Physiological dynamics of dormancy in apple buds grown in areas with insufficient winter chill

Thesis (PhDAgric)--Stellenbosch University, 2020.

ENGLISH ABSTRACT: Apple trees, that do not fulfil their winter chill requirement, undergo inadequate dormancy release resulting in poor budbreak with irregular and delayed flowering that impact negatively on fruit production and tree architecture. Under mild winter conditions, such as most production sites in the Western Cape region of South Africa, the application of chemical rest breaking agents is standard practise to artificially release dormancy and synchronise budbreak in spring to ensure sustainable and profitable apple production. However, rest breaking chemicals can be harmful to the buds and may result in phytotoxic damage if not applied optimally, therefore a good understanding of the dormancy status of the buds is valuable and necessary to mitigate this risk when applying a rest breaking agent. Furthermore, little information is available on the physiological and biochemical dynamics of commercially produced apple buds when under these milder conditions and no information is known for the ‘Cripps Pink’ variety when grown under climatically contrasting South African conditions. The main aim of this study was to investigate the effect of inadequate winter chill and the application of a blend of hydrogen cyanamide and mineral oil (HCo) as a rest breaking agent on the respiration rate and lipid composition of the terminal buds of full bearing ‘Cripps Pink’ apple trees. Dormant apple buds from two climatic regions, Elgin (insufficient winter chill) and the Koue Bokkeveld (sufficient winter chill) were compared in terms of their respiration rate and lipid composition. Buds from the Elgin region were also tested and compared after receiving a commercial HCo treatment at budswell. Shoots were excised and given a specified amount of artificial chill in the laboratory to act as a point of reference. Apple buds exposed to sufficient chill (orchard and laboratory) showed an early dormancy entrance, high maximum dormancy level and an early release, while buds from the milder area generally showed atypical dormancy behaviour. During winter, the total respiration rate and the rate of the main respiratory pathways (tricarboxylic acid cycle (TCA) and cytochrome C (CYT)) were reduced with the decline in temperature. Their levels increased again in the cold area at the beginning of spring to provide energy for growth resumption. In contrast, the main respiration levels remained low in the warm area and the pentose phosphate pathway (PPP) and alternative pathway (ALT) tested higher suggesting an attempt by the plant to compensate for the deficiency in energy production. The HCo treatment induced hypoxia in the buds and immediately decreased the total respiration as well as the main respiratory pathways (TCA and CYT). After this initial response the treated buds showed a significant increase in respiration and reached high levels towards budbreak and green-tip stage. This increase was not observed in the untreated buds from the mild winter climate, in contrast, these buds maintained a higher use of the PPP and ALT pathways. It was also seen that the HCo treatment followed by warmer spring temperatures hastened the occurrence of the green-tip stage. Both cold and warm winter temperatures also affected the lipid composition in the apple buds at the end of the winter period towards the onset of spring. Warmer winter conditions reduced the desaturase process preventing the desaturation of linoleic acid to linolenic acid, a polyunsaturated fatty acid with three double bonds. Lower free phospholipids and higher free sterols content were also detected in buds from the warmer area and is thought to reduce the membrane fluidity and permeability hampering budbreak and growth resumption even under favourable spring conditions. The high saturated and monounsaturated fatty acids in buds from the mild winter area confirmed the results and again pointed towards less fluid and permeable membranes. The HCo treated buds from a mild winter area showed results similar to buds that received sufficient winter chill, characterised by induced desaturation and higher linolenic acid levels enabling earlier budbreak. It is suggested that the artificial rest breaking of terminal apple buds, via the use of HCo, targets similar biological pathways and physiological mechanisms as the natural accumulation of sufficient chill when inducing growth resumption. In general, the physiological aspects under investigation in this study provided evidence that treatment with HCo, in the area with insufficient winter chill, act very similar to sufficient winter chill. It enhanced the main respiration pathways and stimulated the production of polyunsaturated fatty acid, which in turn, provides more fluid and permeable membranes that increase energy production needed for growth resumption in spring. Results from this study provide evidence that future development of artificial ways to target these same systems may improve apple cultivation in marginal production areas, especially in the Western Cape of South Africa.

