Mineral nutrition of citrus trees in relation to flowering, fruit set and yield

Thesis (MScAgric)--Stellenbosch University, 2022.

ENGLISH ABSTRACT: Fruit set is considered as the determining factor for adequate fruit yield in citrus species. Cultivars such as ‘Midknight’ Valencia [Citrus sinensis (L.) Osbeck] and ‘Orri’ mandarin (Citrus paradisi Macfad) produce an abundance of flowers, but struggle to obtain sufficient fruit set, a problem that can be related to mineral nutrition. The effect of excessive fertilisation in combination with fruit thinning on the growth responses and mineral nutrient composition of ‘Midknight’ Valencia and ‘Orri’ mandarin was therefore investigated. Experimental trees received double the amount of the standard fertiliser recommendation of mobile mineral elements including nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg), in combination with a fruit thinning treatment that included no fruit thinning, moderate fruit thinning and severe fruit thinning. ‘Midknight’ Valencia trees were the most reactive to over-fertilisation of N and K. The trees were most reactive to the N applications as it led to a higher vegetative response compared to the other mineral nutrients, but it led to a decrease in fruit set, final yield, and fruit quality. The N treatment also led to higher N content in the leaves, flowers, and fruit of ‘Midknight’ trees. The treatments were also reactive in ‘Midknight’ Valencia trees as it resulted in larger fruit over time as well as the production of the largest fruit at harvest. The K treatment led to a higher K concentration in the leaves, but varying results were found for the K concentration in the flowers and fruit. Over-fertilisation had little effect on the growth responses of ‘Orri’ mandarin trees, which could be due to the trees being in an already over- fertilised state. The K treatment had a positive influence on the fruit set of ‘Orri’ mandarin trees, but led to a decline in the quality of the fruit even though it did produce the largest fruit. The K treatment also led to increasing the K content in the leaves, flowers, and fruit. Fruit thinning treatments had no effect on the growth responses and mineral nutrient composition of ‘Midknight’ Valencia and ‘Orri’ mandarin trees, possibly due to the practice being time-dependent. The results illustrated that trees responded to excessive fertilisation according to the demand for each nutrient, therefore, responses to N and K applications were seen because of the high demand plants have for these elements. It is more likely that the effect of fertilisation and crop load manipulation will influence tree mineral nutrient composition and vegetative growth when trees have a nutritional deficit. Furthermore, the results from this study showed that fertiliser recommendations should not only rely on mineral nutrient norms, and that the crop yield together with the norms should be used to determine the fertilisation requirements. Results from the mineral nutrient analysis of trees indicated that norms should be adjusted for each citrus cultivar. Further research should be conducted to investigate the effect of the time-dependence of manual fruit thinning to justify if it would be practical to perform this practice commercially to improve fruit set of citrus trees.

AFRIKAANSE OPSOMMING: Vrug set word beskou as die bepalende faktor vir optimale vrug opbrengs in sitrus spesies. Kultivars soos ‘Midknight’ Valencia [Citrus sinensis (L.) Osbeck] en ‘Orri’ mandaryne (C. reticulata Blanco) produseer oorgenoeg blomme, maar sukkel met voldoende vrugset, ʼn probleem wat met minerale voeding gekoppel kan word. Daarom was die effek van oormatige bemesting, in kombinasie met vrug uitdunning op die groei reaksies en samestelling van minerale voedingstowwe van ‘Midknight’ Valencia en ‘Orri’ mandaryne ondersoek. Eksperimentele bome het dubbeld die hoeveelheid van die standaard hoeveelheid bemesting ontvang vir die mineraal elemente stikstof (N), fosfaat (P), kalium (K) en magnesium (Mg), in kombinasie met ʼn vlak van uitdunning: geen uitdunning, matige uitdunning of strawwe uitdunning. Die ‘Midknight’ Valencia bome was die mees reaktiefste gewees in reaksie op oorbemesting van N en K. Die bome wat oorbemes is met N, het die sterkste vegetatiewe groei reaksie getoon teenoor die ander behandelings, maar dit het wel gelei tot laer vrugset, finale opbrengs en vrug kwaliteit. Die N behandeling het ook gelei tot hoër konsentrasies van N in die bome se blare, blomme en vrugte. Die ‘Midknight’ bome het ook ʼn reaksie getoon teenoor die K behandeling aangesien dit gelei het tot die produksie van groter vrugte. Die K behandeling het ook gelei tot hoër konsentrasies van K in die blare, maar variërende resultate was in die blomme en vrugte se K konsentrasies verkry. Oorbemesting het nie so ʼn groot effek gehad op die groei reaksies van ‘Orri’ mandaryne nie, dit kan wees as gevolg van die bome wat alreeds in ʼn oorbemesting toestand is. Die K behandeling het ʼn positiewe effek gehad op die vrugset, maar ʼn negatiewe effek op die interne kwaliteit van vrugte alhoewel dit tot die produksie van die grootste vrugte gelei het. Die K behandeling het ook gelei tot ʼn toename in die K konsentrasie van die blare, blomme en vrugte van ‘Orri’ mandaryne. Die vrug uitdunning behandelings het geen effekte gehad op die groei reaksies en die mineraal voedingstof konsentrasies van ‘Midknight’ Valencia en ‘Orri’ mandaryn bome nie, moontlik omdat hierdie praktyk tydafhanklik is. Die resultate van hierdie studie het aangetoon dat bome teenoor oorbemesting sal reageer na gelang van die aanvraag van elke voedingstof, daarom is reaksies op N- en K-toedienings waargeneem as gevolg van die hoë aanvraag vir hierdie elemente. Dit is meer waarskynlik dat die effek van oorbemesting en oes-lading manipulasie die mineraal-voedingstof-samestelling en groei reaksies sal beïnvloed wanneer bome ʼn mineraaltekort het. Die resultate het ook verder gewys dat bemestingsaanbevelings nie net op die voedingstofnorme moet staatmaak nie, en dat die oesopbrengs tesame met die norme gebruik moet word om bemestingsvereistes te bepaal. Resultate van hierdie studie het ook aangedui dat die oesonttrekkingsnorme aangepas moet word vir elke sitruskultivar. Verdere navorsing moet gedoen word om die tydafhanklikheid van uitdunning te ondersoek om dit prakties moontlik uitvoerbaar te maak ten einde die vrugset van sitrusbome te verbeter.

Masters