A survey of Siem mandarin trees (Citrus reticulata Blanco) grown in different soil types and given different fertiliser regimes showed that applications of a foliar fertiliser reduced the symptom expression of trees infected with huanglongbing by~40%. In contrast, infected trees growing in a sandy soil, or in a clay-loam, did not differ in their level of symptom expression, despite differences in the fertility of the two soils. These data suggest that infection restricts either nutrient uptake or transport and, therefore, that foliar applied minerals may prolong tree life and reduce yield losses.
Some plants are able to tolerate salinity using salt glands on the leaf surface, which secrete excess salts transported into the leaves. Rhodes grass (Chloris gayana Kunth) reportedly possesses such salt glands, but the features of secretion remain unclear. In the present study, we compared the ability of Rhodes grass salt glands to secrete sodium and potassium with the aim of clarifying the preference of cations for secretion. In both whole plant experiments and detached leaf experiments, NaCl treatment increased Na + secretion and KCl treatment increased K + secretion. When the ratios of the amount of secreted ions to their concentration in the leaves were compared between Na + and K + , the ratio was greater with Na + than K + .
Alfalfa (Medicago sativa L.) was grown in hydroponic culture to investigate adaptation to Fedeficiency. Root exudates released into the nutrient solution from Fe-deficient plants were trapped and condensed on an amberlite XAD-4 resin column. The diethyl ether fraction of these exudates dissolved ferric phosphate remarkably. The dissolving capability was about 62 times higher than that of root exudates obtained from Fe-sufficient plants in complete nutrient solution. The Fe-dissolving compound was separated and identified. It was a new natural compound with molecular formula C~4H~005 and was identified as 2-(3',5'-dihydroxyphenyl)-5,6-dihydroxybenzofuran by means of mass spectrometry and 1H-nuclear magnetic resonance. This new compound worked as a phytoalexin and inhibited completely the fungal growth of Fusarium oxysporum f. sp. phaseoli.
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