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Leaf senescence is one of the most limiting factors to plant productivity under salinity. Both the accumulation of specific toxic ions (e.g. Na+) and changes in leaf hormone relations are involved in the regulation of this process. Tomato plants (Solanum lycopersicum L. cv Moneymaker) were cultivated for 3 weeks under high salinity (100 mM NaCl) and leaf senescence-related parameters were studied during leaf development in relation to Na+ and K+ contents and changes in abscisic acid (ABA), cytokinins, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the auxin indole-3-acetic acid (IAA). Na+ accumulated to a similar extent in both leaves 4 and 5 (numbering from the base of the plant) and more quickly during the third week, while concurrently K+ contents sharply decreased. However, photosystem II efficiency, measured as the Fv/Fm ratio, decreased from the second week of salinization in leaf 4 but only at the end of the third week in the younger leaf 5. In the prematurely senescent leaf 4, ABA content increased linearly while IAA strongly decreased with salinization time. Although zeatin (Z) levels were scarcely affected by salinity, zeatin-riboside (ZR) and the total cytokinin content (Z+ZR) progressively decreased by 50% from the imposition of the stress. ACC was the only hormonal compound that increased in leaf tissue coincident with the onset of oxidative damage and the decline in chlorophyll fluorescence, and prior to massive Na+ accumulation. Indeed, (Z+ZR) and ACC contents and their ratio (Z+ZR/ACC) were the hormonal parameters best correlated with the onset and progression of leaf senescence. The influence of different hormonal changes on salt-induced leaf senescence is discussed.

In citrus, the relative contributions of chloride and cations to growth disturbances induced by salinity are a matter of controversy. Chloride salts (15 mol m -3 CaCl 2 , 30 mol m -3 CaCl and 30 mol m -3 KCl) reduced growth and gas exchange parameters, increased leaf damage and abscission and produced anatomical disarrangements and mineral imbalances in seedlings of sensitive Carrizo citrange (Citrus sinensis x Poncirus trifoliata) and tolerant Cleopatra mandarin (Citrus reshni). In both cultivars, Ca 2+ was more beneficial, and K + more detrimental, for growth than sodium. Photosynthesis and growth disturbances were highly correlated (P ≤ 0·001) with leaf Clbuild-up. In the sensitive genotype, Cl -was also significantly correlated with several leaf anatomical disarrangements, such as increase in succulence. In comparison with sodium, both calcium and potassium increased leaf Clcontent (up to 25 and 69%, respectively). Protective calcium effects were not linked to improvement of photosynthesis, reduction of leaf anatomical disarrangements, or prevention of Cl -and Na + increases. It is proposed that the ameliorative effects of calcium on citrus grown under salinity are mostly related to reduction of leaf abscission. Collectively, the data suggest a cause-effect relationship between Cl -build-up and reduced growth, whereas chloride correlations with declines in photosynthesis or increases in succulence appear to be indirect.

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After infecting a susceptible host, pathogens spread throughout the plant. Depending on pathogen type and strain, the severity of symptoms varies greatly. In the case of pepper mild mottle virus (PMMoV) infection in Nicotiana benthamiana, newly developing leaves display visual symptoms (symptomatic leaves). In this study, two PMMoV strains were used, the Spanish strain (PMMoV-S) being more virulent than the Italian strain (PMMoV-I). Plants infected with PMMoV-I could recover from the virus-induced symptoms. Leaves that were fully developed at the start of PMMoV infection remained symptomless. In these asymptomatic leaves, an increase in temperature, initiating from the tissue adjacent to the main veins, was observed 7 d before the Chl fluorescence pattern changed. Virus immunolocalization on tissue prints matched well with the concomitant pattern of Chl fluorescence increase. The temperature increase, associated with the veins, was shown to be related to stomatal closure. Upon PMMoV-I infection, the appearance of thermal and Chl fluorescence symptoms as well as virus accumulation were delayed by 3 d compared with PMMoV-S-induced symptoms. The temperature increase of whole symptomatic leaves was also correlated with a decrease in stomatal aperture. In contrast to the persistent increase in symptomatic leaf temperature observed during PMMoV-S infection, the temperature of symptomatic leaves of PMMoV-I-infected plants decreased gradually during recovery. We propose that the earliest temperature increase is caused by a systemic plant response to the virus infection, involving the control of water loss. In conclusion, thermography has potential as an early reporter of an ongoing compatible infection process.

Annals of Botany 79 (1997) 153-160. doi:10.1006/anbo.1996.0326Received by publisher: 1996-04-18Harvest Date: 2016-01-04 12:22:11DOI: 10.1006/anbo.1996.0326Page Range: 153-16

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