Is sampling influential on the variations in 13C stable isotopes whilst screening for drought stress in coffee?

IS SAMPLING INFLUENTIAL ON VARIATIONS IN 13C STABLE ISOTOPE WHILST SCREENING FOR DROUGHT STRESS IN COFFEE

           Janice Nakamya^1,2, Roel Merks³, Rebbeca Hood-Nowtry², Gerd Dercon¹Jeremiah Mwangi⁴, Silas Makune4

1 Soil and Water Management & Crop Nutrition Laboratory, Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, 2444 Seibersdorf, Austria

2University of Natural Resources and Life Sciences Vienna, Peter-Jordan-Straße 82, 1190 Vienna

3 Division of Soil and Water Management, Faculty of Bioscience, KU Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium

4 Kaweri Coffee Plantation, Plot 1 Kitemba 264, Mubende Uganda

An enormous coffee yield gap and subsequent decline in income has been realised from drought, triggering a need for faster screening tool for water stress and estimation of water use efficiency for the commonly grown coffee varieties. However, determining the critical thresholds of drought stress in perennial crops especially in coffee is challenging. Stable isotope measurement of coffee leaves could avail instantaneous measurements as they track the changes at leaf level, there has been limited use of isotopes to monitor drought stress in coffee. Furthermore, using punch leaf samples instead of bulk leaf samples would be faster and cheaper by eliminating grinding.  The method requires homogenous sampling to provide representative samples and ensure quality results, suggesting detailed understanding of variations in d¹³C at coffee leaf level.  The study was conducted on commonly grown 6 old clones’ varieties of Robusta coffee, each in a 16-year-old 15 by 15m trail plot at Kaweri Coffee plantation Mubende, Central Uganda. With a specific aim of establishing (1) Whether d¹³C varies within the different leaf ages, leaf symmetry and sample position on the leaf (Fig.1). Separate punch leaf samples were obtained on young fully grown, below, and old pair of leaves on the left and right side of the apex, middle and rare of the lamina.  The results reveal a significant variation in d¹³C between young and old leaves but not with the pair in between them. Furthermore, a leaf pair is not the same in terms of d¹³C but it does not matter which position on the leaf the punched sample is obtained from.  Implying, for the best representation, coffee punch sample would be from any position of an old leaf.