Abstract

Uganda lies within the drier end of the natural distribution range ofCoffea canephora and contains unexplored genetic material that could be drought-adapted and useful for developing climate-resilient varieties. Using experimental treatments, (i) ample and (ii) restricted-water, response of 148 genotypes were studied comprising wild, feral and cultivated C. canephora. Biomass allocation, standing leaf area and leaf area growth data were collected. Linear mixed effect models and PCA were used to analyse effect of drought on genotypes from different: (i) cultivation status, (ii) genetic groups and (iii) locations. We assessed the relationship between drought tolerance for relative growth rate in leaf area (RGRA), total number of leaves (TNL), total leaf area (TL) and total leaf dry weight (TLDW) of genotypes at final harvest. Restricted-water reduced RGRA across genetic groups (3.2 – 32.5%) and locations (7.1 – 36.7%) but not cultivation status. For TNL, TL and TLDW, genotypes that performed well in ample-water performed worse under restricted-water, indicating growth-tolerance trade-off. Drought tolerance in RGRA and TNL were negatively correlated with wetness index suggesting some degree of adaptation to local climate. Findings indicate a growth-tolerance trade-off within this tropical tree species and drought tolerance of Uganda’s C. canephora is somewhat associated with local climate.
Keywords: Intraspecific variation, drought stress, growth tolerance trade-off, local adaptation Coffea canephora.
INTRODUCTION
Water availability is a major factor limiting global coffee production largely because of the drought sensitivity of Coffea species and because a large fraction of the production is sustained by small-holder farmers who usually lack resources to establish irrigation facilities (DaMatta and Cochicho Ramalho, 2006; Wintgens, 2009; Craparo et al. , 2015). Problems of water limitation in coffee production are expected to be aggravated by climate change. This is because across the coffee production belt, a temperature increase of 2.1° C has been predicted by 2050 (Parry et al. , 2007; IPCC, 2014) and this warming can directly result in increased vapour pressure deficits, higher potential evapotranspiration and hence drought stress in plants. Indirectly, the increase in global average temperatures is expected to result in shifts in the annual precipitation with more frequent occurrences of severe droughts (Schiermeier, 2008). The changes in temperature and precipitation together may have strong negative effects on coffee production (Bunn et al. , 2015), although Verhage et al. (Verhage, Anten and Sentelhas, 2017) reported that the CO2 fertilization effect arising from elevated CO2 concentrations could offset the negative effects of climate change on average coffee yields by a small net increase. The global distribution and production of coffee is therefore likely to be significantly affected by climate change (DaMatta and Cochicho Ramalho, 2006; Davis et al. , 2012; Jassogne et al. , 2013). There is a need for finding or developing drought-tolerant genotypes, and one way of working towards this is to explore the natural diversity in wild coffee populations.
C. canephora Pierre ex A. Froehner is a tree native to African tropical lowland forests stretching from Guinea in West Africa through the Congo River Basin to Uganda in East Africa (Berthaud, 1986; Coste, 1992; Montagnon, Leroy and Yapo, 1992; Davis et al. , 2006). Generally, these tropical forests are characterized by abundant rainfall (precipitation > 2000 mm y-1) with a short or no dry season, high atmospheric humidity and stable average temperatures between 24 °C and 26 °C (Coste, 1992; DaMatta and Cochicho Ramalho, 2006; Damatta et al. , 2018). However, even in these moist tropical forests, there occur periodic water shortages due to dry spells (Engelbrecht et al. , 2006). Furthermore, the natural geographical distribution of C. canephora extends into the somewhat drier areas (Masih et al. , 2014), e.g. in Uganda. Tree growth (e.g. biomass or leaf area increment, referred to as performance hereafter) is commonly observed to decrease with drought intensity (Grime and Hunt, 1975; Chapin, 1980; Garnier and Poorter, 2007). Across tree species (at interspecific level), there tends to be a negative correlation between growth under well-watered conditions and drought tolerance which is defined as the extent to which plants can maintain these growth rates under water-stressed conditions (i.e ., drought tolerance in growth, the ratio of growth under stressed and unstressed conditions) (Chapin, 1980; Garnier and Poorter, 2007; Ouédraogo et al. , 2013). Growth and survival under dry conditions tend to be associated with traits such as low specific leaf area (leaf area/mass ratio), fewer or smaller stomates, small stem vessel diameter, high fractions of dry mass in roots, low leaf area to root mass ratio and low leaf area to sapwood ratio which tend to reduce growth rates under well-watered conditions (Lambers, Chapin and Pons, 2008).
