Theresults of our study found a negative correlation between trichome density andstomatal index within Arabidopsisthaliana (Fig. 2A), and a reduced stomatal density in trichome-producingplants in a wild population of Arabidopsishalleri (Fig. 3A). This suggests a cost associated with the production oftrichomes, specifically a trade-off between trichome and stomata formation. However,within accessions of Arabidopsisthaliana, we found no significant correlation between stomatal index andtrichome number, likely due to the small sample size. Exploring the benefit oftrichomes in Arabidopsis, weconducted a drought experiment, however the results were of little significance.
4.1 Negative correlation between trichomenumber and stomatal index Our findingssuggest that across a range of accessions of Arabidopsis thaliana there is a significant negative correlationbetween trichome density and stomatal density (-0.242, p = 0.02). This suggeststhat during development, the plants are undergoing a trade-off between theformation of stomata and trichomes.
Previous studies have indicated a cost oftrichomes in relation to growth, with trichome producing plants facing a growthpenalty in control treatments compared to glabrous plants (Sletvold and Ågren 2012). Our observations regarding increasedtrichome number correlating with reduced stomatal density could explain thisdetrimental effect of trichomes under herbivore and drought free conditions. Areduction in stomatal density may lead to reduced stomatal conductance, thereforereducing carbon fixation (Tanaka et al. 2013), and negatively impacting uponplant growth. However,the number of trichomes is not consistent throughout a plants life. In Arabidopsis, leaves produced early inrosette development lack abaxial trichomes, but in older plants, newlydeveloping leaves possess trichomes on both abaxial and adaxial surfaces (Telfer et al. 1997).
Additionally, trichome production on stem internodes and floral organs has beenshown to be gradually repressed following bolting, via temporal control of SPLgenes (Yu et al. 2010). For further research it would beof interest to measure the correlation between trichomes and stomata at multiplestages throughout development, to see if plant age has any effect.4.2 Positive correlation between trichomenumber and plant mass and negative correlation for leaf size Inthis study we used four different accessions of Arabidopsis thaliana to provide a range of trichome densities.
However, this introduced a number of confounding variables affecting growth,most notably in the accession WS. WS grew to a much larger size, and possiblyskewed the correlation between mass and trichome number, which was shown to bestrongly positive across accessions (0.401, p = 0.000). Future research shouldbe conducted within one accession, polymorphic for trichome production, inorder to control for these variables. Inour study, we found a significant negative correlation between leaf size andtrichome density (-0.
342, p = 0.001). Trichome formation occurs early indevelopment and during leaf maturation the distance between trichomes increasesdue to mitotic divisions in the epidermal cells, separating trichomes (Hülskamp and Schnittger 1998) and reducing density, explainingthis negative trend. 4.3 Tolerance to drought Thedrought experiment, although showing significant differences between accessionsin above ground mass and area, was severely limited.
In the control treatment(Fig. 4A, Fig. 4B), Col0 had a significantly greater mean mass and mean areathan C24.
This indicates that other factors influencing growth are present, andthat the significant difference in mass between the accessions is due to othereffects. If we were to repeat the study, a single accession would be used,polymorphic for trichome production to control for these other factors.Additionally, the plants were grown in a growth chamber lacking in humiditycontrol and under time constraint, meaning that the severity of drought washard to control. Finally, due to time constraints we were unable to record thetrichome density of the individual plants, which would have provided informationabout the relationship between drought tolerance and trichome density.Despiteour findings, previous field experiments indicate that tolerance of plants todrought conditions are affected by trichome density (Sletvold and Ågren 2012). Leading on from our findings thatincreased trichome production is correlated with reduced stomatal density (Fig.
2A, 3C), this could provide a possible mechanism for the higher tolerance todrought found by Sletvold. To explore whether these differences in tolerance todrought are due to the presence of trichomes or other confounding effects,further investigation into a range of physiological traits, such as stomatalconductance, photosynthesis and evapotranspiration, could expose the mechanismsaffecting drought tolerance. 4.4 Wild population of Arabidopsis halleri showed reducedstomatal density in trichome producing morphs Thewild population of Arabidopsis halleri studiedwas formed from a single subspecies, polymorphic for trichome production, andexplores whether the trend we observed in laboratory conditions is present in awild population. We found a significantly reduced stomatal index amongtrichome-producing plants when compared to their glabrous counterparts (Fig. 3C).
This suggests that a correlation between trichomes and stomata exists withinwild populations. However, we have no indication of the density of trichomes,and due to other unaccounted environmental effects (Fig. S1) this could varysignificantly between samples. Additionally, Arabidopsis halleri is self-incompatible, maintaining high geneticdiversity which could cause differential responses to the environment (Clauss and Koch 2006).Previousstudies have shown that interactions between stomatal complexes and trichomes duringdevelopment are usually competitive, acting in a complex web of positive andnegative interactions. In Arabidopsisthaliana, mutants in the gene STOMATAL DENSITY AND DISTRIBUTION 1 (SDD1)controlling stomatal patterning and density have up to afourfold increase in stomatal density (Von Groll et al. 2002), and demonstrate a strongreduction in trichome number (Berger and Altmann 2000). Additionally, in tobacco plants,ectopic expression of the MIXTA gene from Antirrginummajus results in a significant increase in trichomes, coupled with areduction in stomatal density (Glover et al.
1998). Possible linkage of stomataldensity and trichome number through gene expression (e.g. SDD1) could explainthe negative correlation that we have observed in Arabidopsis (Fig.
2A, Fig. 3C).Over-expressionof SDD1 shows reduced stomatal density, and subsequently reduced photosynthesisdue to limited CO2 at high light (Büssis et al.
2006). In agriculture, a major field ofresearch is the production of drought resistant crops, with maximisation ofphotosynthesis and reduction of unnecessary transpiration being the main aim. Previousresearch into Arabidopsis thalianahas shown increased tolerance to drought through a reduction in stomataldensity (Hepworth et al. 2015). However, without considering thecomplex set of biochemical interactions occurring on the leaf surface, a changein stomatal density may have unforeseen impacts on the plant, possibly evenreducing crop yield.
This explains trends observed in previous studies, whereglabrous plants grew larger than trichome-producing plants in the controltreatment (Sletvold and Ågren 2012). More research into the effects oftrichome density on rate of transpiration is required, as it is possible thatthe presence of trichomes is having a significant effect on plants ability toaccumulate biomass. Trichomesprovide many benefits in agriculture through resistance to a range of arthropodherbivores, and also increasing drought tolerance. However, the possible costof trichomes in relation to stomatal conductance should be considered whenbreeding for insect resistance. Selection for increased herbivore resistance inagronomically important crops, such as Solanumberthaultii (Kalazich and Plaisted 1991), has been shown to come withmany unwelcome traits, such later maturing plants, and even reductions in yield.4.
5 Conclusions Insummary, a negative correlation between stomata and trichomes was found to beexpressed in Arabidopsis thaliana, andalso in a wild population of Arabidopsishalleri. These results indicate that plants may be undergoing a trade-offin development of trichomes and stomata, which may have an impact of transpiration,and ultimately stress tolerance in relation to drought.