) Penny, Pauline ElizabethA method was developed to measure every minute the growth of a single segment excised from the elongating region of a plant. The method was used to determine the short term kinetics of growth in response to auxin addition. The method is not dependant on the use of hollow coleoptile tissue and the results are plotted as a growth rate against time. The technique has a resolution an order of magnitude higher than those in current use. The results show that there is a latent phase before auxin-induced increase in elongation rate occurs. After this latent phase, there is a rapid rise in rate to a maximum followed by a decrease and then usually a rise to a second maximum. Three hypotheses for explaining the growth rate curve are considered. It was found that neither RNA nor protein synthesis were required for the initial action of auxin but that protein synthesis became necessary within a few minutes after auxin addition. The apparent half-life of the protein whose synthesis is stimulated by auxin is about 12 min. This short half life suggests that, after the synthesis of the protein, there is a limited time during which it can act with auxin to increase elongation. A model which incorporated these results has been proposed and its relationship to the three hypothesis for explaining the growth rate curve is discussed.Item Hormonal control of branching and flowering in Zantedeschia species : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University, Palmerston North, New Zealand(Massey University, 2011) Subbaraj, Arvind KumarCalla lilies (Zantedeschia sp. Family: Araceae) are perennial herbaceous geophytes, gaining commercial importance as a cut flower and potted flowering plant. Stimulating branching in Zantedeschia would equate to higher floral productivity via increasing tuber size/weight and/or via triggering the sympodial flowering cascade. Bud outgrowth is however controlled by an autonomous developmental programme, executed via different degrees of para- (apical dominance) and/or endodormancy. Based on visual clues that represent underlying changes in the shoot apical meristem, the growth cycle of Zantedeschia was demarcated into three phases, which coincide with the transition of buds from apical dominance to endodormancy. Application of 6-benzylaminopurine (BAP; an aromatic cytokinin) was successful in stimulating branching in phase 1. This equated to an increase in tuber size/weight, which in turn resulted in increased floral productivity in the next growth cycle. . Efficacy of BAP alone to stimulate branching declined from phase 1 to phase 3, and the need for a sequential application of gibberellin (GA3) increased concomitantly. GA3 alone had no effect on branching. Efficacy of GA3 alone to stimulate flowering declined from phase 1 to phase 3, and the need for a sequential application of BAP increased concomitantly. BAP alone had no effect on flowering. Stimulation of branching and enhanced flowering achieved by the reciprocal cross-talk between cytokinin and gibberellin may have major commercial implications. When applied with unlabelled BAP, a significant decline in the uptake of [8-14C] BAP ([8-14C] BAP + BAP) was observed in phase 3, resulting in a decline in radioactivity available in the buds and upper region of the tuber. With unlabelled GA3 ([8-14C] BAP + GA3) however, increased radioactivity was available in these parts in phase 3. Meta-topolin (mT) was identified as a metabolic product of BAP. Application of [8-14C] BAP + BAP resulted in a decline in the amount of mT from phase 1 to phase 3. However, application of [8-14C] BAP + GA3 resulted in an increase in the amount of mT in phase 3. mT and 6-benzylaminopurine riboside (BAR) were also identified in natural plants. Further studies on branching control in phase 1 involving topolins and strigolactones, elucidating the mechanisms of cross-talk between cytokinin and gibberellin in phase 3, reevaluating the relationship between branching and floral productivity, and corroborating the common mechanisms between dormancy and flowering are recommended.Item Acetyl-CoA carboxylase in photosynthetic tissue : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biochemistry at Massey University(Massey University, 1981) Nikolau, Basil JohnAcetyl-CoA carboxylase catalyses the rate-limiting reaction in de novo fatty acid biosynthesis in a wide variety of organisms. In plants however, the significance of this enzyme in regulating de_ novo fatty acid biosynthesis is unknown. In this investigation acetyl-CoA carboxylase of maize, barley and spinach leaves has been studied in order to compare some features of this enzyme in the three plants. In maize, acetyl-CoA carboxylase is located in chloroplasts, where it occurs as a soluble enzyme in the stromal fraction. Both mesophyll and bundle sheath cells also exhibit acetyl-CoA carboxylase activity. Activities of acetyl-CoA carboxylase in isolated chloroplasts have been compared to rates of lipid synthesis from acetate in order to investigate the role of acetyl-CoA carboxylase in regulating de novo fatty acid biosynthesis from acetate. Although acetyl-CoA carboxylase activity was higher than that expected from the rate of acetate