Changes in leaf epicuticular wax load and its effect on leaf temperature and physiological traits in wheat cultivars (Triticum aestivum L.) exposed to high temperatures during anthesis

Authors: HUGGINS, TREVIS - Texas A&M University; MOHAMMED, SUHEB - Texas A&M University; SENGODON, PADMA - Texas A&M University; IBRAHIM, AMIR - Texas A&M University; Tilley, Michael - Mike; HAYS, DIRK - Texas A&M University

Submitted to: Journal of Agronomy and Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2017
Publication Date: 7/20/2017
Citation: Huggins, T., Mohammed, S., Sengodon, P., Ibrahim, A., Tilley, M., Hays, D. 2017. Changes in leaf epicuticular wax load and its effect on leaf temperature and physiological traits in wheat cultivars (Triticum aestivum L.) exposed to high temperatures during anthesis. Journal of Agronomy and Crop Science. 204:49-61. https://doi.org/10.1111/jac.12227.
DOI: https://doi.org/10.1111/jac.12227

Interpretive Summary: The ability of a plant to survive high temperature conditions depends in part on its ability to reduce the amount of irradiation entering the leaf, and water loss. Plants generally cool their canopies through transpiration, which occurs by way of stomata when soil water is available Efficient reproductive development requires higher gas exchange, which occurs by way of open stomata, putting plants in a constant battle for survival against severe environments. Epicuticular wax (EW) is an important adaptive trait that covers leaf surfaces forming a barrier between the environment and the plant, protecting the plant from both biotic and abiotic stress. EW reduces incident irradiation and by providing a barrier against water loss through epidermal transpiration in hot and dry environments, however, during stress changes in the photosynthetic pigments do occur and can be detected through changes that occur in the reflectance signatures of leaves. This study was conducted to determine the relation between increased EW and leaf temperature depression and stomatal conductance and improved photosynthetic function. Additionally, we wanted to determine if an early deposition of EW prior to the onset of heat stress rather than in response to improved tolerance in terms of physiology and yield. Twelve wheat cultivars were grown in a greenhouse under optimal growth conditions under natural sunlight. At the primary inflorescence stage, when the ear is clear of leaf sheath, and no flowers are visible, half the pots for each cultivar were subjected to high temperature stress conditions under natural sunlight at ~400C/200C day/night. Measurements were collected from seven growth stages for both the high temperature and control treatments. The results indicate that heat tolerant wheat cultivars can effectively use EW as one of the physiological components used to adjust stomatal conductance, chlorophyll fluorescence and leaf temperature. Tolerant cultivars were also able to maintain their seed weight under stressed conditions.

Technical Abstract: The physiological functions of epicuticular wax (EW) include reflectance of irradiation and the reduction of water loss. When a plant experiences stressful conditions, most notably, high irradiance and temperature, damage to the photosynthetic apparatus can occur and is signaled by a decrease in the Fv / Fmax ratio. In the present study, we examined the influence of increased EW on physiological function in terms of chlorophyll fluorescence (ChFl), stomatal conductance (gs), leaf temperature and spectral reflectance indices (SRI) of bread wheat (Triticum aestivum L.) cultivars. The wheat cultivars were subjected to high temperature stress (HT) (38-40 ºC) under greenhouse conditions when the primary inflorescence was fully emerged to determine its effect on leaf EW deposition. Leaf temperature depression (LTD) was generally lower in control (2.30C- 2012, 0.940C- 2013) compared to HT stress (3.130C- 2012, 4.05 ºC- 2013). Cultivars in control (0.69 to 0.74 Fv/Fmax) had significantly higher ChFl compared to HT (0.58 to 0.74 Fv/Fmax). HT treatment resulted in higher EW (1.28 – 2012, 4.59 mg dm-2 – 2013) compared to control treatment (1.04 - 2012 to 4.56 mg dm-2 - 2013). Leaves devoid of EW showed significant variation among cultivars at reproductive stages for water index (WI), normalized phaeophytinization index (NPQI), and simple ratio index (SRI). In HT stress conditions, significant correlations were observed between EWL and SRI only at 3DAFE (days after full emergence), suggesting that increased EWL induced by HT and irradiation in early development may provide relief and prevent grain loss. EWL significantly associated with the physiological traits ChFl, gs, LTD and spike temperature depression (SpTD). These observations suggest that EWL may lessen the effect of high irradiance, thereby, effectively adjusting stomatal conductance, ChFl and leaf temperature, limiting the risk of over excitation of photosystem II.