A study of the dynamics of sunburn reduction in apple (Malus domestica) using foliar applications of boron and calcium

Mwije, Anthony (2020-03)

Thesis (PhDAgric)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Apple fruit sunburn browning (SBB) is largely a climate-related disorder that reduces fruit quality leading to serious financial losses. The aim of this research was to unravel the mode-of-action by which post-full-bloom foliar boron plus calcium (B+Ca) suppress fruit SBB incidence in apple orchards. Scientific elucidation of such mode-of-action is essential for successful recommendation and adoption of this low-cost approach at farm level. Therefore, in this study, four varying foliar B+Ca formulations were applied post-full-bloom in orchards of a bi-colour (‘Cripps Pink’) and two green (‘Golden Delicious’ and ‘Granny Smith’) apple cultivars. The experimental sites / orchards were located in Elgin and Stellenbosch in Western Cape Province, South Africa, and the three cultivars chosen for this study sharply contrast in SBB susceptibility. A preceding and related study found that peel tissue anatomical differences and mineral concentrations did not to explain the SBB incidence reduction, hence this study focused on investigating the possibility of a biochemical-based mode-of-action. Thus, peel tissue biochemicals (phenolics, chlorophyll, carotenoid and total peroxides) were determined, and compared to SBB incidence at harvest. In 2014/15 season, fruit peel was sampled at four early maturity stages / days after full bloom (DAFB). In 2015/16, samples were collected at four bi-weekly intervals towards fruit harvest maturity window. Results showed that treatments influenced peel biochemical levels in varying degrees and with cases of significant (p < 0.05) interaction effects with fruit maturity stages. Significant SBB suppression occurred for ‘Cripps Pink’ (2015/16) and ‘Golden Delicious’ experiments, where the control (no B+Ca) had highest incidence. In ‘Granny Smith’, the SBB differences within treatments did not differ significantly. No treatment-induced changes in any peel biochemical occurred in patterns that could yield the sought mode-of-action; for instance, all treatments reduced SBB considerably in ‘Cripps Pink’, whereas it was the high B treatments (0.08 and 0.17 g.l-1) in ‘Golden Delicious’. This highlighted the influence of cultivar and treatment formulation in respective SBB suppression outcomes and corresponding in peel biochemical levels. With treatment formulation, inclusion of zinc (Zn) significantly lowered photosynthetic pigment attributes, and associated with SBB incidence that was on par with the control (no B+Ca). Overall, this study concluded that different cultivars require different foliar B+Ca formulations to supress SBB, and generally, Zn should not be included. Further results from a multivariate analysis indicated that the sought mode-of-action could be under an additive-metabolite-action phenomenon, involving contributory roles of several peel biochemicals as influenced by particular foliar B+Ca treatments. However, further studies are required to establish the physiological mechanism underlying this postulated mode-of-action.

AFRIKAANSE OPSOMMING: Sonbrandverbruining in appels is grootliks ‘n klimaatsgebonde defek wat vrugkwaliteit verlaag en lei tot ernstige finasiële verliese. Die doel van die navorsing was om die metode-van-werking van die boor plus kalsium (B+Ca) vooroesblaarspuite, wat sonbrandverbruining (SBB) voorkoms in appelboorde verminder, te ontrafel. Die wetenskaplike toeligting van die metode-van-werking is essensieël vir die suksesvolle aanbeveling en implementering van die lae-koste benadering op plaas vlak. Dus is vier B+Ca formulasies na volblom toegedien op ‘n twee-kleur (‘Cripps Pink’) en twee, groen (‘Golden Delicious’ en ‘Granny Smith’) appel kultivars. Die proefpersele/ boorde was geleë in Elgin en Stellenbosch in die Wes-Kaap provinsie, Suid-Afria en die drie kultivars wat vir die studie gekies is, verteenwoordig ook skerp kontraste in SBB voorkoms. ‘n Voorafgaande en verwante studie het gevind dat die skilweefselanatomie en minerale voedingselement-konsentrasies nie SBB voorkoms verlaging verklaar nie, daarom het hierdie studie gefokus op die moontlike biochemies – gebasseerde metode-van-werking. Dus is skilweefsel biochemiese komponente (fenole, chlorofil, karotenoïede en totale peroksiede) bepaal en vergelyk met die SBB voorkoms by oes. In die 2014/15 seisoen, is vrugskille gemonster op vier vroeë volwassenheid stadia/ dae na volblom (DNVB). In 2015/16 is monsters ingesamel op vier twee-weeklikse intervalle van die oesrypheidsvenster van vrugte. Resultate het getoon dat behandelings die vrug biochemiesevlakke in variërende grade beïnvloed het, asook met betekenisvolle (p < 0.02) interaksie effekte met vrugvolwasenheid stadia. Betekenisvolle SBB vermindering het voorgekom net in ‘Cripps Pink’ (2015/16) en ‘Golden Delicioius’ proewe, waar die kontrole (geen B+Ca) die hoogste voorkoms gehad het. In ‘Granny Smith’, het geen betekenisvolle verskille in SBB tussen behandelings voorgekom nie. Geen behandelings-geïnduseerde veranderinge in die skilbiochemie het voorgekom in patrone wat die verlangde metode-van-werking kon verklaar nie; byvoorbeeld het al die behandeling SBB aansienlik verlaag in ‘Cripps Pink’ (2015/16), tenoor slegs behandelings met verhoogde B vlakke (0.08 en 0.17 g.l-1) in ‘Golden Delicious’. By formulasies vir die behandelings, het die insluit van sink (Zn) alle fotosintetise eienskappe oor alle kultivars verlaag en was dit geassosieer met SBBB voorkoms wat soortgelyk was as die kontrole (geen B +Ca). Oorkoepelend het die studie bepaal dat verskillend kultivars verskillende behandelings vir effektiewe SBB-onderdrukking benodig, en dat Zn normaalweg nie ingesluit moet word nie. Verdere resultate van ‘n veelvuldige analise het aangedui dat die verlangde metode-van-werking onder ‘n fenomoon van bykomende-metaboliet-aksie kan funksioneer, wat die bydraende rolle van verskeie skil biochemiese komponente behels van spesifieke blaar B+Ca behandelings. Nietemin is verdere studies nodig om die onderliggende fisiologiese meganisme van die gepostuleerde metode-van-werking te bepaal.

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