Enhancement of vegetative growth in young citrus plantings

Mudzunga, Maluta J. (2000-03)

Thesis (MSc)--Stellenbosch University, 2000.

Thesis

ENGLISH ABSTRACT: Enhancement of vegetative growth in young citrus plantings The establishment of citrus involves high input costs, with break-even usually only attained after six years, making early returns imperative for economic survival. Early production is inseparably associated with good growth of the trees in the non-bearing years. In cool and cold production regions, coupled with a high frequency of wind, the initial tree growth is unsatisfactory. Trials were conducted to evaluate the effect of foliar application of Progibb® (GA3), Promalin'" (G~+7 + BA), Kelpak® (seaweed extract containing auxins and cytokinins) and soil applications of Temik® (aldicarb) as a means to promote vegetative growth. GA3 and/or aldicarb significantly increased tree height, without increasing the stem diameter in comparison to the control trees on 'Eureka' lemon, but not on 'Lisbon' lemon. G~+7 + BA or Kelpak® treatments did not have an effect on tree growth or fresh weight distribution. In conclusion, growth stimulation by GA3 and/or aldicarb could lead to quicker filling of the allotted space, and consequently higher, earlier yields, but further trials are required to verify these results. Insufficient carbohydrate and nitrogen reserves are often implicated in poor establishment performance of nursery trees. The effect of tree size at planting and different topping heights on early growth in the field were evaluated; also the physical and chemical profile of tall nursery whip trees were quantified. Significant correlations were found between the initial stem diameter and final stem diameter and initial tree height and final tree height at the end of the first growing season. Stem diameter increase was significantly reduced in topped trees relative to untopped trees. Roots comprised approximately 22%, and the aboveground portion about 78% of total dry weight. Roots nearly always contained higher concentrations of carbohydrates and nitrogen reserves. However, the aboveground sections, comprising a higher proportion of total dry weight, contained more than two thirds of total carbohydrates and nitrogen reserves. Topping of nursery whip trees at 60 cm aboveground thus would result in a loss of 33 to 37% dry matter, 29 to 33% carbohydrates and 37 to 46% nitrogen. Therefore, planting large, untopped nursery trees enhance initial tree growth in the orchard. The use of well-branched nursery trees for orchard planting can shorten the time to commercial fruit production. Citrus nursery trees do not develop lateral shoots adequately in the nursery or in the field and therefore necessitate various branch induction techniques. Two trials on newly-planted (one or five months after planting) trees were conducted to evaluate various branch induction techniques, viz., girdling, Promalin® (G~+7 + BA) (at 1000 or 2000 mg/I) and/or leaf removal and notching either to individual buds or the whole trees. The first trial, on l-month-old trees did not yield significant results. In the second trial, notching and girdling significantly increased the number and total length of lateral branches, but notching was the most effective technique. As the first trial, on the very young trees, yielded no significant results, it seems that trees have to be well established before they will respond. In young, non-oearing trees a lot of energy is invested in the flowering process, which results in an inhibition of vegetative growth. Gibberellic acid (GA3) and heavy mineral oil (Bac-oil) treatments, either individually or in combination, applied during early winter were evaluated as a means to reduce flowering levels in young nonbearing citrus trees. GA3 and mineral oil either separately or in combination were sprayed from April to July to the whole tree. GA3 and mineral oil either individually or in combination applied late in May to late June markedly reduced flowering. GA3 application in mid April had a minimal effect on flowering levels. Peak responses to GA3 coincided with a significant reduction in bud sprouting. Although optimum application time proved to be around May to July, this time is likely to vary from year to year depending on the influence of the prevailing temperature and other climatic conditions. The winter application of GA3, specifically, and possibly in combination with mineral oil to inhibit flowering and early vegetative growth may be utilised commercially. However, response may vary dramatically from season to season. Effect of time of application and concentration of 2,4-dichlorophenoxypropionic acid (2,4-DP) and l-naphthaleneacetic acid (NAA) as potential fruit thinning agents to reduce fruit set soon after flowering and enhance vegetative growth were evaluated over a two year period. 2,4-DP at 150 and 300 mg/I and NAA at 200 and 400 mg/! were sprayed on 2- and 4-year-old 'Mihowase' Satsuma, as well as on 2-year-old 'Marisol', 'Nules' and 'Oroval' Clementine trees at two times (late October and early November). The higher concentration of 2,4-DP and NAA generally did not result in increased fruit abscission in the first year. However, in the case of 2,4-DP the higher concentration resulted in stronger thinning in the second year. 2,4-DP and NAA can be used as potential fruit thinning agents to reduce fruit set on young non-bearing trees.

