Aspects of mineral nutrition affecting fruit quality of ‘Nadorcott ...

Aspects of mineral nutrition affecting fruit quality of

¡®Nadorcott¡¯ mandarin

by

Helen Marais

Thesis presented in partial fulfilment of the requirements for the degree of Master of

Science in Agriculture (Horticultural Science) in the Faculty of Agrisciences at

Stellenbosch University

Supervisor: Dr. P.J.R. Cronj¨¦

Citrus Research International

Dept. of Horticultural Science

Stellenbosch University

South Africa

Co-supervisor: Dr. E.W. Hoffman

Dept. of Horticultural Science

Stellenbosch University

South Africa

Co-supervisor: Prof. L. Zacar¨ªas

Extraordinary professor: Dept. of Horticultural Science

Stellenbosch University

South Africa

Instituto de Agroqu¨ªmiqa y Tecnolog¨ªa de Alimentos (IATA)

Consejo Superior de Investigaciones Cientificas (CSIC)

Valencia, ESPA?A

December 2017

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Declaration

By submitting this thesis electronically, I declare that the entirety of the work contained therein is my

own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated),

that reproduction and publication thereof by Stellenbosch University will not infringe any third party

rights and that I have not previously in its entirety or in part submitted it for obtaining any

qualification.

December 2017

Copyright ? 2017 Stellenbosch University

All rights reserved

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Acknowledgements

Firstly, to our Heavenly Father, for being there every step of the way and teaching me many

invaluable life lessons during my master¡¯s.

To my supervisors, Dr Paul Cronj¨¦ and Dr Lynn Hoffman, thank you for your understanding, patience

and for remaining supportive throughout my master¡¯s. Without your guidance in writing I would have

given hope up long ago. I have learnt a lot from both of you.

To my parents, Willie and Charlene, thanks for your love and support which I have had for as long

as I can remember. To all my siblings, Chan¨¦, Mia and Hennie, and brother-in-law Stephen, as well

as Carla-Mart thanks for your support and encouragement. I love you all very much.

To all the Horticulture department lecturers throughout my studies, thank you for inspiring me to

continue studying horticulture, without you I would not have been here. Jakkie Stander, your help

and ideas inspired me and made me excited about my research again whenever I was struggling. Dr

Michael Schmeisser, your encouragement whenever I ran into you is very much appreciated.

I would like to thank all Stellenbosch University staff from other departments who helped me, Prof.

Daan Nel, with my statistical analysis, and Angelique Laurie, Madeleine Frazenburg and Lize

Engelbrecht at the Central Analytical Facilities (CAF).

To Gustav L?tze and the lab staff, thank you for your assistance throughout my trials.

Jade North, thanks for all your help in the field and all the interesting stories you always told which

made driving to Citrusdal much less tedious.

Andr¨¦ Swartz and Tikkie Groenewald for their assistance in the field and for always brightening up

my days with their jokes.

Dr Elizabeth Rohwer and Dr Remy Rosalie for your assistance and irreplaceable knowledge in the

lab.

The Citrus Academy and The Harry Crossley Foundation for financial assistance during my studies.

To Mouton Citrus, the Smit brothers and B&B Boerdery, Willem Kievet and Indigo farms, Duppie

and Dome Citrus and CRI thank you for making these trials possible.

Thanks to all the other MSc students, past and present. Johan¨¦, Robert, Rudolph and Du Toit, thanks

for your support ¨C I wish you guys came earlier and I¡¯ll miss you very much.

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iii

To all my other friends, Hel¨¦ne, E¡¯louise, Morgana, Imke, Sumarie, Sonja, Dalene, Marike, Tersia,

Jonathan, Jacques, Adriaan, Ryan, Muaz and many many more. Thanks for always being there

whenever I need you.

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Summary

Mineral nutrition of Citrus is one of the key controllable factors influencing fruit quality. The mineral

nutrients known to have the greatest impact on Citrus fruit quality, in order of magnitude of effect,

are nitrogen (N), potassium (K) and phosphorous (P). The timing of application together with the

amount of fertiliser applied is of critical importance in determining the impact it will have on Citrus

fruit quality, including any possible long-term effects on tree growth, development and phenology.

Three experiments were conducted to study fruit quality of ¡®Nadorcott¡¯ mandarin, taking both these

above-mentioned considerations into account. The first experiment explored the known influence of

P on decreasing citric acid content of Citrus fruit. In production areas with cold winters, harvest dates

can be delayed by high fruit acid content, to such an extent that flowering is detrimentally impacted

and hence also the fruit yield of the following season. Results obtained indicated that the citric acid

content of fruit was significantly decreased by mono-ammonium phosphate (MAP) sprays, applied

at seven and/or eight weeks after full bloom (WAFB). The second and third experiments concentrated

on the influence of late N application on ¡®Nadorcott¡¯ fruit quality when studied in combination with

different irrigation regimes and under different climatic conditions, respectively. The second

experiment showed that, when the influence of N is considered in combination with irrigation, it has

a more pronounced effect on Citrus fruit quality than exerted by N alone. Over-irrigation can have

the most serious negative effect on fruit quality when all the factors studied were considered, therefore

it is proposed that irrigation is likely to be the main determinant for success when managing N

fertilisation. In the third experiment where the effect of late N in different forms on fruit quality was

studied, the influence thereof was found to be insignificant. In addition, none of the N applications

negatively affected rind quality, including that of colour development or the incidence of disorders.

Beneficial effects on flowering for the following season were, however, also not recorded. Currently

it can be concluded that since the application of late N had negligible effects on fruit quality in this

study it may be used in an orchard with a known N deficiency, later in the season, subject to the

prerequisite that the orchard is not over-irrigated. This study confirmed the complexity of mineral

nutrition in citriculture, based on the large number of factors that influence fruit quality. Future

research should attempt to establish the ideal time of MAP application, also taking different

concentrations and production areas into account. Late N application trials should be expanded to an

in-depth study of the effect of the N status of the tree on flower initiation, whilst establishing the

production conditions under which this practice should be recommended.

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