Critical factors concomitant to the physiological development of alternate bearing in citrus (Citrus spp.)

Stander, Ockert Petrus Jacobus (2018-12)

Thesis (PhDAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The significance of carbohydrates, mineral nutrients and phyto-hormones was investigated in relation to their possible roles in selected phenological events in alternate bearing ‘Nadorcott’ mandarin (C. reticultata Blanco) trees. Crop load in ‘Nadorcott’ mandarin trees was influenced by flowering intensity. The most important determinants of flowering intensity were the amount of new vegetative shoot growth and resulting number of new potential floral buds that developed during summer, and the influence of fruit on floral bud development during winter. The lack of development of summer vegetative shoots in “on” trees was not related to leaf carbohydrate concentration. In “off” trees, root sugar concentration peaked during full bloom and high root growth activity was observed prior to the vegetative shoot flush in summer. In “on” trees, fruit were the major carbohydrate sinks and probably disturbed the balance between vegetative shoot development and root growth. Sugar concentration in roots in “on” trees was 3-fold lower, root growth was absent, and shoot growth was halved. The concentration of mineral nutrients in leaves was a response to fruit load and not related to parameters of flowering or vegetative shoot growth. Measurements of phyto-hormones in leaves and roots confirmed that the inhibition of summer vegetative shoots was related to a high concentration of 1 H-indole-3-acetic acid (IAA) in leaves. High concentrations of dihydrophaseic acid and the abscisic acid (ABA) glucose ester suggested that IAA might have acted synergistically with ABA to create a growth inhibition in fruiting shoots. As a result, cytokinins did not contribute to the development of new summer vegetative shoots. High gibberellin concentration in leaves in May and June contributed to limited flowering in “on” trees. Consistent with this interpretation, treatment of “off” trees with 40 mg·L-1 gibberellic acid inhibited flowering, whereas soil and foliar treatments of “on” trees with 1000 mg·L-1 paclobutrazol or uniconazole, gibberellin biosynthesis inhibitors, increased flowering and resulted in fruit development from buds of “on” shoots.

AFRIKAANSE OPSOMMING: Die verband tussen die konsentrasies van koolhidrate, minerale nutriente en fito-hormone, en belangrike fenologiese gebeure is ondersoek in ‘Nadorcott’ mandaryn (C. reticulata Blanco) bome met ‘n alternerende drag patroon. Vruglading was beinvloed deur blomintensiteit. Intensiteit van opvolgblom is bepaal deur die aantal beskikbare blomposisies wat gedurende die voorafgaande seisoen se somer ontwikkel het, asook deur die invloed van vrugte op blomontwikkeling gedurende winter. Die gebrek aan somer vegetatiewe lootgroei in “aan”-bome was nie verwant aan die konsentrasie van blaarkoolhidrate nie. Die suikerkonsentrasie in wortels was die hoogste in “af”-bome en tydens volblom, en wortelgroei is waargeneem voor die vegetatiewe lootgroei-stuwing in die somer. Vrugte was die sterkste koolhidraat sink in “aan”-bome en het waarskynlik die balans tussen loot- en wortelgroei versteur. Die suikerkonsentrasie in wortels van “aan”-bome was laer, wortelgroei was afwesig en lootgroei gehalveer. Die inhoud van makro-elemente in blare was’n reaksie op vruglading en nie verwant aan vegetatiewe lootgroei of blom nie. Bepaling van fito-hormoon vlakke in blare en wortels het bevestig dat indool-3-asynsuur (IAA) primêr verantwoordelik was vir die inhibisie van somer vegetatiewe lootgroei. Hoë konsentrasies van dihidrofaasuur en die absisiensuur (ABA) glukose-ester in blare kon moontlik sinergisties met IAA opgetree het om te lei tot die lootgroei-inhibisie in “aan”-bome. Gevolglik het sitokinien toedienings nie somer vegetatiewe lootgroei gestimuleer nie. Hoë gibberellien inhoud in blare gedurende die vroeë winter het bygedra tot die ontwikkeling van min of geen blomme in “aan”-bome. Behandeling van “af”-bome en lote met 40 mg·L-1 gibberelliensuur gedurende winter het opvolgblom inhibeer, terwyl behandelings met 1000 mg·L-1 paclobutrazol of unikonasool op dieselfde tyd gelei het tot blomvorming en vrugontwikkeling vanaf knoppe op “aan” lote.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105137
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