Non-stomatal factors limit cotton photosynthesis under silverleaf whitefly stress

Reduced plant productivity induced by silverleaf whitefly (SLW), Bemisia argentifolii Bellows and Perring, is often associated with reduced net photosynthetic rate (P(n)). This study was aimed at characterizing the effects of SLW on photosynthetic activity in cotton and the underlying mechanisms. Cotton (Gossypium hirsutum L. cv. Siv'on) plants were examined under SLW-infested and non-infested conditions and subjected to measurements of gas exchange, chlorophyll content and chlorophyll fluorescence. SLW infestation significantly reduced the P(n) of cotton by 22-54% throughout the day as well as throughout most of the examined ranges of photosynthetic photon flux density (PPFD) and CO₂ concentrations. Stomatal conductance was also reduced by SLW injury, while intercellular CO₂ concentration (C(i)) was not affected, suggesting that stomatal limitation is not the major factor limiting P(n). This conclusion was further supported by estimates of stomatal limitation, based on P(n)/C(i) curves, which were not affected by SLW infestation. On the other hand, non-stomatal limitation induced by SLW was over 2-fold higher than stomatal limitation. Chlorophyll a and b contents were not modified by SLW infestation. Fluorescence ratio (F(v)/F(m)) and the effective quantum yield of PSII were reduced by less than 10%; thus, impaired photochemical reaction is probably not the only factor limiting P(n) under SLW infestation. In conclusion, SLW-induced P(n) reduction in cotton was due to non-stomatal factors, possibly induced by impaired carbon fixation and/or carbohydrate export.