Studies on the phenology and carbohydrate status of alternate bearing ‘Nadorcott’ mandarin trees

Van der Merwe, Izak Schalk (2012-12)

Thesis (MscAgric)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: Alternate bearing is a common phenomenon in most commercial perennial fruit trees. In citrus, the “on” year consists of a heavy crop load with mostly small fruit, often followed by an “off” year with few, large and coarse fruit. Carbohydrates play an important role in affecting alternate bearing, especially during fruit set, but also flowering and fruit maturation, and are essential in maintaining a regular bearing habit. Changes in starch and total sugar accumulation in the leaves of the alternate bearing prone „Nadorcott‟ mandarin were followed over an entire season for both “on” and “off” trees to evaluate the possibility of using carbohydrate levels to predict bearing potential. Starch accumulation followed a distinct pattern with differences between “on” and “off” trees visible in April and May. Starch concentrations in April showed a moderate negative correlation with yield and a moderate positive correlation with return bloom. Rapid starch accumulation started prior to harvest with a peak at the beginning of flowering. Thereafter a sharp decrease in starch levels occurred until after full bloom followed by a steady decrease from physiological fruit drop towards fruit maturity. “On” trees bore 53% more fruit than “off” trees, but the return bloom of “off” trees was 140% more than “on” trees, thus illustrating the negative effect that a large crop has on the next season‟s bloom. It was concluded that for „Nadorcott‟ mandarin, leaf starch concentration in April can be used as an indication of bearing potential the following season. Pruning is a well-established management tool to control alternate bearing. Summer pruned trees had more spring flush vegetative shoots, more nodes per shoot and also more growth per parent shoot overall, compared to unpruned, control trees. Control trees had higher light levels inside the tree compared to summer pruned trees. However, no differences in leaf starch or total sugar levels during April were measured between treatments. Production of new bearing sites should therefore be considered in this experiment. It was concluded that pruning during November followed by early regrowth management gave the best balance between light penetration and production of new bearing units. Pruning in November, rather than during winter, also allowed selective pruning of shoots with or without flowers, depending on whether it was an “on” or an “off” year. When fruit thinning chemicals are applied at the optimum time and concentration, it is an effective way of moderating an alternate bearing cycle. Unfortunately no significant differences were obtained in this experiment even though the thinning treatments did show slightly higher starch levels in April 2012, indicating that the demand for energy was lower in these trees. This response was most likely due to the slightly lower yield and fruit number of the thinning treatments compared to the control. The dichlorprop treatment also showed a higher fruit growth rate, and future research should focus on timing of chemical thinning sprays in late mandarin cultivars

AFRIKAANSE OPSOMMING: Alternerende drag is 'n algemene verskynsel by die meeste meerjarige kommersiële vrugtebome. In die “aan” jaar by sitrus word 'n swaar oeslading gedra wat hoofsaaklik uit klein vrugte bestaan gevolg deur 'n “af” jaar met minder, groter en growwer vrugte. Koolhidrate speel 'n belangrike rol, veral gedurende vrugset, maar ook tydens blomtyd en vrugrypwording, en is noodsaaklik om ‟n reëlmatige drasiklus te verseker. Veranderinge in stysel- en totale suiker akkumulasie in die blare van „Nadorcott‟ mandarynbome, is deur die loop van 'n volle seisoen gevolg op beide “aan” en “af” bome om die moontlikheid te ondersoek dat koolhidraatvlakke gebruik kan word om dragpotensiaal te bepaal. Verskille tussen “aan” en “af” bome was in April en Mei sigbaar. Styselvlakke in April het 'n matige negatiewe korrelasie met drag getoon en 'n matige positiewe korrelasie met die volgende seisoen se blom. Styselvlakke het voor oestyd begin toeneem en aan die begin van blomtyd 'n piek bereik waarna 'n skerp daling voorgekom het tot na volblom. Dit is gevolg deur 'n geleidelike afname vanaf fisiologiese vrugval totdat die vrugte ryp was. “Aan” bome het 53% meer vrugte gedra as “af” bome, maar die volgende seisoen se blom van “af” bome was 140% meer. Dit illustreer die negatiewe effek wat ‟n groot oes op die volgende seisoen se blom het. Die gevolgtrekking is dat styselvlakke in blare gedurende April gebruik kan word as 'n aanduiding van die drag-potensiaal vir die komende seisoen vir „Nadorcott‟ mandarynbome. Snoei is 'n gevestigde manier om alternerende drag te beheer. Bome wat in die somer gesnoei is, het 'n groter aantal vegetatiewe lote in die lente, meer knoppe per loot en ook meer groei op ouer-lote gehad in vergelyking met die kontrole bome wat nie gesnoei is. Kontrole bome het hoër ligvlakke binne-in die boom gehad in vergelyking met die bome wat in die somer gesnoei is. Daar is egter in April geen verskille gemeet in die blare se stysel- en totale suikervlakke tussen behandelings nie. Produksie van nuwe dra-posisies moet dus vir hierdie eksperiment in ag geneem word. Die gevolgtrekking was dat, deur in November te snoei en vroeë bestuur van nuwe groei toe te pas, die beste boomvorm verkry is. Deur in November te snoei eerder as in die winter, kon daar ook selektief gesnoei word aan lote met of sonder blomme, afhangende of dit ‟n “aan” of “af” jaar was. Korrekte chemiese vruguitdunning is een van die mees effektiewe maniere om ‟n alternerende drag-siklus te verminder. Ongelukkig is geen betekenisvolle verskille in hierdie eksperiment verkry nie, ten spyte van die feit dat die uitdunningsbehandelings wel ietwat hoër styselvlakke in April 2012 getoon het. Dit dui daarop dat die behoefte aan energie in hierdie bome laer was. Die reaksie was waarskynlik te wyte aan die effens laer oes en vruggetalle as gevolg van die uitdunningsbehandelings in vergelyking met die kontrole. Die dichlorprop-behandeling het ook ‟n hoërvruggroeitempo gestimuleer. Navorsing in die toekoms behoort te fokus op die tydberekening waarvolgens die chemiese uitdunningsmiddels op laat mandarynkultivars toegedien word.

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