Abstract
Calcium and potassium are essential for cell signaling, ion homeostasis and cell wall strength in plants. Unlike nutrients such as nitrogen and potassium, calcium is immobile in plants. Localized calcium deficiencies result in agricultural losses; particularly for fleshy horticultural crops in which elemental imbalances in fruit contribute to the development of physiological disorders such as bitter pit in apple and cork spot in pear. Currently, elemental analysis of plant tissue is destructive, time consuming and costly. This is a limitation for nutrition studies related to calcium in plants. Handheld portable x-ray fluorescence (XRF) can be used to non-destructively measure elemental concentrations. The main objective was to test if handheld XRF can be used for semi-quantitative calcium and potassium analysis of in-tact apple and pear. Semi-quantitative measurements for individual fruit were compared to results obtained from traditional lab analysis. Here, we observed significant correlations between handheld XRF measurements of calcium and potassium and concentrations determined using MP-AES lab analysis. Pearson correlation coefficients ranged from 0.73 and 0.97. Furthermore, measuring apple and pear using handheld XRF identified spatial variability in calcium and potassium concentrations on the surface of individual fruit. This variability may contribute to the development of localized nutritional imbalances. This highlights the importance of understanding spatial and temporal variability in elemental concentrations in plant tissue. Handheld XRF is a relatively high-throughput approach for measuring calcium and potassium in plant tissue. It can be used in conjunction with traditional lab analysis to better understand spatial and temporal patterns in calcium and potassium uptake and distribution within an organ, plant or across the landscape.
Highlights
Calcium and potassium are critical macronutrients for plants and essential for cell wall synthesis, signaling processes and cellular homeostasis (Marschner, 2011)
The relationship between calcium and the normalized counts becomes linear in the absence of high concentrations of potassium indicating that there is an interference from potassium on the analysis of calcium only when potassium is present in high concentrations
While destructive sample homogenization of dried samples or NIST standards used in previous studies is more reproducible (Reidinger et al, 2012), recreation of water-cellulose matrix spiked with known standards provided validation of nondestructive, whole tissue elemental analysis using a handheld portable x-ray fluorometer (PXRF)
Summary
Calcium and potassium are critical macronutrients for plants and essential for cell wall synthesis, signaling processes and cellular homeostasis (Marschner, 2011). Low calcium in fruit tissue has been associated with disorders such as Handheld XRF Elemental Analysis in Fruit blossom end-rot in tomato (Solanum lycopersicum L.) (Ho and White, 2005) and pepper (Capsicum annuum L.) (Marcelis and Ho, 1999), hollow-heart in potato (Solanum tuberosum L.) (Palta, 2010), bitter pit in apple (Malus domestica Borkh.) (Ferguson and Watkins, 1992; de Freitas et al, 2010), and cork spot in pear (Pyrus communis L.) (Mason and Welsh, 1970; Raese and Drake, 2006), among others These disorders limit the productivity of horticultural crops. Advances in the rate of elemental measurements are required to increase the capacity to understand the complexity of localized nutritional imbalances in fleshy horticultural plants that contribute to calcium-related disorders
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