4.3. Slow growth as a strategy to cope with drought stress and
evidence of a trade-off between growth and drought tolerance.
Genotypes that had low RGRA TNL,
TL and TLDW values in ample-water
conditions were comparatively less affected by drought, a scenario which
indicates a trade-off between growth and drought tolerance across the
study populations (Fig. 3). This finding concurs with an established
ecological paradigm that there is a trade-off between the capacity of
plants to grow fast when resources are abundant and their capacity to
tolerate resource shortages (Bazzaz and Bazzaz, 1996; Aerts and Chapin,
1999; Grime, 2006). The trade-off between growth and tolerance has been
linked to a conservative resource-use strategy in which slow growth
results in slow tissue turnover (i.e. conservative use of
resources) and subsequently less dependency on the environment for
acquisition of new resources. On the contrary fast growth is associated
with high resource turnover rates, intensive resource acquisition, high
dependency on the environment and ultimately shorter lifespan (Chapin,
1980; Chapin III, Autumn and Pugnaire, 1993; Grime et al. , 1997;
Reich et al. , 2003; F. J. Sterck, Poorter and Schieving, 2006;
Sterck et al. , 2011). Ecologically, slow growth has been reported
as an adaptive strategy for plants in resource limiting conditions.
Poorter, (1989) studied the ecological consequences of the interspecific
variation in relative growth rate (RGR) of plants and concluded that
differences in potential RGR between species were habitat-related
whereby fast-growing species were found in resource-rich habitats while
slow growers could be found in any adverse environmental condition. In
response to drought, a growth-tolerance trade-off could be expected
because several traits and mechanisms that confer tolerance in dry
conditions (e.g. low specific leaf area, low stomatal size or
number) reduce water loss but also reduce rates of net photosynthesis
per unit area, which, in turn, results into slower growth under
favourable water availability (Lambers, Chapin and Pons, 2008; Stercket al. , 2011).
Although the growth-tolerance trade-off has been widely studied and
established across species (interspecific), including tropical forest
trees (Poorter and Jong, 1999; F J Sterck, Poorter and Schieving, 2006;
Sterck et al. , 2011; Amissah et al. , 2018) much fewer
studies (Pallardy and Kozlowski, 1981; Silva et al. , 2013;
Menezes-Silva et al. , 2015) have been conducted to explore the
intraspecific variation of tropical trees to drought and the
manifestation of the growth-tolerance trade-off. Pallardy and Kozlowski,
(1981) revealed a probable growth-tolerance trade-off amongPopulus clones: fast-growing clones had a larger initial rate of
decline in leaf water potential with transpirational flux density but
reduced the rate of decline more than slow-growing clones as the
transpirational flux density increased. Similarly, Menezes-Silva et al.,
(2015) and Silva et al., (2013) studied eight clones of cultivatedC. canephora (variety Conilon) and found that wood density, a
trait that partially influences the plant’s water-conducting capacity,
was higher in drought-tolerant clones, and was associated with greater
resistance to cavitation. This adaptation however could limit growth
under favourable water conditions as dense wood is more costly to
produce and the associated smaller xylem have lower maximum water
conductance (Silva et al. , 2013; Menezes-Silva et al. ,
2015).
In our study, the relatively low RGRA and high tolerance
of genotypes from Kibale, Itwara and Zoka locations (Table 4; Appendix
Fig. A.3.) suggests that those populations employ a more conservative
resource-use strategy, while genotypes from Mabira, Malabigambo,
Kalangala and Budongo employ a more rapid resource-acquisition strategy.
Similar to our results, Silva et al.,(2013) and Menezes-Silva et
al.,(2015) also found that across a set of cultivated C.
canephora clones, the most drought-tolerant ones tended to be slow
growers. Slow growth in stressful conditions could in the long term be
more adaptive than fast growth because fast growth results in larger and
more resource-demanding plants that could eventually die off if the
resource demand is not met. Here, we showed the existence of a
growth-tolerance trade-off across a large set of wild accessions of a
perennial crop species, suggesting that intraspecific variation in
tolerance may be related to selection in natural environments. Evidence
of a growth-tolerance trade-off in our study is further corroborated in
our related experiment by Kiwuka (2020) where we studied fewer (15)
genotypes with more response traits and found that slow-growing
genotypes were more drought tolerant and less plastic for most of the
response traits.
In interpreting our findings, it should be noted that our experiment was
a pot experiment and pots have limited volume. Firstly, this could cause
a so-called pot-binding effect (Poorter et al. , 2012; Sinclairet al. , 2017); pots holding insufficient water to support
transpiration and therefore growth. This could be more severe for
fast-growing plants than for slow-growing ones. However, in our set up
we accounted for this effect as we determined the relationship between
water consumption and plant size and adjusted the amount of water gift
in restricted water treatment to correct for larger plants consuming
more water (see section 2.4). Therefore, we are confident that larger
plants did not suffer greater drought stress than smaller ones in the
water-restricted treatment and that the pot-binding effect was minimised
as seen in Appendix Plate. A.1 and Appendix Plate. A.2. Secondly, in the
field, rooting depth can be a drought adaptive trait as it allows access
to deeper moister soil layers. This effect evidently could not be
mimicked in pots. Association of rooting depth with growth potential
could be either positive (fast-growth facilitating deeper roots) or
negative (larger deeper root systems imposing greater metabolic costs
and therefore, slowing growth). Altogether, it is important to determine
whether the drought-tolerance trade-off found in our study also occurs
in the field. If the observed growth-tolerance trade-off occurs under
field conditions, it would pose a dilemma for breeding on what to select
for if one cannot have both. For instance, selecting fast growth could
result in low drought tolerance which poses a challenge especially for
small scale coffee farmers who may not have irrigation facilities to
deal with drought spells. Therefore, to sustain C. canephoraproduction in drought-prone environments, breeders should break the
negative correlation between poor performance and tolerance (Table 4;
Fig. 3). This proposition agrees with Damatta et al., (2018) who
suggested that breeding for drought tolerance in coffee should aim at
developing tolerant genotypes with ”acceptable yields”. Despite the
adaptive advantage of slow growth (conservative resource-use strategy),
its positive association with low performance is also a challenge as
farmers are interested in good yields. Selection for either slow or
fast-growing genotypes should therefore be done in consideration of
whether the intended production is in stressful or optimal conditions.