Leaf starch and nutrient responses to stem girdling and drought stress with respect to understanding HLB (greening) symptoms in citrus

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 stemgirdling 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.