Cell-bound phosphatase activity in cotton (Gossypium hirsutum L.) roots
A study was made of phosphorus nutrition in cotton cultivars derived from Pakistan with particular reference to phosphatase activity. Cultivars used were Qalandri, Rehmani, Reshami and TH-3/83; Qalandri was used for most of the phosphatase characteristic studies, since this cultivar exhibited the highest germination percentage and seedling establishment. Investigations of soil parameters from cotton growing region of Pakistan were also carried out. Soil pH was above 8.0 in all sites studied. Soil organic component levels were very variable between sites and within sites at different depths. Soil potassium was found to be at levels regarded as being sufficient for sustaining plant growth at all sites. However, levels of combined nitrogen and inorganic phosphate were in the range of values which could be regarded as being limiting to plant growth. The importance of phosphatase in the mobilization of organic phosphorus was therefore apparent. Phosphomonoesterase (PMEase) activity, assayed using two substrates p- nitrophenyl phosphate (pNPP) and 4-methylumbelliferyl phosphate (4-MUP), was investigated and was found associated with roots of hydroponically-grown seedlings. Staining studies indicated that the enzymes were present in the cell wall. Phosphodiesterase activity was also detected in roots, using bis-pNPP as the substrate. PMEase activity was found to have its greatest activity at pH values between 4.0 and 6.0 for all cultivars. In cv Qalandri the optimum pH for PMEase activity was 5 .5 and the optimum temperature was 55 C when assays were incubated over a 1h period using pNPP or 4-MUP as the substrate. Levels of activity were higher using pNPP than with 4-MUP, but an absolute direct comparison could not be made since the concentrations of the individual substrates used were different. Plants grown in phosphate-limitation conditions showed enhanced PMEase activity. Enhancement of PMEase activity was associated with the levels of phosphate in the plant growth nutrient solution rather than with those in the plant itself This activity was highest in 19-day old seedlings, under the conditions of growth employed here. A comparison between cultivars showed some differences in the level of PMEase activity and in the specific pH optimum for enzyme activity. Limitation of combined nitrogen to seedling also resulted m enhanced PMEase even when plants were under phosphate-sufficient conditions. A similar effect of potassium-limitation was also found and this was associated with reduced uptake of phosphate into the seedlings. No effect of magnesium-limitation on PMEase activity was found. A comparison was also made with other plants; cotton. Primula farinosa, rice, wheat and algae. All had associated phosphatase activity, with cotton showing significantly higher root acid phosphatase activity than the other higher plant species. Acidic phosphatase activity was found in the alga Drapamaldia whilst Stigeoclonium had alkaline phosphatase.