Assessment of Root Trait Variability Among Graft Combinations in Coffea arabica Under Water Stress

Coffee (Coffea arabica) is a major global commodity, yet its productivity is
increasingly challenged by water scarcity, especially in rainfed cultivation systems. This
study investigated the physiological and morphological responses of 16 grafted coffee
combinations and their respective parental lines under 70% water deficit conditions using a
rain-out shelter over three consecutive years (2015–2018). Significant genetic variability was
observed in root traits, total dry matter (TDM) and water use efficiency (WUE). Among the
grafts, S.4538/Sln.6 recorded the highest TDM (112.8 g plant⁻¹), root biomass (27.0 g
plant⁻¹) and shoot biomass (44.9 g plant⁻¹). The highest root-to-shoot ratio was found in
Sln.6/Sln.10 (1.05), while S.4538/Sln.6 showed the lowest (0.60), indicating diverse carbon
allocation patterns under stress.Water use efficiency varied significantly among
combinations, with Sln.11/Sln.11 achieving the maximum WUE (5.84 g kg⁻¹), followed
closely by Sln.11/Sln.6 (5.82 g kg⁻¹), Sln.10/Sln.6 (5.73 g kg⁻¹) andTaffarikela/Sln.6 (5.61 g
kg⁻¹). The highest Net Assimilation Rate (NAR) and Mean Transpiration Rate (MTR) were
recorded in Taffarikela/C x R (4.39 g dm⁻² day⁻¹ and 23.27 mg dm⁻², respectively). Z-
score-based classification further revealed that grafts like S.4538/Sln.6, Sln.6/Taffarikela and
Sln.10/Sln.6 consistently combined high WUE with strong root development. A negative
correlation between MTR and WUE (r = –0.68) confirmed that efficient stomatal regulation
contributed to water-saving strategies under drought.These findings underscore the potential
of grafting drought-sensitive but agronomically elite arabica lines onto physiologically
resilient rootstocks. Such combinations offer a dual advantage of improved water acquisition
and utilization, providing a viable strategy for enhancing coffee resilience and yield stability
under increasing climatic variability.