Physiological Performance of Asymptomatic and Yellow Leaf Syndrome-affected Sugarcanes in Venezuela

Serological analyses revealed the presence of the sugar- cane yellow leaf virus (ScYLV) in asymptomatic (S±) and symptomatic (S+) yellow leaf syndrome-affected sugar- cane plants of the cultivars PR.692176, C.323±68, V.64± 10, V.71±47, V.75±6, SP.72±2086, SP.72±1210, SP.74± 2005, C.323±68, B.80±549 and B.82±363. Tests for the presence of the sugarcane yellows phytoplasma, carried out by Dr P. Jones (IACR-Rothamsted), gave negative results in all cultivars. Physiological analyses were performed in the top visible dewlap (TVD) leaf of S± and S+ plants of the cultivar PR.692176. All plants were at the second ratoon and ¯owering.When compared with S± plants, the S+ plants showed: (a) a marked reduction in the area of the leaf and internodes; (b) a high accumulation of total reducing sugars (TRS), glucans and a-amino-N in the leaf blade and of TRS in the corresponding leaf sheath; (c) a decrease in the chloro- phyll, phosphorus and nitrogen content in the leaf; (d) the disappearance of the leaf diurnal ¯uctuations in TRS accumulation and export as well as the daily oscillations of TRS and glucans between dawn and dusk; and (e) major ultrastructural alterations in the companion cells of the phloem, including the accumulation of ScYLV particles in the cytoplasm. In S± plants, none of the growth and physiological alterations described above were observed, in spite of the high density of ScYLV particles in the cytoplasm. The location of S± and S+ plants close to each other without a discernible pattern of distribution in plots subjected to optimal irrigation and fertilization rule out the possibility that environmental conditions underlay the appearance of symptoms. In plots under severe drought for 3 months, however, all S± plants become S+. Symptom expression did not affect the acid phosphatase activity in the rhizosphere of S+ plants.