|Authors: ||G. Cimo, R. Lo Bianco, P. Gonzalez, W. Bandaranayake, E. Etxeberria, J.P. Syvertsen|
|Keywords: ||blotchy mottle, leaf boron, 'Cleopatra' mandarin, 'Swingle' citrumelo|
The most important problem in world citrus production is the bacterial disease Huanglongbing (HLB; greening) which is caused by a phloem-limited bacterium that is vectored by a phloem-feeding psyllid.
The earliest visible symptoms of HLB in leaves are an asymmetrical chlorosis referred to as “blotchy mottle”, thought to be from starch accumulation from a phloem dysfunction and a decline in root health.
We tested the hypothesis that such visible symptoms are not unique to HLB by stem-girdling two year-old seedling trees of 'Cleopatra' mandarin and 'Swingle' citrumelo rootstocks in the greenhouse.
Girdling induced a 4-fold greater starch concentration in leaves on well-watered trees while starch in woody roots of girdled trees decreased up to 19 fold relative to non-girdled trees.
Drought stress cycles induced some starch accumulation in non-girdled roots but there were no effects of drought stress on root starch in girdled trees.
Girdling reduced leaf transpiration in well-watered trees.
Leaves on girdled trees clearly had HLB-like visible blotchy mottle symptoms but no visible symptoms developed on non-girdled trees.
The up to 40% increase in leaf starch increased leaf dr wt per leaf area (DW/LA) and consequently reduced many leaf nutrients on a leaf DW basis.
Most of these differences disappeared when expressed on a LA basis except for the girdle-induced decreases of leaf phosphorous and sulphur.
Leaf boron (B) was inversely related to leaf starch when both were expressed on a LA basis.
In the absence of HLB, girdling increased leaf starch, decreased root starch, and duplicated the asymmetric blotchy mottled visual leaf symptoms that have been associated with HLB-infected trees.
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