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Transport of photoassimiJates from source to sink is mainly determined by both source and sink strength, both of which can be affected by water stress. A series of experiments were conducted in which different levels of water stress together with different labelling methods and translocation times were employed. The experiments were carried out in glasshouse or growth cabinet using a completely randomized design. Water stress was induced by withholding water from the pots or by lowering osmotic potential of the culture medium by the addition of PEG 8000 to the medium. Plants were
labelled with 14C. Water stress level causing visible reversible wilting led to a significant reduction in photosynthetic rate, but did not affect distribution or labelled assimilates in plants. Neither photoassimilates import nor their convertion to starch were affected by this treatment. The same results \vere observed even when plants were kept at wilting stage for some 20 or 40 hr.
Degree of water stress causing severe wilting in leaves led to a marked reduction in photoassimilate import by grains (sinks), although this reduction was not attributable to reduced sink activity. Addition of PEG to culture medium almost inhibited photoassimilate import by sinks. Again this reductIon was not due to diminished sink activity. The lack of sink response to these nearly severe water stress conditions is explained in relation to experimental methods employed. These results suggest that translocation of assimilated to the grains and its conversion to starch is resistant to short water stress conditions. However, photosynthesis is inhibited under such stress conditions, Causing source limitation.