The effect of different low delivery rate drip irrigation systems on the physiological responses of citrus reticulata Blanco (Nadorcott)

Abstract

Thesis (MScAgric)--Stellenbosch University, 2025.van Zyl, G. B. 2025. The effect of different low delivery rate drip irrigation systems on the physiological responses of Citrus reticulata Blanco (Nadorcott). Unpublished masters thesis. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/87fa0c33-d1e1-4c07-9d7f-aec9a3038bd9ENGLISH ABSTRACT: This study investigates the dynamics of soil water distribution in relation to root distribution under varying drip irrigation delivery rates for citrus trees. The research focuses on three irrigation rates 0.4 L/h, 0.7 L/h, and 1.6 L/h and their impact on soil water distribution within the root zone development, salt accumulation, and overall tree performance. Results indicate significant differences in soil water distribution within the rootzone across the different studied drip irrigation rates. The 0.4 L/h treatment exhibited the highest volumetric water content (θv) in the upper root zone at 200 mm, with a subsequent decrease below 300 mm, leading to potential leaching. In contrast, the 0.7 L/h and 1.6 L/h treatments maintained more consistent θv levels throughout the root zone, facilitating better water distribution and root activity. The 0.4 L/h treatment, however, showed increased salt accumulation at 600 mm depth, negatively impacting root function, and limiting vertical root distribution, which restricted the canopy volume and yield. The 0.4 L/h treatment had a high concentration of fine roots at 300 mm, spreading horizontally but with limited vertical distribution, restricting the root zone and canopy growth. The 0.7 L/h treatment had denser vertical roots at 700 mm, while the 1.6 L/h treatment showed extensive root distribution both horizontally and vertically, supporting greater canopy volume Moreover, the study found that the 0.7 L/h and 1.6 L/h treatments demonstrated superior Production Water Use Efficiency (WUEP), with the 1.6 L/h treatment yielding the highest efficiency at 13.87 kg/m³.The 0.4 L/h treatment resulted in the lowest WUEP of 11.42 kg/m³, and was also associated with the smallest canopy volume together with lower fruit production. Despite lower total water inputs, the 0.4 L/h treatment presented higher risks of equipment failure and mechanical issues, which could exacerbate water deficits during periods of prolonged power outages. The 0.4 L/h treatment had fewer flowers per shoot, likely due to competition for carbohydrates in its smaller canopy. This led to more fruit per shoot but a lower fruit growth curve, ultimately reducing fruit size and yield. In conclusion, the 0.7 L/h irrigation system shows the most promising results for commercial citrus farming, optimizing water use and minimizing risk. The study highlights the importance of balancing irrigation rates to achieve optimal performance while considering factors such as root health, water distribution, and salt management.AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die dinamika van grondwaterverspreiding in verband met wortelverspreiding onder verskillende besproeiingstelsels vir sitrusbome. Die navorsing fokus op drie besproeing behandelings 0.4 L/h, 0.7 L/h en 1.6 L/h en hul impak op waterinhoud, wortelontwikkeling, die opbou van soute en algehele boom prestasie. Resultate toon betekenisvolle verskille in waterverpreiding oor die onderskeie besproeiingstelsels. Die 0.4 L/h behandeling het die hoogste volumetriese waterinhoud (θv) in die boonste wortelsone by 200 mm gehad, met 'n daaropvolgende afname onder 300 mm, wat moontlike uitloging tot gevolg het. In teenstelling hiermee het die 0.7 L/h en 1.6 L/h behandelings meer konstante θv vlakke deur die wortelsone gehandhaaf, wat beter waterverspreiding en wortelaktiwiteit bevorder het. Die 0.4 L/h behandeling het egter verhoogde soutakkumulering by 600 mm diepte getoon, wat wortelfunksionaliteid negatief beïnvloed en die vertikale wortelverspreiding beperk het, wat die boomvolume en produksie geaffekteer het. Die 0.4 L/h behandeling het 'n hoë konsentrasie fyn wortels op 300 mm gehad, wat horisontaal versprei het maar met beperkte vertikale verspreiding, wat die wortelsone en boomvolume beperk het. Die 0.7 L/h-behandeling het digter wortels op 700 mm gehad, terwyl die 1.6 L/h-behandeling 'n uitgebreide wortelverspreiding beide horisontaal en vertikaal getoon het, wat 'n groter boomvolume ondersteun het. Verder het die studie getoon dat die 0.7 L/h en 1.6 L/h behandelings Produksie Water Verbruik Effektiwiteit (WVEP) verbeter het teenoor die 0.4 L/h behandeling. Die 1.6 L/h behandeling het die hoogste WVEP van 13.87 kg/m³ gelewer. Hoewel die 0.4 L/h behandelings die laagste WVEP van 11.42 kg/m³ behaal het, was dit geassosieer met die kleinste boomvolume en laer vrugproduksie. Ondanks laer totale waterinsette, het die 0.4 L/h behandeling groter risiko’s van toerustingfoute en meganiese probleme, wat watergebreke tydens lang periodes van kragonderbrekings kan vererger. Die 0.4 L/h behandeling het minder blomme per loot gehad, waarskynlik as gevolg van kompetisie vir koolhidrate in die kleiner blaredak. Dit het gelei tot meer vrugte per loot, maar 'n laer vruggroei-kurwe, wat uiteindelik vruggrootte en opbrengs verminder het In gevolgtrekking, het resultate van die 0.7 L/h besproeiingstelsel die mees belowende potensiaal vir kommersiële sitrus verbouing getoon, wat watergebruik optimaliseer en risiko’s verminder. Die studie beklemtoon die belangrikheid om besproeiings lewerings in balans te bring om optimale prestasie te behaal, terwyl faktore soos wortelgesondheid, waterverspreiding en soutopbou in ag geneem word.Master