The impact of shade netting on the microclimate of a citrus orchard and the tree's physiology

Prins, Michael du Toit (2018-12)

Thesis (MScAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: This study aimed to quantify the change in microclimate under shade netting and its effect on the leaf physiology of ‘Nadorcott’ mandarin (Citrus reticulata Blanco) in a winter rainfall production area. The first experiment quantified the change in orchard microclimate. The reduction in solar radiation affected the ambient, soil temperature and the number of hours for specific physiological and phenological temperature ranges under the shade netting. A reduced wind speed under the netting potentially led to less removal of moisture from the air and slightly increased relative humidity leading to a decrease in vapour pressure deficit (VPD) of the air. Therefore, the atmospheric evaporative demand was reduced and increased the volumetric soil water content. The second experiment focused on how the changes in microclimate affected a citrus leaf’s physiology. A reduction in VPD, especially in summer, led to increased stomatal conductance and resulted in increased CO2 assimilation rates of leaves under the shade netting. Therefore, the shade netting did not influence the leaf physiology negatively. Thirdly the total carbohydrate assimilation and distribution of different carbohydrate components, i.e., reducing sugars, polysaccharides, and starch, in leaves and roots were investigated as glucose equivalents. The most notable change under the shade netting in carbohydrate levels was in leaves, with an increased starch content especially noted after harvest. Root carbohydrates showed some differences between treatments, however, these could not only be attributed to the shade net treatment. The fourth experiment investigated how the change in microclimate under shade netting influenced the tree water status or tree water potential as well as fruit and canopy growth. This was accomplished by conducting pre-dawn pressure chamber readings of the main treatments as well as additional irrigation treatments. An increased tree water potential for trees under the shade netting was recorded as well as increased tree canopy volume and final fruit size. It can, therefore, be concluded that 20% white shade netting altered the orchard microclimate without negatively affecting the leaf physiology associated with photosynthesis in ‘Nadorcott’ mandarin trees. Leaves under the shade netting favoured the production of storage carbohydrates and the reduction in solar radiation and increased soil water content that led to a less water stressed environment, increasing the tree water potential. Further research should include how the microclimate can be altered in citrus production areas with different climatic conditions in South Africa.

AFRIKAANSE OPSOMMING: Die gebruik van skadunette in sitrusverbouing is besig om ʼn populêre tegnologie te word om die mikroklimaat van ʼn boord te verander. Sonstraling is ʼn belangrike faktor wat verskeie klimaat parameters asook fisiologiese prosesse in die plant beheer. Die verlaging van sonstraling kan dus die mikroklimaat verander en ʼn direkte of indirekte impak maak op die boomfisiologie wat geassosieer is met die fotosintese van ʼn sitrusboom. Vier eksperimente was uitgevoer om die moontlike veranderinge in mikroklimaat deur skadunette te dokumenteer en die invloed van dié veranderinge op die fisiologie van ‘Nadorcott’ mandaryn in ʼn winter reënval area te bepaal. Die eerste eksperiment was gemik om die verandering in mikroklimaat onder die skadunet waar te neem. Die opvallende verlaging in sonstraling het veranderinge in lug-, grondtemperatuur asook die hoeveelheid ure van temperatuur reekse vir belangrike fisiologiese en fenologiese prosesse te weeg gebring. ʼn Vermindering in windspoed onder die skadunet het gelei tot minder lugvermenging en vogverwydering en kon die verhoging in relatiewe humiditeit (RH) en verlaging in waterdampdruk verskil (WDV) veroorsaak het. Dus is die atmosfeer se aanvraag tot vog verlaag onder die skadunet en dit het daartoe gelei dat ʼn verhoogde voginhoud in grond oor die twee seisoene waargeneem was. Die tweede eksperiment was gefokus om die veranderinge van klimaatparameters wat normaalweg as beperkend beskou word vir ʼn sitrusblaar se fisiologie te dokumenteer en vas te stel hoe hierdie verandering fotosintese impakteer. Die resultate dui daarop dat die skadunet ʼn verlaging in WDV veroorsaak het, veral in die somer maande, wat gelei het tot verhoogde huidmondjiegeleiding en as gevolg daarvan ʼn verhoogde CO2 assimilasie tempo. Die skadunet het dus nie die blaar fisiologie negatief beïnvloed nie. Die totale koolhidraat-assimilasie en verspreiding van reduserende suikers, polisakkariedes en stysel in blare en wortels, was ondersoek. Verhoogde styselvlakke in die blare onder skadunet was veral waarneembaar na-oes. Wortelkoolhidrate het in spesifieke periodes veranderinge getoon, maar dit kon nie alleenlik aan die skadunet behandeling toegeskryf word nie. Laastens was ondersoek hoe die skadunet die boom se waterstatus asook vrug en vegetatiewe groei beïnvloed het deur drukbomlesings voor dagbreek te neem. Die resultate toon dat die bome onder die skadunet ʼn verhoogde, minder negatiewe, stamwaterpotensiaal gehad het. Die blaredak asook die finale vruggrootte onder die skadunet was verhoog. Daar kan dus ʼn voorlopige gevolgtrekking gemaak word dat 20% wit skadunet die mikroklimaat van ʼn boord verander het met geen negatiewe uitwerking op blaarfisiologie wat met fotosintese van die ‘Nadorcott’ mandaryn boom verband hou nie. Die blare onder die skadunet het die produksie van reserwe koolhidrate begunstig en die verhoogde voginhoud in die grond dui op ʼn laer vogstremming wat die boom se waterpotensiaal verhoog. In verdere navorsing kan verskillende produksie areas en klimaatstoestande waar sitrus in R.S.A verbou word ingesluit om verskille in die fisiologiese respons te bepaal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105226
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