incorporation into lipids of isolated chloroplasts of maize and barley, the opposite was found for chloroplasts isolated from spinach. In chloroplasts from the emerging leaves of maize seedlings, lipid synthesis was maximal with chloroplasts isolated from the leaf segment corresponding to leaf greening. However, a relationship between leaf development and acetyl-CoA carboxylase activity was less apparent. Consequently a regulatory role for acetyl-CoA carboxylase in de novo fatty acid biosynthesis could not be directly established from a comparison of the rates of lipid synthesis and acetyl-CoA carboxylase activities. An alternative approach to the establishment of a regulatory role for acetyl-CoA carboxylase was to purify the enzyme from maize and barley leaves, and study its response to variable concentrations of substrates, products and certain cellular metabolites. Purification of both enzymes was performed by identical procedures, including polyethylene glycol fractionation, hydrophobic chromatography and gel filtration. Acetyl-CoA carboxylase from both maize and barley leaves appeared to be an integral enzyme, as no evidence for its dissociation was found, contrary to the findings of the bacterial enzyme. The kinetic properties of the partially purified enzyme from maize and barley were very similar. The apparent Michaelis constants for the substrates, acetyl-CoA and HCO-3, were about 0.lmM and 2mM, respectively for both enzymes. The enzymically active form of the substrate, ATP, was found to be Mg.ATP. Furthermore, free ATP inhibited enzymic activity, while free Mg2+, activated the enzyme from both plant sources. Monovalent cations, particularly K+, were positive effectors of acetyl-CoA carboxylase, on the other hand, the products of the acetyl-CoA carboxylase reaction, malonyl-CoA and ADP were inhibitors. ADP inhibition was competitive with respect to ATP, but uncompetitive with respect to acetyl-CoA. Inhibition of acetyl-CoA carboxylase activity by CoA was noncompetitive with respect to acetyl-CoA, while palmitoyl-CoA inhibition was uncompetitive with respect to acetyl-CoA. From the view point of regulation of acetyl-CoA carboxylase activity, the observed response of activity to changes in pH, and the concentrations of Mg2+, K+, ATP and ADP in vitro, may be of significance. Changes in the chloroplastic levels of these effectors of acetyl-CoA carboxylase activity have been reported to occur during light-dark transition of chloroplasts, and would be consistent in regulating acetyl-CoA carboxylase activity in the light and dark.Item Studies on the mechanism of plant cell expansion : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Botany at Massey University(Massey University, 1983) Pearce, David WilliamThe mechanism of elongation of segments of hypocotyl of light-grown seedlings of lupin (Lupinus angustifolius cv. N.Z. Bitter Blue) has been investigated. The approach was three-fold: biophysical analysis of growth responses; an investigation of the role of individual tissues in elongation; and tests of predictions of the acid-growth hypothesis. In biophysical studies, a method was developed to measure the half-times of transients in elongation rate in response to application of a compressive load. For loads of 4-18g (equivalent to applied pressures of about 0.1-0.5 bars) half-times for the return of elongation rate to a steady value after loading were 3-15 minutes for segments incubated without IAA, and 6-13 minutes for IAA-treated segments. Half-times after removing the load were 2-7 minutes for non-IAA-treated segments. Results were analysed according to the diagnostic scheme of Cosgrove (1981, Plant Physiol. 68:1439-1446), and suggested that IAA promoted elongation through an effect on either the tissue free energy diffusivity of water (D), or on extensibility. It was not possible to distinguish between these alternatives on the evidence available. In studies on the role of different tissues in elongation, the effect of removing specific tissues from non-IAA-treated segments was first determined. The epidermis apparently limited elongation of intact segments, since a burst of extension occurred when it was removed by peeling. In peeled segments, the stele (vascular tissue and pith) apparently limited the rate of extension since its removal resulted in very rapid extension of the remaining cylinder of cortex. On IAA treatment, the response of segments with the stele removed was initially similar to that obtained with intact segments, suggesting that the epidermis and cortex only were involved in the initial response. In segments where the epidermis had previously been removed this initial response to IAA was absent, but there was a lenger term response. These results suggest that the response of intact segments to IAA consisted of two superimposed phases. The first was the result of epidermal "relaxation", and the second was an independent elongation controlled by the cortex. The acid-growth hypothesis predicts that treatment with acid solutions will promote elongation to IAA-induced levels. Tests of this prediction with hollow cylinders and peeled segments of lupin hypocotyl showed that the most IAA-responsive preparation (hollow cylinders with the