- HUANGLONGBING: DEVASTATING DISEASE OF CITRUS 331
Mean leaf CLas titer (log CN)0123456Starch (% dry wt.)02468101214
Leaf
RootFig. 7.4. Citrus leaf and root CLas titer determined by PCR (CLas 16S rDNA log cycle
number, CN) plotted against starch content (McCollum and Hall, unpublished).
et al. 2009; Achor et al. 2010; Fan et al. 2010; Fan et al. 2012). Starch
accumulates not only in photosynthetic cells of CLas-infected leaves,
but also in phloem elements and vascular parenchyma in leaves and
petioles, in xylem parenchyma, and in the phelloderm (Etxeberria et al.
2009). In contrast to aerial plant parts, starch is depleted in roots of
CLas-infected trees (Etxeberria et al. 2009; Aritua et al. 2013; Johnson
et al. 2014; McCollum and Hall, unpublished). It has been suggested
(Johnson et al. 2014) that in roots, carbohydrate starvation is not respon-
sible for the HLB-associated root decline because the starch content
of these roots only decreased significantly when canopy decline was
severely affected by HLB. Fig. 7.4 shows the accumulation of starch in
leaves compared to roots in relation to CLas titer determined by PCR
(McCollum and Hall, unpublished).
Because accumulation of starch precedes foliar chlorosis (Achor et al.
2010), it is possible that the reduction in availability of fixed carbon
for roots takes place well before HLB symptoms become obvious in the
canopy. Root extension growth falls by more than 50% in 10 h in Ara-
bidopsis plants subjected to an unexpected extension of the night. After
starch reserves are exhausted, inhibition of growth is not immediately
reversed when carbon becomes available again (Yazdanbakhsh et al.
2011). Tree growth can be limited by the availability of carbon within
the plant (carbon limitation), and sequestration of recently fixed carbon
as starch may cause carbon limitation (Wiley and Helliker 2012).