Evidence across 4 accessions (-0.242, P = 0.02).

Evidence exists for arelationship between formation of stomata and trichomes in plant development.

However, no study has considered the cost of trichome development in regards tostomatal density. Four accessions (WS, C24, Col0 and Lansberg erecta) of Arabidopsis thaliana that varied intrichome density were grown under 12-h /12-h photoperiod and key plant traits(stomatal density, mass, trichome density and area) were recorded. Arabidopsis halleri was observed in thefield and stomatal density recorded for glabrous and trichome-producing morphs.A drought experiment was performed with two accessions (C24 vs Col0), however hadserious limitations.

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In Arabidopsisthaliana, stomatal density was negatively correlated with trichome densityacross 4 accessions (-0.242, P = 0.02). In Arabidopsishalleri glabrous plants had higher stomatal density than trichome-producingplants (0.181 vs 0.161 respectively; P < 0.05).

A negative correlation betweentrichome density and stomatal index within Arabidopsisthaliana, and also a reduced stomatal density in trichome-producing plantsin a wild population of Arabidopsishalleri suggests a cost associated with trichomes in relation to stomata.This relationship should be considered in breeding of agronomically importantcrops.1.IntroductionPlants have developed adiverse range of traits against unfavourable biotic and abiotic conditions: secondarymetabolites, which reduce palatability (Howe and Jander 2008), and  morphological structures, such as trichomesand thorns, which disrupt movement of herbivores (War et al. 2012). Within a population,there is quantitative variation within these traits providing some individuals withgreater resistance whilst others are more susceptible.

This variation poses aninteresting question; if there was no cost to resistance, what is preventingall plants from developing resistance to abiotic or biotic stresses? A possibletrade-off between defences and growth could serve as an explanation for this. Defenceis a costly process, and resource allocation optimised for growth orreproduction (Mauricio 1998; Wise 2007).The flowering plant Arabidopsis is polymorphic for trichomeproduction, with glabrous and trichome producing morphs. Within populations andeven within individual plants, trichomes vary considerably in their morphologyand density. In Arabidopsis thaliana, trichomesare large, single, non-glandular cells that begin development in the epidermisof young leaves (Dalin et al. 2008). These smallhairs are able to store metals (Sarret et al.

2002), provide a rangeof anti-herbivorous functions (Dalin et al. 2008; Levin 1973) and even conferresistance to drought (Sletvold and Ågren 2012). Previousresearch on the possible costs of trichomes has found that they conferresistance to both drought conditions and herbivory (Sletvold and Ågren 2012), but in controltreatments they pose a significant cost in growth (Mauricio 1998; Sletvold et al. 2010). A possible mechanism for thisreduction in growth could be a reduction in stomatal density. During leaf development,there are constraints on space, and for epidermal functioning, a balancing actof cell types is required (Dow et al.

2014). Previousresearch on patterning of stomata and trichomes in the epidermis in Arabidopsis thaliana has shown thatformation of stomata and trichomes are not independent events, but interactionsbetween numerous genes (J. Bean et al. 2002). As space islimited to a single field of cells, it is conceivable that an increased numberof trichomes could lead to a reduced stomatal density, resulting in a possible trade-offbetween the two leaf traits. We conducted ourexperiment to understand the relationship between trichome density and stomataldensity: we wanted to determine whether (1) there is a trade-off associatedwith trichome formation, (2) does the trade-off exists in a wild population,and (3) if tolerance to drought is positively related to trichome density.Greenhouse experiments looked at Arabidopsisthaliana, which varies quantitatively in trichome density betweenaccessions, providing a range of allocation to defence.

The wild population wasformed of Arabidopsis halleri.