AFRIKAANSE OPSOMMING: Verbetering van vegetatiewe groeie by jong sitrus-aanplantings Die vestiging van sitrus behels hoë insetkoste en die gelykbreekpunt word gewoonlik eers na ses jaar bereik. Vroë opbrengste is dus noodsaaklik vir ekonomiese oorlewing. In koel en koue produksie-areas, met baie wind, is die aanvanklike boomgroei onvoldoende. Studies is uitgevoer om die effek van blaarbespuitings van Progibb'" (GA3), Promalin® (G~+7 + BA), Kelpak (seewier-ekstrak wat ouksien en sitokiniene bevat) en grondtoedienings van Temik® (aldicarb), as metodes om vegetatiewe groei te verbeter te evalueer. GA3 en/of aldicarb het boornhoogte betekenisvol laat toeneem, sonder om stamdeursnit te bevoordeel op 'Eureka' suurlemoen, maar nie op 'Lisbon' suurlemoen nie. G~+7 + BA of Kelpak'" behandelings het geen effek op boomgroei of varsmassa-verspreiding gehad nie. Groeistimulasie deur GA3 en/of Aldicarb kan lei tot vinniger vul van spasie, en hoër en vroeër oeste, maar verdere studies is nodig om die resultate te bevestig. Onvoldoende koolhidraat- en stikstofreserwes word dikwels gekoppel aan swak vestigingsprestasie van kwekerybome. Die effek van boomgrootte by planttyd en verskillende tophoogtes op vroeë groei in die veld is geëvalueer; die fisiese en chemiese profiel van lang ongetopte kwekerybome is ook gekwantifiseer. Betekenisvolle korrelasies is gevind tussen die aanvanklike stamdeursnit en en finale stamdeursnit en tussen aanvanklike boomhoogte en finale boomhoogte aan die einde van die eerste groeiseisoen. Stamdeursnit-toenarne is betekenisvol verminder in getopte bome relatief tot ongetopte bome. Wortels het uit ongeveer 22% en die bogrondse porsie ongeveer 78% van die totale droë massa bestaan. Wortels het bykans altyd hoër konsentrasies koolhidrate- en stikstofreserwes bevat. Die bogrondse gedeeltes, wat ook 'n hoër proporsie van die totale droë massa bevat, het meer as twee derdes van die totale koolhidraat en stikstofreserwes. Die top van kwekerybome op 60 cm bo die grond sal lei tot 'n verlies van 33 tot 37% droë massa, 29 tot 33% koolhidrate en 37 tot 46% stikstof. Dus, die plant van groot, ongetopte kwekerybome sal die aanvanklike boomgroei in die boord verbeter. Die gebruik van goedvertakte kwekerybome by planttyd kan die tyd tot komrnersieële vrugteproduksie verkort. Sitrus-kwekerybome gee nie voldoende laterale vertakking in die kwekery of in die boord nie. Twee studies op pas-aangeplante bome (een en vyf maande na plant) is uitgevoer om verskillende tegnieke om laterale lootgroei te stimuleer te evalueer, nl. Promalin® (G~+7 + BA) (teen 1000 of 2000 mg/I) en/of blaarverwydering, en die maak van kerfies op individuele knoppe of op bome as geheel. In die eerste studie, op die een-maand-oue bome, is geen betekenisvolle resultate verkry nie. In die tweede studie het kerfies en ringelering die hoeveelheid en lengte van laterale takke betekenisvol vermeerder, maar kerfies was meer effektief. Die bome moet egter blykbaar goed gevestig wees voor dit reageer. In jong, nie-draende bome word baie energie gebruik in die blomproses en dit lei tot die inhibisie van vegetatiewe groei. Gibberelliensuur (GA3 ) en 'n swaar mineralolie (Bac-oil) wat individueel of in kombinasie toegedien is tydens die vroeë winter is geëvalueer as 'n tegniek om blomvlakke in jong nie-draende sitrusbome te verminder. GA3 en minerale olie individueel of in kombinasie, is gespuit vanaf April tot Julie op die bome as geheel. GA3 en minerale olie toegedien individueel of in kombinasie laat in Mei tot Junie het blomvlakke verminder. GA3 toegedien in middel April het 'n minimale effek gehad. Die beste reaksie op GA3 het saamgeval met 'n betekenisvolle vermindering in die bot van knoppe. Alhoewel die optimum tyd van toediening rondom Mei tot Julie is, mag hierdie tyd wissel van jaar tot jaar afhangende van die effek van heersende temperature en ander klimaatstoestande. Die wintertoediening van GA3, spesifiek en moontlik in kombinase met minerale olie om blomvlakke en vroeë vegetatiewe groei te verminder kan kommersieel gebruik word. Die reaksie op hierdie behandelings mag egter drasties wissel van seisoen tot seisoen. Die effek van die tyd van toediening en konsentrasie van 2,4- dichlorofenoksipropioonsuur (2,4-DP) en 1-naftaleenasynsuur (NAA) as potensiële vruguitdunagente om vrugset kort na blom totaal te verminder en om vegetatiewe groei te verbeter, is geëvalueer oor 'n twee-jaarperiode. 2,4-DP teen 150 en 300 mg/l en NAA teen 200 en 400 mg/l is gespuit op 2- en 4-jaaroue 'Mihowase' Satsuma, en ook op 2-jaaroue 'Marisoi', 'Nules' en 'Oroval' Clementines op twee tye (laat Oktober en vroeg November). Die hoër konsentrasies van 2,4-DP en NAA het oor die algemeen nie gelei tot 'n verhoging in vrugafsnoering in die eeste jaar nie. Met 2,4- DP het die hoër konsentrasie gelei tot strawwer uitdunning in die tweede jaar. 2,4-DP en NAA kan as potensiële vruguitdunagente op jong, nie-draende bome gebruik word.

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