Do thick leaves avoid thermal damage in critically low wind speeds?
New Phytol. 2012 Apr; 194(2):477-87
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Leigh et al. assess the importance of leaf thickness in ameliorating the effects of transient heat stress events and show that thermal damage in thick leaves that experience transient heating.
In recent years, growing interest has been given to the functional and evolutionary significance of the ratio of leaf mass per unit area among contrasting plant species adapted to harsh and favourable environments (Wright et al. 2004; Poorter et al. 2009). Such studies have highlighted the importance of inter- and intra-specific variations in leaf density and thickness in determining leaf life span and metabolic rates, with slow-growth and long-life span being linked to leaves that are dense and often very thick. Such species typically occur in stressful habitats where resource availability is low. In the current paper, Leigh et al. consider a further potential adaptive benefit of increased leaf thickness – that being the extent to increased thickness dampens rapid increases in leaf temperature in hot environments were wind speeds are low. Building on past work highlighting links between heat capacity per unit leaf area and leaf succulence (Ball et al. 1988) and published data on leaf traits and thermotolerance (e.g. Knight and Ackerly 2002), the authors use data modelling show that even small increases in leaf thickness can markedly reduce peak leaf temperatures under conditions of low windspeed, particularly in leaves that lack other mechanisms of limiting rapid rises in leaf temperature. The results raise the exciting prospect that there has been selection for increased leaf thickness in hot environments to reduce leaf temperatures, and in doing so, further increase leaf life span in some of the hottest, driest parts of the world (via limiting thermal damage to metabolic processes).
Atkin O: F1000Prime Recommendation of [Leigh A et al., New Phytol 2012, 194(2):477-87]. In F1000Prime, 04 Jul 2012; DOI: 10.3410/f.717598018.793052931. F1000Prime.com/717598018#eval793052931
F1000Prime Recommendations, Dissents and Comments for [Leigh A et al., New Phytol 2012, 194(2):477-87]. In F1000Prime, 20 Jun 2013; F1000Prime.com/717598018
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Transient lulls in air movement are rarely measured, but can cause leaf temperature to rise rapidly to critical levels. The high heat capacity of thick leaves can damp this rapid change in temperature. However, little is known about the extent to which increased leaf thickness can reduce thermal damage, or how thick leaves would need to be to have biological significance. We evaluated quantitatively the contribution of small increases in leaf thickness to the reduction in thermal damage during critically low wind speeds under desert conditions. We employed a numerical model to investigate the effect of thickness relative to transpiration, absorptance and leaf size on damage avoidance. We used measured traits and thermotolerance thresholds of real leaves to calculate the leaf temperature response to naturally occurring variable low wind speed. Our results demonstrated that an increase in thickness of only fractions of a millimetre can prevent excursions to damaging high temperatures. This damping effect of increased thickness was greatest when other means of reducing leaf temperature (transpiration, reflectance or reduced size) were lacking. For perennial desert flora, we propose that increased leaf thickness is important in decreasing the incidence of extreme heat stress and, in some species, in enhancing long-term survival.
© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.
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