The use of plant growth regulators to improve apple pedicel length and reduce fruit cracking

Van der Westhuizen, Marno Wiehan (2020-04)

Thesis (MScAgric)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Maintaining apple fruit quality is complex and factors such as short fruit pedicels and fruit cracking reduce quality. Fruit pedicels of certain cultivars in the Elgin-Grabouw-Vyeboom-Villiersdorp (EGVV) region are problematic as they tend to be short, stubby and rigid and cause losses pre- and post-harvest. Fruit pedicel dimensions also vary within trees, complicating the control of the problem. Gibberellin A4 and A7 (GA4+7) on its own and in combination with 6-benzyladenine (6-BA) were evaluated to improve the pedicel lengths of ‘Nicoter’, ‘Fuji’ and ‘Cripps’ Pink’, without having significant side-effects on fruit set, yield, fruit quality and return bloom. We also evaluated the effect of three bearing shoot types (terminal on spur, short shoot and long shoot) on the inflorescence composition as well as the effect of the three shoot types and two flower positions in the inflorescence (king and lateral) on pedicel dimensions and flower, and subsequent fruit quality. Gibberellins A4+7 successfully increased the pedicel length of ‘Nicoter’, ‘Fuji’ and ‘Cripps’ Pink’ apples when applied multiple times between tight cluster and full bloom. Although higher rates of GA4+7 (100 and 200 mg·L-1) significantly increased the pedicel length, the lower rates (10 to 50 mg·L-1) gave fewer negative side-effects on fruit set, yield and return bloom. The recommended treatments to increase pedicel length are 50 mg·L-1 GA4+7, applied twice, 10 mg·L-1 GA4+7, applied four times, and 20 mg·L-1 GA4+7, applied twice for ‘Nicoter’, ‘Fuji’ and ‘Cripps’ Pink’, respectively. Regarding the variable pedicel dimensions, neither the flower position, nor the type of bearing shoot had strong effects on the pedicel size and further research is needed to identify the origin of the intra-plant variation in pedicel lengths. Fruit cracking on ‘Fuji’ (pedicel-end) and ‘Rosy Glow’ (calyx-end) has become a problem in the Ceres region during the past few seasons. GA4+7 on ‘Fuji’ and GA4+7 + 6-BA on ‘Fuji’ and ‘Rosy Glow’ were thus applied at an early and later period during the cell division phase of fruit growth to reduce cracking without causing side-effects on fruit set, yield, fruit quality and return bloom. Unfortunately, due to specific weather conditions and adjustment of management practices, no cracking was observed in the two seasons, except for low levels of pedicel-end cracking on ‘Fuji’ after controlled atmosphere storage in the second season. The return bloom of ‘Fuji’ was, however, reduced by all the GA4+7 and GA4+7 + 6-BA rates (10, 20 and 40 mg·L-1) during the first season and decreased linearly with increasing rate of GA4+7, applied seven, 14 and 21 days after full bloom (d.a.f.b). It also decreased linearly with increasing rate of GA4+7 + 6-BA, applied seven, 21 and 35 d.a.f.b. during both seasons. Higher rates of GA4+7 and GA4+7 + 6-BA should thus be used with caution when considered for cracking control.

AFRIKAANSE OPSOMMING: Handhawing van appelvrugkwaliteit is kompleks en faktore soos kort vrugstingels en vrugkrake benadeel vrugkwaliteit. Stingels van sekere kultivars in die Elgin-Grabouw-Vyeboom-Villiersdorp (EGVV) produksiearea neig om kort, dik en rigied te wees wat lei tot voor- en na-oesverliese. Die stingelafmetings varieer ook binne bome wat die beheer van die probleem verder kompliseer. Gibberellien A4 en A7 (GA4+7), op sy eie, of in kombinasie met 6-bensieladenien (6-BA) was geëvalueer vir die vermoë om stingellengtes van ‘Nicoter’, ‘Fuji’ en ‘Cripps’ Pink’ te verleng sonder om enige negatiewe effekte op vrugset, opbrengs, vrugkwaliteit en opvolgblom te hê. Ons het ook die effek van drie loottipes (terminaal op spore, kort lote en lang lote) op die bloeiwyse samestelling bestudeer, sowel as die effek van die drie loottipes en twee blomposisies in die bloeiwyse (terminaal en lateraal) op stingelafmetings, en die kwaliteit van blomme en die daaropvolgende vrugte. Opeenvolgende GA4+7 toedienings het die stingels van ‘Nicoter’, ‘Fuji’ en ‘Cripps’ Pink’ suksesvol verleng wanneer dit tussen die katpoot en volblom stadiums toegedien is. Al het die hoë konsentrasies (100 en 200 mg·L-1)‘n aansienlike toename in die stingellengtes veroorsaak, is dit nie die aanbevole behandelings nie aangesien die laer konsentrasies minder negatiewe effekte gehad het op vrugset, opbrengs en opvolgblom. Die aanbevole behandelings om stingels te verleng vir ‘Nicoter’, ‘Fuji’ en ‘Cripps’ Pink’ is dus onderskeidelik 50 mg·L-1 GA4+7 tweemaal toegedien, 10 mg·L-1 GA4+7, viermaal toegedien, en 20 mg·L-1 GA4+7 tweemaal toegedien. Met betrekking tot die varierende stingellengtes, het nóg die blomposisie, nóg die tipe loot beduidende effekte op stingelafmetings gehad. Verdere navorsing is dus nodig om die oorsprong van die intra-plant variasie in stingellengtes te identifiseer. Vrugkrake op ‘Fuji’ (stingel-ent) en ‘Rosy Glow’ (kelk-ent) het die afgelope paar seisoene ʼn probleem geword in die Ceres area. Bespuitings van GA4+7 op ‘Fuji’ en GA4+7 + 6-BA op ‘Fuji’ en ‘Rosy Glow’ was dus geëvalueer tydens ʼn vroeë en latere stadium van die seldelingsfase van vruggroei om die krake te verminder, sonder om negatiewe effekte te hê op vrugset, opbrengs, vrugkwaliteit en opvolgblom. Weens spesifieke weerstoestande en aanpassings aan bestuurspraktyke het geen krake in beide seisoene ontwikkel nie, behalwe vir ʼn klein hoeveelheid stingel-ent krake op ‘Fuji’ na beheerde atmosfeer opberging in die tweede seisoen. Die opvolgblom van ‘Fuji’ was egter verminder deur al die GA4+7 en GA4+7 + 6-BA konsentrasies (10, 20 en 40 mg·L-1) gedurende die eerste seisoen en het liniêr afgeneem met toenemende konsentrasie van GA4+7 wat sewe, 14 en 21 dae na volblom toegedien is. Verder het die opvolgblom van ‘Fuji’ in beide seisoene ook liniêr afgeneem met toenemende konsentrasie van GA4+7 + 6-BA wat sewe, 21 en 35 dae na volblom toegedien is. Hoë konsentrasies van GA4+7 en GA4+7 + 6-BA moet dus versigtig gebruik word wanneer dit oorweeg word om krake met hierdie plantgroeireguleerders te beheer.

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