epidermis intact) was the least acid-responsive, with little elongation response at pH 5. Treatment at pH 4 was needed to promote elongation to IAA-induced rate. The cortex alone responded strongly to acid treatment (pH 5), suggesting that the epidermis was limiting response when it was present. Peeled segments elongated in response to IAA treatment, but did not elongate in response to acid treatment (pH 5) (if pretreated in water), perhaps because response was limited by restricted diffusion of hydrogen ions through the starch sheath and into the stele. However, peeled segments elongated rapidly initially after treatment with acid if first pretreated in buffer (1 mM K2HPO4-citric acid, pH 6.6). These results show that acid-induced elongation of segments may be influenced by differential response of tissues, by barriers to diffusion of hydrogen ions, and by treatment with buffered solutions. The results suggest that unless IAA action in intact segments causes pH in the walls of the outermost cell layers to fall to to about pH 4, then it is unlikely that IAA-induced elongation is mediated (initially) by hydrogen ions.Item Trace elements in New Zealand plants : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University(Massey University, 1969) Lyon, Graeme LesterIn biogeochemical studies initiated for the first time in New Zealand, some trace elements in indigenous plants and in the soils supporting these plants, were determined by emission spectrography and atomic absorption spectrophotometry. A test of the biogeochemical method of prospecting was made by studying elemental concentrations in leaves of three tree species and in the corresponding soils from mineralised and non-mineralised ground at Copper-stain Creek, North-West Nelson. All samples were analysed for copper and molybdenum. Olearia rani was also analysed for zinc. Elemental contents in plants and soils were compared by correlation calculations on a computer. The molybdenum content of the ash of O. rani leaves showed a highly-significant correlation with the content of the same element in corresponding soil samples. This indicated that O. rani could be used as a biogeochemical indicator for molybdenum mineralisation. No other significant plant-soil correlations were found for molybdenum., copper or zinc. For a further set of O. rani samples, leaves, twigs, wood and flowers were all analysed for zinc, copper and molybdenum. This data showed that the leaves of O. rani were better than the other parts of the plant, and that analyses based on ash weight were better than dry weight values for indicating molybdenum mineralisation in the soil. Cumulative frequency diagrams gave values for threshhold concentrations used to delineate the anomalous areas at Copperstain Creek. Molybdenum contents in the ash of plants showed wide variations, ranging from one to 1600 parts per million. Copper and zinc contents showed less variation but there were significant differences in the mean values for each species. A New Zealand serpentine flora and the associated soils from near Dun Mountain on the Nelson Mineral Belt was studied. In an initial orientation survey, it was found that seventy-one samples of twenty-six species showed that wide variations existed in the concentrations of the elements chromium, nickel, cobalt and copper. Six of these species were sampled further and analysed for chromium, nickel, copper, cobalt, calcium and magnesium. The species Cassinia vauvilliersii, Hebe odora and Leptospermum scoparium were sampled both randomly and from a localised area of serpentine where soils were more uniform. Plant elemental contents were found to vary up to several orders of magnitude in both sets of samples. Specimens of the same species sampled from near the boundary of serpentine with sedimentary rocks and from an andesitic area at Mt. Egmont, contained much lower amounts of chromium, nickel, cobalt and magnesium and higher amounts of calcium than samples from serpentine. Correlation coefficients were calculated for the relationships between pairs of elements and showed that for C. vauvilliersii, there were highly-significant correlations between plant ash and soil contents for chromium, nickel and cobalt. H. odora and L. scoparium showed similar but less pronounced correlations for the same three elements. It was concluded that these species, especially C. vauvilliersii would be useful for biogeochemical prospecting. Twenty samples of each of the serpentine-endemic species, Myosotis monroi, Notothlaspi australe and Pimelea suteri were also collected for comparison with the other, more common species. P. suteri, in particular, is a strong accumulator of chromium, nickel and cobalt although L. scoparium, also accumulates chromium to a greater degree than the other species. The highest concentrations found included 2.6% chromium in the ash of a P. suteri and 9% chromium in a soil in which L. scoparium grew. These values are higher than any previously observed in other parts of the world. Good correlations were found for some pairs of elements showing that chromium, nickel and cobalt are strongly related in soils and in most plant species. The range of calcium and magnesium, contents in plant ash was from