Physiological measurements of daily daylight fertigated citrus trees

Schoeman, Stephanus P. (Stephanus Philippus) (2002-12)

Thesis (MScAgric)--University of Stellenbosch, 2002.

Thesis

ENGLISH ABSTRACT: Daily daylight fertigation regimes (involvinq drip fertigation, on a daily basis, during the daylight hours) are becoming widely used in the South African citrus industry in the endeavor to enhance tree productivity. Such regimes could provide sensitive nutrient and moisture management, reducing plant stress in general as well as the response time to root-directed cultural activities. There is a need to evaluate the efficacy of daily daylight fertigation systems relative to conventional irrigation systems. Standard horticultural evaluation of orchard management practices is very time consuming. We opted for physiological studies comparing plant stress levels, in an attempt to quantify plant performance under each system. Citrus trees under daily daylight fertigation and conventional micro-jet and drip-irrigated regimes were monitored to establish plant stress levels as indicated by sap flow, xylem water potential, stomatal conductance and chlorophyll a fluorescence. Plants under a daily daylight fertigation regime are believed to have good soil water conditions in their rooting volume, and therefore experience negligible baseline levels of stress. The trees do, however, experience midday depression in stomatal conductance, to a lesser degree, but not unlike trees under conventional regimes. It appears as if a larger rooting volume of microjet irrigation regimes enhances recovery from the midday depression. It is recommended that producers optimise productivity during the moming hours, by early irrigation, so that plants can function optimally, whilst environmental conditions are most favourable for high physiological activity. We also assessed the effect of withholding water from trees adapted to a daily daylight fertigation regime to evaluate the risk involved with short-term water deficits in trees adapted to this regime, as well as the usefulness of physiological techniques for identifying water stress. Stomatal conductance and xylem water potential indicated water stress sooner than the other physiological parameters. Citrus trees seem to be relatively insensitive to water deficit stress as measured by sap flow and chlorophyll a fluorescence. Sap flow is buffered by tree capacitance, and although mediated via stomatal conductance, atmospheric conditions and not the soil water content primarily determine it. As daily fertigation is applied to trees under DOF regimes, they exhibit more optimal levels of xylem water potential and stomatal conductance, compared to trees from which water is withheld. Although alleviating it to a degree, daily irrigation did not mitigate the midday depression in these values. Seen over a season, even small enhancements of stomatal conductance (and with it photosynthesis and possibly, growth) and xylem water potential, could incrementally produce higher yields.

AFRIKAANSE OPSOMMING: In die strewe na verhoogde boomproduktiwiteit, word daaglikse sproeibemesting (deur 'n drupbesproeiingsstelsel toegedien tydens die dagligure) al meer algemeen in die Suid Afrikaanse sitrusbedryf gebruik. Hierdie praktyk verminder algemene plantstres deur baie spesifieke voedingsen vogbeheer, en verkort ook die plant se reaksietyd op wortelgerigte bewerkingsaktiwiteite. Dit is nodig om die relatiewe voordeel van daaglikse sproeibemesting teenoor konvensionele besproeiingssisteme te evalueer. Huidige tuinboukundige evaluering van boord-bestuurspraktyke is baie tydrowend. In 'n poging om plantreaksie onder verskillende praktyke te beskryf, het ons besluit om die plantstresvlakke met fisiologiese metodes te vergelyk. Sitrusbome onder daaglikse sproeibemesting, en konvensionele mikro- en drupbesproeiing, is onderskeidelik gemonitor om die plant se stresvlakke vas te stel, soos aangedui deur sapvloei, xileem-waterpotensiaal, stomatale geleiding en chlorofil a f1uoresensie. Die plante onder daaglikse sproeibemesting ondervind lae viakkke van waterstremming, waarskynlik weens hoë grondvogtigheid in die wortelsone. Die bome ondervind wel, soos dié onder konvensionele besproeiing, middagdepressie in stomatale geleiding, hoewel tot 'n mindere mate. Dit blyk asof die groter wortelvolume van mikrospuit besproeide bome die herstel na middagdepressie bespoedig. Produsente word aangeraai om die oggendure optimaal te gebruik deur vroeg te besproei sodat plantproduktiwiteit hoog is terwyl die omgewingsfaktore op hul gunstigste is en wanneer die hoogste fisiologiese aktiwiteit voorkom. Ons het ook die effek van wateronthouding gemeet op die bome wat aangepas is vir daaglikse sproeibemesting. Sodoende is die risiko verbonde aan 'n korttermyn watertekort op hierdie bome ge-evalueer, asook die bruikbaarheid van fisiologiese tegnieke om waterstremming in sitrus te identifiseer. Stomatale geleiding en xileem-waterpotensiaal het waterstremming vroëer aangedui as die ander fisiologiese parameters. Sitrusbome blyk redelik onsensitief te wees teenoor droogtestremming soos gemeet deur sapvloei en chlorofil a fluoresensie. Sapvloei word gebuffer deur boom-kapasitansie, en alhoewel sapvloei gereguleer word deur stomatale geleiding, is dit die atmosferiese toestande (hoofsaaklik dampdruk verskil) wat dit primêr beïnvloed, en nie die grond-water inhoud nie. Omdat bome daagliks sproeibemes word, het hulle meer optimale vlakke van xileemwaterpotensiaal en stomatale geleiding in vergelyking met bome waarvan water weerhou is. Alhoewel daaglikse sproeibemesting die middagdepressie verlaag het, is dit nie daardeur opgelos nie. Oor die typerk van 'n seisoen kan selfs minimale verhogings in stomatale geleiding (en daarmeesaam fotosintese en moontlik groei) en xileem-waterpotensiaal, hoër opbrengste tot gevolg hê.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/53000
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