Calcium uptake and distribution in relation to periods of active white root growth in young, potted apple trees (Malus domestica Borkh. cv. ‘Golden Delicious’) in the Western Cape

Cameron, Anouska (2019-02)

Thesis (MScAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Two-year-old ‘Golden Delicious’/M7 apple trees were chosen as the experimental material for this study. Potted trees were fertigated with a low calcium (Ca), balanced nutrient solution over two consecutive seasons (2015/16 and 2016/17). In the first season, trees were either left to drop their leaves naturally (NLD) during autumn and winter or were completely defoliated by hand earlier in autumn (AD), during April 2016. At the beginning of the second root flush, in winter (May 2016), calcium nitrate (Ca(NO3)2) was applied to the soil at three different rates: no additional Ca (control), moderate Ca (1X) and high Ca (2X). At the end of winter (August 2016), all trees were destructively sampled for macro mineral analysis of the different plant organs. Although an increase in soil Ca supply did not have a significant impact on white root numbers during winter (determined by minirhizotrons), early leaf loss via AD caused a drastic decline in white root numbers during winter. Despite this, Ca uptake and reserve accumulation in the roots and stems of the 2X(AD) treatment was not compromised, indicating possible active uptake by older, brown roots. Sap flow rates in leafless trees (determined by dendrometers) also remained above zero during winter, suggesting that leaf transpiration-driven sap flow is not essential for root Ca uptake and translocation in the xylem. In the NLD trees, an increase in autumn soil Ca supply resulted in a significant increase in leaf Ca concentration following the extended leaf drop period. At the same time, scanning electron microscopy and wavelengthdispersive x-ray spectroscopy (SEM-WDS) results indicated rapid uptake of soil-applied Ca by apple white root tips during winter. As 50 % leaf drop was reached late in winter (July), a substantial fraction of total Ca uptake in the 2X(NLD) treatment was lost via leaf abscission at the expense of Ca allocation to the reserve tissues during winter. The preferential allocation of Ca to the leaves was possibly the result of relative high rates of leaf transpiration-driven sap flow prior to 50 % leaf drop. Calcium partitioning in relation to periods of active white root growth, after harvest in the second season, were also investigated. Treatments comprising Ca(NO3)2 soil applications included a summer-only (2X) treatment, an autumn-only (2X) treatment and a summer (1X)/autumn (1X) treatment. Although fruit Ca concentrations were satisfactory at harvest, no significant differences were found between treatments. Whilst confirming primary xylem transport to the more dominant leaf and shoot sinks, the lack of response to summer applications (current season, 2016/17) indicates the predominant role of Ca reserve accumulation in the trees in supporting new growth the following season. In contrast to the summer/autumn treatment, a significantly higher % of total Ca content was found in the fruit of the autumn-only treatment. These results suggest that under local conditions of insufficient winter chilling, relatively high rates of soil-applied Ca(NO3)2, both in summer and autumn during active white root growth, may benefit the next season’s crop through remobilization of stored Ca in the roots and reserve tissues of the stems.

AFRIKAANSE OPSOMMING: Twee-jaar ‘Golden Delicious’/M7 appelbome is as eksperimentele materiaal vir hierdie studie gekies. Bome is uitgeplant in potte en sproeibemes met ‘n lae Ca, gebalanseerde voedingsoplossing oor twee seisoene (2015/16 en 2016/17). In die eerste seisoen, is bome of gelaat om normale blaarval (NBV) te ondergaan gedurende die herfs en winter, of per hand ontblaar in die herfs (HO), gedurende April 2016. Aan die begin van die tweede witwortel groeifase, in die winter (Mei 2016), is bome behandel met drie verskillende konsentrasies van kalsiumnitraat (Ca(NO3)2) as grondtoedienings: geen addisionele Ca (kontrole), matige Ca (1X) en hoë Ca (2X). Aan die einde van die winter (Augustus 2016), is alle bome destruktief gemonster vir minerale (makro) analises van die verskillende plant organe. Al het ‘n toename in grond Ca toevoer nie ‘n betekenisvolle impak op witwortel getalle in die winter gehad nie (bepaal deur minirhizotrons), het vroeë blaarverlies deur HO tot ‘n drastiese afname in witwortel getalle gedurende die winter gelei. Ten spyte daarvan, is Ca opname en reserwe akkumulasie in die wortels en stamme van die 2X(HO) behandeling nie negatief geaffekteer nie, wat dui op moontlike aktiewe opname deur ouer, bruin wortels. Sapvloeitempo in blaarlose bome (bepaal deur dendrometers) het ook bokant nul gebly gedurende die winter, wat daarop dui dat blaar transpirasie-gedrewe sapvloei nie noodsaaklik is vir Ca opname en translokasie in die xileem nie. Na die verlengde blaarval tydperk in die NBV bome, het ‘n toename in grond Ca toevoer tot ‘n beduidende toename in blaar Ca konsentrasie gelei. Terselfdertyd, dui skanderingselektromikroskopie en golflengte-dispersiewe x-straal spektroskopie (SEM-WDS) resultate op vinnige opname van grondtoedienings van Ca in appel witwortelpunte gedurende die winter. Aangesien 50 % blaarval eers laat in die winter (Julie 2016) bereik is, was ‘n aansienlike fraksie van die totale Ca opname in die 2X(NBV) behandeling deur middel van blaarval verloor, ten koste van Ca allokasie na die reserwe weefsels gedurende die winter. Die hoë persentasie Ca wat na die blare geallokeer is, is die moontlike gevolg van relatiewe hoë blaar transpirasie-gedrewe sapvloeitempo’s in die bome voor 50 % blaarval. Die verwantskap tussen Ca opname en verspreiding teenoor periodes van aktiewe witwortelgroei na oes, in die tweede seisoen, is ook ondersoek. Behandelings bestaande uit Ca(NO3)2 grondtoedienings, het ‘n somer-alleen (2X), ‘n herfs-alleen (2X) en ‘n somer (1X)/herfs (1X) behandeling ingesluit. Vrug Ca konsentrasies tydens oes was bevredigend en daar was geen betekenisvolle verskille in vrug Ca konsentrasie tussen behandelings nie. Terwyl primêre xileem vervoer na die meer dominante blaar- en lootsinke bevestig is, dui die gebrek aan reaksie op somer grondtoedienings (huidige seisoen, 2016/17) op die oorheersende rol van Ca opberging in die bome om nuwe groei in die volgende seisoen te ondersteun. In teenstelling met die somer/herfs behandeling, is ‘n noemenswaardige hoër % van totale Ca inhoud in die vrugte van die herfs-alleen behandeling gevind. Hierdie resultate dui daarop dat onder plaaslike toestande van onvoldoende winterkoue, mag relatiewe hoë vlakke van Ca(NO3)2 grondtoedienings, beide in die somer en die herfs tydens aktiewe witwortelgroei, moonlik die daaropvolgende seisoen se oes bevoordeel deur remobilisasie van opgebergte Ca in die wortels en reserwe weefsels in die stamme.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105632
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