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Temperature effects on the response to sulphur of barley (Hordeum vulgare L.), peas (Pisum sativum L.) and rape (Brassica campestris L.) Herath, Herath Mudiyanselage Walter

Abstract

The effects of temperature and sulphur nutrition on the growth, yield and mineral composition (N, NO₃-N, S and SO₄-S) of Hordeum vulgare L. cv Olli, Pisum sativum L. cv Dark Skin Perfection and Brassica campestris L. cv Arlo, were investigated in controlled environments. The net CO₂ exchange rates and compensation points were also determined at two S levels (0 and 64 ppm) under various temperature regimes. When barley and rape plants were grown at 0 ppm S, deficiency symptoms developed in about two weeks, whereas pea plants at the same level developed deficiency symptoms in about three weeks. Plants at the lowest S level and the highest temperature took the shortest time to develop S deficiency symptoms. Fresh and dry weights, shoot length, number of nodes and number of fertile fruit increased with increasing S levels. Shoot growth in all three species was more depressed by S deficiency than root growth. Optimum growing temperature regimes for barley and peas were found to be 24/16 at the vegetative stage and 18/10°C at the mature stage as evident from increased weights, maximum fruit set and mineral uptake. Optimum temperature for rape plants was 29/21°C at both stages of growth. Detrimental effects of cotyledon or endosperm removal tended to mask the effects of temperature and S levels. This method was thus found to be unsatisfactory for the study of S nutrition in plants. Higher mineral concentration was observed at the vegetative stage than at the mature stage in peas and rape plants, while in barley the mineral concentration remained constant at both stages of growth. With increase in S supply there was an increase in uptake of both total S and SO₄-S. Uptake also increased with increasing temperatures. This increase was largely due to "concentration effects". Hence the use of SO₄-S level as a criterion for diagnosis of S deficiency may be unsatisfactory, unless plants are grown at optimum temperatures. S deficient plants had increased total N and NO₃-N concentrations in all three species. NO₃-N concentration also increased with an increase in temperature. The total N concentration did not increase appreciably with temperature. Consequently, at low S level (0 and 8 ppm) total N:total S ratios (N:S) tended to increase or decrease depending on low or high growing temperatures respectively. These changes in ratios were independent of actual size of the plants. Furthermore the ratios for all S levels at the vegetative stage were lower than those at the mature stage. Therefore both temperature and stage of growth are important factors to be considered in interpreting S deficiency from N:S ratios in plants. The net C0₂ exchange rates were generally higher at 20 days than at 30 days. At 0 ppm S level and at high temperature, the decline in net C0₂ exchange rate with age was greater. Maximum CO₂ exchange rates were observed at the optimum growing temperatures for both S levels. Increasing the measuring temperature above the growing temperature caused no further stimulation in CO₂ uptake, and at high temperatures there was a decrease in uptake. When CO₂ exchange rates were measured at two 5.5°C intervals above and below the growing temperatures the maximum rates were recorded at or below growing temperatures in all the species at both S levels. The CO₂ compensation values were higher with lower S level in the leaf tissue than at higher S levels. Increase in growing temperatures also caused larger CO₂ compensation values than at lower temperatures. Negative correlations between CO₂ compensation point and leaf tissue S level and positive correlations between CO₂ compensation point and temperature were observed in barley and peas.

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