Spatiotemporal distribution of essential elements through Populus leaf ontogeny

Abstract

We examined the spatiotemporal distribution and accumulation of calcium (Ca), potassium (K), and zinc (Zn) during the growth and maturation of grey poplar (Populus tremula × alba) leaves covering plastochrons 01 through 10. This period spans the sugar sink-to-source transition and requires coordinated changes of multiple core metabolic processes that likely involve alterations in essential and non-essential element distributions as tissues mature and effect a reversal in phloem flow direction. Whole-leaf elemental maps were obtained from dried specimens using micro X-ray fluorescence spectroscopy. Additional cross-sections of fresh leaves were scanned to check for tissue specificity in element accumulation. The anatomical distribution of Zn and K remains relatively consistent throughout leaf development; Ca accumulation varied across leaf developmental stages. The basipetal allocation of Ca to the leaf mesophyll matched spatially and temporally the sequence of phloem maturation, positive carbon balance, and sugar export from leaves. The accumulation of Ca likely reflects the maturation of xylem in minor veins and the enhancement of the transpiration stream. Our results independently confirm that xylem and phloem maturation are spatially and temporally coordinated with the onset of sugar export in leaves.

Keywords: Calcium; Populus; X-ray fluorescence spectroscopy.; essential elements; leaf development; leaf maturation; phloem loading.

Fig. 1.

Composite µ-XRF maps of the abundance of Ca (green), K (red), and Zn (blue) (see colour gradients at the bottom of the figure) in developing leaves of grey poplar at LPI 01 (CHESS scan cycle/run 2014–2/102), LPI 02 (CHESS 2014–2/107), LPI 03 (CHESS 2014–2/120), LPI 04 (CHESS 2014–2/110), LPI 05 (CHESS 2014–3/1530), LPI 06 (CHESS 2014–3/1534), and LPI 10 (CHESS 2014–2/494). Fluorescence intensities of elements were normalized using reference standards, and provide a direct comparison of element abundance across representative leaves for each of the plastochrons shown (see centred scale at bottom for each of the three elements). Maximum pixel brightness corresponds to maximum abundance for each element. Note basipetal accumulation of Ca as leaves mature as shown by green colouration in leaves LPI 05, LPI 06, and LPI 10. Insert A highlights areas characterized by high abundance of Ca and Zn in teeth apices in LPI 01. Insert B highlights areas characterized by high abundance of Ca and Zn at the base of LPI 02 (see arrows). Insert C highlights the absence of co-localized Ca and Zn in teeth apices LPI 10 (see arrows).

Fig. 2.

Elemental µ-XRF map for Zn abundance for developing leaves of grey poplar. Image intensity matches element abundance (see scales in A and C). A Early developing leaf at LPI 03. Maximum Zn = 1615 µg Zn/g dry mass. B Selected area from Fig. 2A highlighting the localization of Zn at leaf teeth apices (see arrows). C Fully mature leaf at LPI 10. Maximum Zn = 1305 µg Zn/g dry mass. D Detail of Zn localization around major veins at LPI 07. E Transversal section of a fully mature leaf at LPI 10 showing the accumulation of Zn in the upper and lower epidermis and in the peripheral vasculature of the midvein.

Fig. 3.

Elemental µ-XRF map for Ca for developing leaves of grey poplar. Image intensity matches element abundance (see scales in A and B). A Early developing leaf at LPI 03, showing Ca localization in teeth apices (see arrows) and major veins. Maximum Ca = 10625 µg Ca/g leaf dry mass. B Fully mature leaf at LPI 10 showing the localization of Ca in higher order veins and in the mesophyll as seen from above (see C for a transverse section). Maximum Ca = 10670 µg Ca/g leaf dry mass C Transversal section of fully mature leaf at LPI 10 showing Ca in mesophyll, the peripheral tissues in the midvein, and peripheral tissues of the midvein vascular strand.

Fig. 4.

Variation in Ca abundance (i.e. g Ca/g dry mass) along longitudinal transects across grey poplar leaves in different stages of development (LPI 01, LPI 03, LPI 06, and LPI 10) (see dashed line in upper insert for transect orientation across each leaf). Solid lines in each graph indicate average leaf Ca abundance for each leaf. Ca abundance peaks in each graph correspond to the locations of major veins; peak maxima increase as leaves mature from LPI 01 to LPI 10. Ca accumulation occurs basipetally (from the apex to the leaf base) (see also Fig. 1).

Fig. 5.

Elemental µ-XRF map for K for developing leaves of grey poplar. Image intensity corresponds to element abundance (see scales in A and B). A Early developing leaf at LPI 03. Maximum K = 57290 µg K/g leaf dry mass. B Fully mature leaf at LPI 10. Maximum K = 55900 µg K/g leaf dry mass. C Transverse section of fully mature leaf LPI 10 showing accumulation of K in the upper epidermis and mesophyll. D Detail of K accumulation in a developing leaf at LPI 04. E Cellulose distribution as shown in D based on predicted leaf thickness (using µ-XRF data and assuming cellulose as the primary source of dry mass absorption). In D and E, the abundance of K in veins appears high because the veins are denser than lamina tissues.

Fig. 6.

Leaf growth parameters and changes in element abundance (i.e. g element/g dry mass) in relation to leaf development and the transition from phloem importing to phloem exporting, as denoted by the shaded blocks (see notation at the bottom of each graph). A Leaf area. B Leaf dry weight. C Leaf mass per area (LMA). D Zn abundance. E Ca abundance. F K abundance (diagonal line denotes ordinary regression line for abundance vs. LPI).