Starch Analysis of HLB-affected and Control Healthy Citrus Leaves Reveal Variations in the Amylose/ Amylopectin Ratio
- plant disease,
- phloem disease,
- starch properties,
- starch accumulation
Leaves of HLB (Huanglongbing or citrus greening)-affected branches contain considerably higher levels of starch than those of HLB-unaffected or “healthy trees.” Over-accumulation of starch takes place in photosynthetic cells, vascular parenchyma, ray parenchyma, and even sieve elements. These observations imply strong disturbances in starch metabolism and photoassimilate partitioning brought about by HLB. The elevated starch content, appearance of starch granules in phloem elements, and previous reports of the pathogen effect on starch properties lead us to hypothesize that starch from HLB-affected citrus differs morphologically, physically, and/or chemically from starch accumulated in otherwise healthy leaves. We investigated starch morphology using brightfield, polarized light and SEM and found no morphological differences between HLB-induced starch and that from healthy girdled trees. When reacted with 2% I2, whole starch fractions also showed no significant difference in the absorption spectra (λmaxfor HLB = 604.1 and girdled 606.2 nm, respectively; n = 6; P ≤0.05) nor in their amylose/amylopectin ratio (HLB = 1.4 ± 0.17 and girdled = 1.16 ± 0.07, P ≤0.05) after chromatographic separation. Nevertheless, λmaxfor individual fractions of HLB-affected leaves increased between 11 to 14 nm indicating a significant increase in the degree of polymerization of chain lengths estimated between 12 to 45 glucose units. The increase in amylopectin chain length was verified by the rise in gelatinization temperature of approximately 10 °C observed by polarized light microscopy. Our results indicate that starch grains from leaves affected by HLB, although morphologically similar, differed biochemically from those formed by healthy trees after phloem blockage caused by mechanical injury.