Improving crop load and fruit quality in deciduous fruit using plant growth regulators

Thesis (MScAgric)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: Crop load in deciduous fruit trees plays an important role in fruit quality, yield and alternate bearing. Young ‘Packham’s Triumph’ trees fail to achieve adequate yields, while apple, peach, and nectarine trees, set fruit abundantly, requiring annual thinning. Crop load can be manipulated using plant growth regulators to enhance or reduce abscission. Hand thinning remains the popular thinning method in stone fruit. Due to increasing labor costs alternatives are needed. The ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC) has recently shown potential as a chemical thinner. While ACC thinned at various phenological stages (pink bud, balloon, early flower and late flower), later stages produced more ethylene resulting in greater thinning. In ‘Keisie’, ACC at 450 µl.L⁻¹ applied when fruitlets were 8-10 mm in diameter achieved adequate thinning, leading to an increased fruit weight but a slight reduction in yield. In nectarine (‘Alpine’, ‘August Red’, ‘Zee Fire’, ‘Ruby Rose’ and ‘Luciana’), ACC thinned successfully when applied at pink bud – 20% flowering to a greater or lesser extent. Second applications to the upper ⅓ of the canopy can enhance thinning, if applied at the correct phenological stage. Where ACC successfully thinned, an increase in fruit weight was observed. ACC did not affect yield or harvest distribution. Generally, two applications at 450 + 225 µl.L⁻¹ ACC or a single application at 450 µl.L⁻¹ ACC where trees bloom uniform, seems promising in nectarine. No leaf toxicity or split pit was induced by ACC. Future studies in stone fruit should evaluate ACC closer to full bloom. In apple, chemical thinners are considered standard commercial practice. However, existing thinners are only effective until 15 mm fruitlet diameter and therefore the need remains for a reliable “rescue thinner”. Recently, ACC has shown thinning potential in this late window. When 450 µl.L⁻¹ ACC was applied at 15-20 mm fruitlet diameter it successfully thinned ‘Royal Gala’, ‘Cripps’ Pink’, ‘Rosy Glow’, ‘Royal Beaut’ (Gala type) and at 560 µl.L⁻¹ ACC in ‘Cripps’ Red’. Results in ‘Fuji’ varied between seasons, although a full bloom application seemed promising in one trial. ACC was combined with existing thinners but did not always thin additively. At the recommended rates, ACC sometimes affected yield, ground color, blush intensity and percentage, seed number or stem-end russet, but variable results were observed between trials. Fruit weight was improved in some, but not all trials. Trees treated with ACC tended to have a better return bloom the following spring. Aminoethoxyvinylglycine (AVG) is an inhibitor of ethylene biosynthesis and can reduce premature fruitlet abscission in young ‘Packham’s Triumph’. The effect of AVG varied between trials due to weather conditions during and after AVG application. Generally, an increase in final fruit set and/or yield was observed when AVG was applied during cool, damp conditions. AVG at rates between 125 mg.L⁻¹ and 250 mg.L⁻¹ was effective one or two weeks after full bloom, therefore application timing should be altered according to weather conditions. An expected decrease in fruit weight was observed in trials were fruit set and/or yield was increased. AVG did not affect fruit firmness, the occurrence of retiform russet or skew calyx-ends.

AFRIKAANSE OPSOMMING: Oeslading speel ‘n rol in vrugkwaliteit asook in alternerende drag in sagtevrugte bome. Jong ‘Packham’s Triumph’ bome sukkel om in drag te kom. In appel, perske en nektarien, set bome ‘n oormatige oeslading en moet dus jaarliks uitgedun word om vrugkwaliteit en gereelde drag te optimiseer. Vrugval kan gestimuleer of verminder word met behulp van plantgroeireguleerders. Handuitdunning bly die populêre keuse in steenvrugte om oeslading te verlaag. As gevolg van stygende arbeidskoste word ‘n alternatief benodig. Die etileenvoorlooper, 1-aminosiklopropaan-1- karboksielsuur (ACC) toon potensiaal as ‘n chemiese uitdunmiddel. ACC het by verskeie fenologiese stadiums (pinkknop, ballon, vroeë blom en laat-blom) uitgedun, alhoewel gevorderde stadiums meer etileen produseer het en gevolglik ook beter uitgedun het. In ‘Keisie’ het ACC teen 450 µl.L⁻¹ toe vrugdeursnee 8-10 mm was voldoende uitgedun wat gelei het tot ‘n verhoogde vruggewig maar effens laer opbrengs. In nektarine (‘Alpine’, ‘August Red’, ‘Zee Fire’, ‘Ruby Rose’ and ‘Luciana’) het ACC suksevol maar varieerend uitgedun toe bome in pinkknop – 20% blom was. Tweede spuite van ACC gerig op die boonste ⅓ van die boom het meer uitgedun, indien dit toegedien was tydens die korrekte fenologiese stadium. Waar ACC suksesvol uitgedun het, was vrugte swaarder. ACC het nie oes of oesverspreiding geaffekteer nie. Oor die algemeen lyk twee toedieings 450 + 225 µl.L⁻¹ ACC of ‘n enkel toediening van 450 µl.L⁻¹ ACC indien bome eweredig blom, belowend op nektariens. ACC het geen fitotoksisiteit of gesplete pitte veroorsaak nie. Toekomsitige studies moet ACC evalueer nader aan volblom. In appelbome is die gebruik van chemiese uitdunmiddels die kommersiële standard praktyk, alhoewel bestaande uitdunmiddels net effektief is tot-en-met 15 mm vrugdeurnsnee, dus is daar ‘n vraag na ‘n betroubare “rescue thinner”. ACC het onlangs uitdunpotensiaal getoon in hierdie laat venster. ACC teen 450 µl.L⁻¹ toe vrugdeursnee 15-20 mm was, het ‘Royal Gala’, ‘Cripps’ Pink’, ‘Rosy Glow’ en ‘Royal Beaut’ (Gala tipe) suksesvol uitgedun asook teen 560 µl.L⁻¹ ACC in ‘Cripps’ Red’. Resultate in ‘Fuji’ het varieër tussen seisoene, alhoewel ‘n volblom toediening belowend gelyk het in een seisoen. ACC was ook gekombineer met bestaande uitdunmiddels maar het nie altyd sterker uitgedun nie. Teen die aanbevole dosis het ACC die opbrengs, agtergrondkleur, blos-intensiteit of -persentasie, aantal sade of stingelentverruwing soms beïnvloed, maar resultate tussen proewe het verskil. Vruggewig was in sommige proewe verbeter. Bome wat met ACC behandel was, was geneig om ‘n beter opvolgblom in die volgende lente te hê. Aminoetoksifinielglisien (AVG) inhibeer etileensintese en kan absissie van jong vruggies in jong ‘Packham’s Triumph’ verminder. Die effek van AVG het verskil tussen proewe as gevolg van die weersomstandighede tydens en na AVG toediening. Oor die algemeen was daar ‘n toename in finale vrugset en/of oes was bereik indien AVG toegedien was tydens koel, klam omstandighede. AVG teen 125 mg.L⁻¹ en 250 mg.L⁻¹ AVG was effektief toe dit een of twee weke na volblom toegedien was. Dus moet toediening geskeduleer word volgens weersomstandighede. ‘n Verwagte laer vruggewig was ‘n gevolg van ‘n toename in vrugset en/of oes. AVG het nie vrugfermheid, of die voorkoms van vrugverruwing of skewe kelkente beïnvloed nie.

Masters