Altered calcium and potassium distribution maps in tomato tissues cultivated under salinity: studies using X-ray fluorescence (XFM) microscopy

Abstract

Calcium (Ca2+) distribution into relatively low-transpiring fruit versus relatively high-transpiring leaves is of concern to the fruit industry. Ca2+ uptake is further compromised under salinity through antagonistic competition with sodium (Na+). Herein, X-Ray Fluorescence (XFM) was applied to elucidate in-situ localization and distribution patterns of Ca2+ and potassium (K+) in tomato leaflet and fruit tissues under salinity versus non-saline controls. Leaflet Ca2+ signal was up to 7 times higher than that in fruit. Highest Ca2+ hotspots were in leaflet blades, notably at their margins. XFM spatial maps revealed striking contrasts in K+ versus Ca2+ patterns along proximal–distal mesocarp tissues. Under compressive stress, proximal fruit flesh tissues were firmer, stiffer and showed higher resilience to mechanical deformation than distal tissues. This intrinsic mechanical gradient property was positively correlated with Ca2+ locality. Added Ca2+ ameliorated mechanical failure induced by Na+ and restored fruit firmness, but not stiffness and peak force. The exocarp had the strongest Ca2+ signal in fruit tissue. The weakest was in the locular cavity. Ca2+ in Blossom End Rot (BER) affected tissue was up to fourfold that in immediately surrounding sound tissue, reflecting cell wall collapse. New insights reported here into Ca2+ and K+ dynamics in tomato mesocarp under salinity offer improved understanding of fruit mechanical properties and Ca2+-deficiency.