Abstract:

The effect of green spot disease (Apatococcus lobatus) parasitization on diurnal variation of photosynthesis of valencia orange trees ( Citrus ‘Olinda’) was studied by portable photosynthesis system LI-6400 with 2-year pot-grown grafting seedlings. The results showed that diurnal course in net photosynthetic rate (Pn) and stomatal conductance (Gs) of leaveswith low severity (5% ) presented two peaks, transp iration rate ( Tr) appeared single peak, intercellular CO₂ concentration (Ci) before noon greatly declined and
lower Ci was kep t afternoon. Leaves with slight severity were similar to healthy leaves in diurnal variation of photosynthesis, their photosynthesis appeared ‘Wuxiu’phenomenon, and mainly controlled by stomatal factors. Diurnal changes in photosynthesis of leaveswith medium severity (40%) and high severity (80% ) were different greatly from the healthy leaves, their diurnal curve of Pn, Gs, and Tr declined sharp ly, Ci decreased slowly before noon and higher Ciwasmaintained in the whole afternoon, their photosynthesis was mainly controlled by non-stomatal factors.

Key words: Orange, Apatococcus lobatus, Parasitization, Photosynthesis, Diurnal variation