AFRIKAANSE OPSOMMING: Appelbome, waarvan die koue vereiste nie beveredig is nie, ondergaan ‘n onvoldoende dormansieopheffing wat lei tot swak knopbreek, onreëlmatige en vertraagde blot en het ‘n negatiewe impak op vrugproduksie en boomargitektuur. Onder matige winterkondisies, soos in meeste van die produksie areas in die Wes-Kaap streek van Suid-Afrika, is die gebruik van chemiese rusbreekmiddels standaardpraktyk om dormasie kunsmatig op te hef, knopbreek in die lente te sinkroniseer en so volhoubare, winsgewende appelproduksie te verseker. Daar is egter min informasie beskikbaar rakende die fisiologiese en biochemiese dinamika van kommersieël geproduseerde appelknoppe onder hierdie matige winterkondisies en geen informasie is beskikbaar oor die ‘Cripps Pink’ kultivar onder klimaatkontrastrerende toestande in Suid-Afrika. Die hoof doel van hierdie studie was om die effek van onvoldoende winterkoue en die toediening van ‘n mengsel van waterstofsianamied en mineral olie (HCo), as ‘n rusbreekmiddel, op die respirasietempo en lipiedsamestelling van terminale knoppe van voldraende ‘Cripps Pink’ appelbome te ondersoek. Dormante knoppe van twee klimaatkonstrasterende gebiede, Elgin (onvoldoende winterkoue) en die Koue Bokkevled (genoegsame winterkoue), is vergelyk in terme van hul respirasietempo en lipiedsamestelling. Knoppe van die Elgin area is ook getoets en vergelyk nadat ‘n kommersieële behandeling van HCo toegedien is tydens knopswel. Lote is ook gesny en ‘n spesifieke hoeveelheid kunsmatige koue in die laboratorium gegee om te dien as verwysingspunt. Appelknoppe wat bootgestel is aan genoegsame koue (boord en laboratorium) het in vroeë ingang in dormansie, ‘n diep maksimum dormasievlak en ‘n vroeë uitgangspatroon getoon terwyl knoppe van die matige area oor die algemeen atipiese dormansiegedrag gehad het. Gedurende die winter het die totale respirasietempo en die tempo van die hoof respirasiepadweë (trikarboksielsuur sikus (TCA) en sitokroom C (CYT)) verlaag met die daling in temperatuur. In die koue area het hul vlakke weer aan die begin van die lente gestyg om so energie te verskaf vir die aanvang van groei. Daarteenoor, het die hoof respirasievlakke van die knoppe in die warmer area laag gebly en die pentosefosfaat padweg (PPP) en alternatiewe padweg (ALT) het hoër getoets wat ‘n poging deur die plant impliseer om te kompenseer vir die tekortkoming in energieproduksie. Die HCo behandeling het moontlik hipoksia in die knoppe geïnduseer wat ‘n onmiddelike afname in die totale repirasie asook die hoof respirasiepadweë (TCA en CYT) veroorsaak het. Na hierdie aanvanklike reaksie het die behandelde knoppe ‘n merkbare toename en hoë vlakke van respirasie getoon naby knopbreek en tydens die groenpunt stadium. Hierdie toename was nie waargeneem in die kontrole (onbehandelde) knoppe van die matige klimaatstreek nie, daarteenoor, het hierdie knoppe hoër vlakke van die PPP en ALT padweë gehandhaaf. Daar is ook gevind dat die HCo behandeling wat deur warmer lente kondisies gevolg is die voorkoms van die groepunt stadium verhaas het. Beide koue en warm winter temperature het ook die lipiedsamestelling in die appelknoppe aan die einde van die winterperiode en begin van die lente beïnvloed. Warm winterkondisies verlaag die desaturasieproses wat die onversadiging van linolïensuur na linoleniese suur, ‘n polionversadigde vetsuur met drie dubbelbindings, verhoed. Lae vry fosfolipied en hoë vry sterool inhoud is ook waargeneem in knoppe van die warmer area en daar word vermoed dat dit die vloeibaarheid en deurlaarbaarheid van die membrane verminder en so knopbreek en die aanvang van groei vertraag selfs onder gewensde lente toestande. Die hoë versadigde en mono-onversadigde vetsure in die knoppe van die matige winterstreek bevestig die resultate en verwys weer na ‘n laer vloeibaarheid en deurlaatbaarheid in die membrane. HCo behandelde appelknoppe van matige winterstreke toon dieselfde resultate as knoppe wat genoegsaam winterkoue ontvang het en is gekarakteriseer deur geïnduseerde desaturasie en hoë linoleniese suur vlakke wat vroeë knopbreek tot gevolg gehad het. Dit stel voor dat die kunsmatige rusbreking van terminale appelknoppe, deur die gebruik van HCo, dieselfde biologiese padweë en fisiologiese meganismes as die natuurlike akkumulasie van genoegsame winterkoue gebruik om groei te induseer. Oor die algemeen bied die fisiologiese aspekte, wat deur die studie ondersoek is, bewyse dat die behandeling van HCo dieselfde meganismes gebruik as genoegsame winterkoue deur die hoof respirasiepadweë en die produksie van poli-onversadigde vetsure te stimuleer wat tot gevolg het dat die membrane meer vloeibaar en deurlaatbaar word en sodoende die energie produksie, wat nodig is vir die hervatting van groei in die lente, aanmoedig.

Doctoral