While multispecies comparisons are useful to understand ecological strategies and community composition, questions regarding natural selection and applications for breeding require additional intraspecific comparisons across wild accessions of a species. When an environmental stress gradient such as water availability acts as a selective force, one may expect tolerance of a genotype to this stress factor to be related to the climate in the site of origin (Alberto et al. , 2013). Analysing such patterns is important as it may provide insights into natural selection but may also provide basic information to assess the adaptive potential to climate change and, for crops, identify drought-tolerant genotypes (Alberto et al. , 2013; Rungwattanaet al. , 2018). However, very few studies have compared wild accessions from different climates for tropical trees such as coffee. Rungwattana et al. (2018) compared wild accessions of rubber (Hevea brasiliensis ) from different locations across a rainfall gradient in the Amazon forest and found no correlation between any of the traits investigated and either temperature or rainfall at the site of origin. In C. canephora’ s congener, C. arabica , comparisons between nine accessions from different Ethiopian forests showed that accessions from drier areas were more plastic in leaf gas exchange traits in response to changes in water availability than those from wetter areas (Beining, 2007) but another study with a similar set of accessions found no correlations between water availability as an experimental factor and leaf gas exchange traits (Kufa and Burkhardt, 2011).
Uganda has been reported to have substantial C . canephoradiversity (Musoli et al. , 2009; Ngugi and Aluka, 2019; Kiwuka, 2020; Kiwuka et al. , 2021) which could be explored to identify functional diversity in regards to drought stress. But to our knowledge, intraspecific comparisons of drought-related traits in C .canephora have been limited to cultivated material e.g. in (DaMatta et al. , 2003; Pinheiro and Var, 2004; Dias et al. , 2007; Silva et al. , 2013; King’oro, 2014; Menezes-Silvaet al. , 2015). While the aforementioned studies give important insights into the morphological and physiological drivers of drought tolerance, exploration of the variation in drought tolerance across wild populations and potential correlations with climate need to be done. Furthermore, none of the studies on tropical trees has explored the extent to which drought tolerance is associated with genetic diversity, a link that would provide helpful information to interpret drought adaptation. Finally, as far as we know, drought tolerance in coffee has also not been explored along a cultivation status trajectory, i.e. comparing wild, feral (second generation or higher of formerly cultivated material and abandoned for over 50 years) and cultivated genotypes. It is therefore unknown whether the cultivation of C. canephora has been selected for or against drought tolerance.
This study was set out to determine: (i) the effect of drought on vegetative growth (biomass and leaf area increment) of C. canephora genotypes, collected across a climatic gradient in Uganda and categorised by (a) cultivation status, (b) genetic groups as characterised by Kiwuka et al., (2021), (c) and location, indicating the different climatic envelopes (for the years 1950 -2000), (ii) the relationship between performance under restricted and ample-water conditions, (iii) the relationship between drought tolerance of genotypes and wetness index (WI) at their native location. WI, the ratio of mean annual precipitation to mean annual potential evapotranspiration (PET) is a reasonable proxy for local climate wetness, whereby high WI indicates wetter climates and vice versa (note that we do not use the original but confusing term, aridity index, from Zomer et al., (2008)). We hypothesized that, since Uganda’s wild C. canephorapopulations occur in different climatic envelopes, genotypes from dry (lower WI) locations characterised by high temperatures, low precipitation, and high PET and will have comparatively higher growth and performance under restricted-water conditions than genotypes from locations with low to moderate temperatures, high precipitation, higher WI and low PET (wet location). Additionally, we expect a trade-off between drought tolerance and performance, whereby the mechanisms that underlie drought tolerance in material from dry locations are associated with slow growth and the inability to exploit favourable conditions (McGill et al. , 2006; Lambers, Chapin and Pons, 2008; Sade, Gebremedhin and Moshelion, 2012; Amissah et al. , 2018).
MATERIALS AND METHODS