Fig. 4. Relationship between performance of C. canephora genotypes
in restricted-water conditions and wetness index of the location in
which they were collected from (A) and the relationship between drought
tolerance as estimated from RGRA and wetness index of the location (B);
wetness index (WI), high WI values indicate moist conditions and low WI
values indicate dry conditions). Both slopes were negative and
significantly different from zero at p = 0.05
There was also a marginally significant negative (p = 0.05) relationship
between drought tolerance of genotypes and wetness index in their
location (Fig. 4 panel B), being defined by: Tolerance = 0.99 – 0.214
(wetness index), R2 = 0.05 and S.E. = 0.106. The
negative relationship between tolerance and wetness index of the
locations possibly indicates a climatic signature related to drought
tolerance of the genotypes. This observed relationship between drought
tolerance and wetness index suggests that on average, genotypes from the
comparatively drier areas, e.g. Zoka, tended to be somewhat more
drought tolerant than genotypes from the wetter area Kalangala. No
difference in terms of goodness of fit was found between the linear and
the other two types of non-linear models. Other traits also tended to
show a similar trend of drought tolerance being negatively correlated
with wetness index although statistically significant relations were
observed in TNL only (Appendix Fig. A.4. and Appendix
Box A.2.).
4. DISCUSSION
In this study, we explored Uganda’s C. canephora genotypic
diversity in a screening experiment with 148 genotypes. We specifically
explored: (i) the effects of drought on growth categorised by
cultivation status (wild, feral and cultivated), genetic groups and the
geographic location, (ii) the relationship between performance under
restricted-water and performance under ample-water conditions and (iii)
the relationship between drought tolerance and wetness index (WI). To
our knowledge, this is the first study to explore intra-specific
variation in drought responses for a large number of genotypes
(> 100) in a tropical tree species.
4.1. Effect of drought on C. canephora in growth response
traits
Drought significantly reduced the RGRA (relative growth
rate of leaf area), TNL (total number of leaves),
TL (total leaf area), TLDW (total leaf
dry weight), SLA (specific leaf area) and increased the
RL (root volume to leaf area) (Table 2). The latter
finding concurs with the optimal partitioning theory which entails that
in response to stress, plants allocate proportionally more resources to
the structure capturing the most limiting resources (Brouwer, 1963;
Bloom, Chapin and Mooney, 1985). Other studies (Ryser and Eek, 2000;
Shipley and Meziane, 2002) and reviews (Hoffmann and Poorter, 2002; Ezizet al. , 2017) also stated that, in response to stress, plants
adjust their biomass allocation in accordance to whether the most
limiting resource is above- or belowground. In our study,
TNL and TL were the most affected traits
(Table 2) implying that genotypes responded to drought stress mainly by
minimising transpirational water loss by reducing the number of leaves
and leaf area. Differential reduction in leaf area as a response to
drought stress has also been observed by other authors (DaMatta et
al. , 2003; Pinheiro et al. , 2004; Dias et al. , 2007;
King’oro, 2014). Our current findings extend these observations to a
wider range of genotypes including wild, feral and cultivated material.