one to 30%. Although the exchangeable-calcium content of soils was about one twentieth of that for magnesium, the Ca/Mg ratio in plants ranged from 0.1 to 5.0. In view of the unusual concentrations of chromium in serpentine plants, the metabolism and uptake of the radioisotope chromium-51 was studied in selected species. Translocation of 51Cr when applied as chromate, to cuts in the stem or the tips of branches of seedlings was observed to be greater in P. suteri and L. scoparium than in C. vauvilliersii or H. odora, as indicated by radioautography. In young L. scoparium (manuka) plants, 51Cr supplied as chromate was translocated as chromate in the xylem sap as shown by high-voltage electrophoresis. Trifoliun pratense seedlings (red clover) which were more readily available and grown in nutrient solution for ease of manipulation, were able to absorb and translocate 51Cr to the leaves when the 51Cr was supplied as either sodium chromate or chromic chloride. In both species (T. pratense and L. scoparium), generally much less than 5% of the 51Cr appeared to be bound to protein or nucleic acids. In the roots of L. scoparium seedlings, 32% of the radioactivity was soluble in boiling 80% ethanol and a further portion was soluble in boiling water. These fractions were examined by high-voltage electrophoresis and a total of 18% of the 51Cr in roots of L. scoparium cultured in nutrient solution existed as the trioxalatochromate(III) ion with lesser amounts as two other soluble anionic compounds. T. pratense did not show the presence of the trioxalatochromate(III) ion. It was concluded that the feasibility of carrying out biogeochemical prospecting in New Zealand has been demonstrated for the first time. Further, the plant chemistry studies have contributed not only to general plant nutrition research, but also have provided basic information for the understanding of the trace element metabolism involved in biogeochemical prospecting.Item Physiological investigations on regeneration from bulb scale leaves of Lilium speciosum Thun. : thesis presented for the degree of Doctor of Philosophy, University of New Zealand(Massey University, 1954) Robb, Sheila M.During the past twenty years a vast amount of research work has been carried out on the culture of plant tissues or organs excised from parent bodies. Haberlandt's (53) theories in 1902 on the possibility of procuring growth in such isolated tissues were not put into practice until some twenty years later, when the work of Kotte (63,64) and Robbins (97,98) was partially successful and encouraged further research along these lines. Later work by white (123-125) on excised roots, and by Gautheret (43-46), Nobecourt (84-87), and white (126) on cambial tissue proved that excised plant material could be cultured on a suitable synthetic medium. From that time, an ever-increasing volume of research work has been done on the culture of plant tissue, and the technique has found application in the investigation of a variety of problems in plant physiology.Item Ecology of pastoral communities in a heterogeneous environment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Pastoral Science at Massey University, Palmerston North, New Zealand(Massey University, 2000) López, Ignacio Fernando; López, Ignacio FernandoA group of studies was conducted to examine environmental variables and pasture components and their relationships in heterogeneous hill country pasture. Four studies were conducted in relation to the hill country grassland ecosystem of New Zealand. 1. The effects of long-term fertiliser-stocking rate and hill country slope category (LS Low slope, 0-12° MS Medium slope, 13-25° HS High slope, ?25°) on soil physical and fertility attributes and pasture production were examined. Field treatments, high fertility-high stocking rate (HH) and low fertility-low stocking rate (LN), have been applied to paddocks since 1975. Soil samples were taken from the slope categories of the two field treatments (microsites) and physical and fertility features were analysed. Dry matter production through the year was also measured from these units. The soil attributes that explained the largest percentage of the differences between microsites were water holding capacity (WHC), water conductivity (Kunsat), slope, soil compressibility (SC), bulk density (BD), Olsen-P, soil total nitrogen (Total-N) and soil rebound after compression (SR). Slope led to greater differences between soil features of microsites than fertiliser and stocking rate history. Dry matter production increased with increasing Total-N, Olsen-P, WHC and SC, and decreasing slope, Kunsat, BD and SR. 2. The presence of plant functional groups, species segregation and their relationship with soil features were analysed. The relationship between field condition and plant functional group was also examined. The evaluation was conducted in the same sites as the first study. The pasture botanical composition for each microsite was measured through the year and plant functional groups determined. The relationship between the presence of plant species and the soil attributes WHC, Kunsat, slope, SC, BD, Olsen-P, Total-N and SR (from the first study) and plant functional groups were studied, as well as the field condition-plant functional groups relationship. Seven functional groups were determined. High fertility grasses and Lolium perenne (Lp) were associated with LS and high availability of resources, while low fertility species were segregated to HS. Groups of species such as Agrostis capillaris (Ac) were indifferent to environmental changes. Functional groups proved to be good indicators of soil development. Field condition and plant functional groups were complementary concepts in grassland dynamic analyses. 