Tropentag, September 10 - 12, 2025, Bonn "Reconciling land system changes with planetary health"

Rationalising the use of δ 13c stable isotope & leaf temperature in screening of coffee variety for drought stress

¹International Atomic Energy Agency, Soil and Water Management & Crop Nutrition Laboratory, Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications, Austria
²CIRAD, UPR GECO, F-97130 Capesterre-Belle -Eau, Guadeloupe, France
³KU Leuven, Dept. of Earth and Environmental Sciences
⁴BOKU University, Inst. of Soil Research, Dept. of Forest and Soil Sciences, Austria
⁵Kaweri Coffee Factory Limited, Sustainabilty, Uganda
⁶Kaweri Coffee Factory Limited, Agronomy, Uganda
⁷International Atomic Energy Agency, Soil and Water Management and Crop Nutrition Laboratory

Abstract

Determining the critical thresholds for drought stress in perennial crops such as coffee is a significant challenge. δ ¹³C isotope measurements of coffee leaves offer assessment of stress by tracking intrinsic changes. In addition, using leaf punch samples for δ ¹³C measurements can enhance efficiency and reduce costs by eliminating the grinding process. Despite this, the use of isotopes to monitor drought stress in coffee remains limited. To note, isotopic sampling requires homogeneity to yield representative results. Therefore, documenting variations in the ¹³C values of coffee leaves and developing a δ¹³C homogeneous sampling method tailored to the coffee is crucial. This study was conducted on 13 Robusta varieties, including six commonly grown old clones and seven Kitunza Resistant varieties (KRs). The six old clone are each on a 16-year-old 15 × 15 m trial plot while the KRs on a four-year-old 12 × 12 m plot, at the Kaweri Coffee plantation in Mubende, Central Uganda.The specific objectives of this study were to determine (i) which leaf provides the best indication of water stress, (ii) whether the leaf temperature in young leaf pairs is higher than that of other leaf pairs and,(iii) whether clones are more drought-resistant than KRs. Leaf temperature was measured before and after midday from young, below the young, and old leaf pairs (left and right). Leaf punch samples were collected from the apex, middle, and rare lamina on the same leaves. The findings reveal that δ¹³C is significantly(p < 0.01) different across different leaf age and variety. δ¹³C, leaf temperature, and Crop Water Stress Index (CWSI) are confirmed to serve as indicators of drought stress in coffee trees. Drought stress significantly influenced the δ¹³C and CWSI values. Notably, there was a significant difference in δ¹³C (0.89 ± 0.2‰, p < 0.001) and predawn CSWI (0.14 ± 0.02, p < 0.001) between the dry and rainy seasons. Old leaves exhibited higher δ¹³C values (28.35± 0.1‰, p < 0.01) and CWSI (0.53 ± 0.2) values than young leaves.

Keywords: ¹³C discrimination, drought stress, leaf temperature, Robusta, variety screening

Contact Address: Janice Nakamya, International Atomic Energy Agency, Soil and Water Management & Crop Nutrition Laboratory, Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications, 2444, 2444  Seibersdorf, Austria, e-mail: janienakagmail.com