Reflectance Spectroscopy for Non-Destructive Measurement and Genetic Analysis of Amounts and Types of Epicuticular Waxes on Onion Leaves

Abstract

Epicuticular waxes on the surface of plant leaves are important for the tolerance to abiotic stresses and plant–parasite interactions. In the onion (Allium cepa L.), the variation for the amounts and types of epicuticular waxes is significantly associated with less feeding damage by the insect Thrips tabaci (thrips). Epicuticular wax profiles are measured using used gas chromatography mass spectrometry (GCMS), which is a labor intensive and relatively expensive approach. Biochemical spectroscopy is a non-destructive tool for measurement and analysis of physiological and chemical features of plants. This study used GCMS and full-range biochemical spectroscopy to characterize epicuticular waxes on seven onion accessions with visually glossy (low wax), semi-glossy (intermediate wax), or waxy (copious wax) foliage, as well as a segregating family from the cross of glossy and waxy onions. In agreement with previous studies, GCMS revealed that the three main waxes on the leaves of a wild type waxy onion were the ketone hentriacontanone-16 (H16) and fatty alcohols octacosanol-1 (Oct) and triacontanol-1 (Tri). The glossy cultivar “Odourless Greenleaf” had a unique phenotype with essentially no H16 and Tri and higher amounts of Oct and the fatty alcohol hexacosanol-1 (Hex). Hyperspectral reflectance profiles were measured on leaves of the onion accessions and segregating family, and partial least-squares regression (PLSR) was utilized to generate a spectral coefficient for every wavelength and prediction models for the amounts of the three major wax components. PLSR predictions were robust with independent validation coefficients of determination at 0.72, 0.70, and 0.42 for H16, Oct, and Tri, respectively. The predicted amounts of H16, Oct, and Tri are the result of an additive effect of multiple spectral features of different intensities. The variation of reflectance for H16, Oct, and Tri revealed unique spectral features at 2259 nm, 645 nm, and 730 nm, respectively. Reflectance spectroscopy successfully revealed a major quantitative trait locus (QTL) for amounts of H16, Oct, and Tri in the segregating family, agreeing with previous genetic studies. This study demonstrates that hyperspectral signatures can be used for non-destructive measurement of major waxes on onion leaves as a basis for rapid plant assessment in support of developing thrips-resistant onions.

Leaf spectroscopy of resistance to Ceratocystis wilt of ‘Ōhi’a

“…As with with other aggressive forest diseases, it is necessary to detect infected individuals, understand how the pathogen affects the host, and then identify resistant individuals. The potential of using remote sensing to screen for resistant individuals has been demonstrated in both agricultural and forest systems [16,20,34,36]. Here, we demonstrate that resistance in M. polymorpha to C. lukuohia is detectable using VSWIR spectroscopy.…”

“…This, in turn, has resulted in heritable differences in reflectance spectra of M. polymorpha across environmental gradients [70]. Disease resistance screening using spectroscopy is in early stages of development, but this technique has been demonstrated to successfully distinguish between resistant and susceptible hosts in a number of forest and agricultural plant species [16,20,34,36]. This study demonstrates that a resistance signal to C. lukuohia infection is present in leaf reflectance data, but because of the influence of site and genetic factors on leaf spectra, it is important to continue considering these factors in future investigations.…”

“…Some methods use the entire visible to shortwave infrared (VSWIR) spectra (350-2500 nm) while others capture only the visible to near infrared (VNIR; 350-1050 nm) spectra. Not only have traits captured in spectroscopic data been used to understand the genetic structure of plants and their evolutionary history [30,32,33], but reflectance spectra have been shown to distinguish individuals based on their resistance to diseases [16,20,[34][35][36][37]. For example, spectroscopy is a promising method for both determining putatively disease resistant germplasm and nondestructively monitoring changes in leaf traits.…”

Onion Germplasm Possesses Lower Early Season Thrips Numbers

“…Research has found that onions with waxy leaves suffer more thrips damage compared to onions with semi-glossy and glossy leaves, since their leaves accumulate only intermediate or sparse amounts of epicuticular waxes and have lighter green colors that result in slower thrips population growth and reduced egg hatching [8,14,[16][17][18][19]. Lower wax phenotypes can be conditioned by a single recessive locus (gl), such as the single recessive locus (gl wp ) found in the glossy phenotype White Persian, which has been mapped to chromosome 8 and been shown to interact with a region of chromosome 1 that acts to increase waxy progeny numbers [20][21][22][23]. Relative to other epicuticular waxes, the amount of hentriacontanone-16 wax may also influence the total amount of leaf wax [24].…”

Genetic Analysis of the Unique Epicuticular Wax Profile of ‘Odourless Greenleaf’ Onion

“…The onion cultivar Odourless Green Leaf (OGL) has a light green leaf color, is less attractive to thrips relative to waxy onion (Cramer et al, 2014), and has a unique profile of epicuticular waxes (Munaiz et al, 2020b). During this research, we determined the genetic basis of the wax profile and glossy phenotype of OGL, and we showed that it is inherited independently from the gl wp locus (Munaiz and Havey, 2020).…”