Premature Senescence
As with most crops, potassium deficiency symptoms of cotton generally occur on older leaves, as plants relocate highly mobile potassium to areas of new growth. However, in the 1990s, symptoms consistent with potassium deficiency began appearing on the newest upper leaves of growing cotton crops. This often occurred on soils which tested high in plant available potassium. The condition was called premature senescence (PS).
Symptoms of PS appear first on young leaves near the top of the canopy with leaves turning red during the early fruiting stage of the crop. As the season progresses symptoms may spread further down the canopy and cause defoliation, ultimately affecting lint yield and quality.
Research in Australia has shown that rather than being related solely to soil supply of potassium, PS is due to a complex interaction of plant uptake, translocation and stress. In cases where root uptake of potassium is reduced (for example because of water-logging) the potassium demand by filling fruit may outweigh supply. Hence, potassium is re-routed from young leaves to the priority reproductive plant parts, resulting in the PS symptoms observed in field. (See photo.)
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This photo illustrates symptoms of premature senescence (PS) in an Australian cotton crop. |
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Susceptibility to PS can be linked to:
Soils which are low in potassium, particularly in the subsoil, are probably more susceptible to PS. In these cases surface applied potash may be sufficient to overcome the problem, but broadcast applications have had only limited success in overcoming PS in most situations.
Research is under way in New South Wales which attempts to link soil potassium supply characteristics with a field's susceptibility to PS.
Plants with heavy boll loads have a greater susceptibility to PS. A large, even boll load creates a high demand for nutrients, particularly potassium.
Consequently those varieties which have a greater boll load potential, or which retain bolls better are more susceptible to PS. Transgenic Bt cotton crops often have a heavier boll load because less fruit are lost from insect damage compared to conventional varieties. They also appear to be more susceptible to PS. It has also been observed that most longer season varieties have a lower susceptibility to PS.
Waterlogging has been shown to reduce root activity and the uptake of potassium and phosphorous from soil. As a result crops are less capable of meeting the demands of rapidly filling bolls. Stresses such as waterlogging are often the trigger for development of PS.