3. Sheep grazing behaviour was examined in relation to slope category and plant species selection. The study was conducted in the same microsites as studies 1 and 2. Transects with marked tillers of Anthoxanthum odoratum (Ao), Ac and Lp were placed in the slope categories as follows: Ac and Lp in LS; Ac, Ao and Lp in MS; and Ac and Ao in HS. The evaluation was carried out during 4 weeks in each of Summer, Autumn, Winter and Spring, and records of grazed and ungrazed tillers were analysed. Pasture growth rates were calculated through the year. During Spring sheep grazed mainly the LS. With decreasing availability of pasture, sheep enlarged their grazing areas towards the HS. Species selection was only present during Winter when pasture availability was low. In Winter sheep also grazed in all slope categories and selected Lp over of Ac but showed no selection for Ao. 4. Ecotype segregation and plant phenotypic plasticity were examined. Plant material was collected from the extremes of the environmental gradient analysed in studies 1, 2 and 3 and grown in glasshouse conditions under five levels of phosphorus and three of nitrogen in the soil. The plants in each pot were cut on three occasions and total dry matter was calculated. Height, plant architecture, plant horizontal expansion and leaf growth were analysed for Cynosorus cristatus (Cc), Holcus lanatus (HI), Ac, Ao and Lp. Morphological and physiological differences were present between genotypes of Lp whereas only physiological genotypic differences existed in Ao and Cc. Consistent differences were not found between HI genotypes. Thus, Ao, Cc and Lp showed ecotype differentiation. Ac genotypes showed high plasticity with no ecotype differentiation.Item Crown development and related changes in morphology and physiology of asparagus plants associated with their productivity : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science, Institute of Natural Resources, Massey University, Palmerston North, New Zealand(Massey University, 2004) Daningsih, EntinThe results are presented of eight experiments designed to investigate the influence of interrelationships between bud population dynamics and carbohydrate supply from root stores on spear production in asparagus (Asparagus officinalis L). These investigations involved studies in the field and the greenhouse, and using aeroponics and hydroponics techniques to facilitate non-destructive studies of plant development. The evidence indicated that spear yield was limited by the number of buds of adequate size for developing into marketable spears, rather than total bud number. It was shown that bud development continues throughout the harvest period. About 14% of these buds contributed to fern production after harvest, but the majority were involved, following a period of dormancy, in development of the next season's spears. Approximately 16% of the new buds contributed to spear yield in the current harvest, 68% were dormant until the following summer and contributed to 18% of total buds at that time. Spear production was most efficient in plants with large crowns, since the effects of correlative inhibition on spear development were greater in small than large crowns. Nevertheless, increase in crown size in terms of root mass is not necessarily accompanied by an equivalent increase in bud number or cluster number, and bud availability is potentially an important yield limiting factor. However, large crowns reduced the period of correlative inhibition within a bud cluster. Crown size and bud population were sensitive to nutrient supply, and it is suggested that control of nutrient supply over the harvest period may be best achieved by use of slow-release fertilizer or split application of nitrogen. Carbohydrate partitioning and possibly photosynthetic rate were also sensitive to daylength, and there was some evidence of genotypic variation in the response to daylength changes and contrasts. Principal component analysis indicated that numbers of buds and bud clusters, plant size and chlorophyll content were the main determinants of spear yield, and cluster analysis demonstrated potentially important genetic variation for these variables in potentially high yielding cultivars. Spear yield is the product of harvest intensity and harvest duration, and harvest duration itself was shown to be sensitive to genotype and management effects on bud initiation and development. A conceptual model is used to illustrate the influence of bud population and bud cluster characteristics on harvest intensity and duration, and on spear yield, and the relative importance of management manipulation of bud dynamics and carbohydrate supply to spear yield.