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In this trial the nitrate concentration of the nutrient solution affected the rate of DM production only when it fell below 0.2 mM. Between 0.2 and 1.5 mM the chemical composition of the plant was altered considerably but production was not affected. The soluble carbohydrate content, which reached a very high value in the above-ground parts when the nitrate supply was low, fell as the nitrate concentration rose; the same tendency was observed in the crude-fibre content and some unidentified components, probably hemicelluloses and pentosans. The organic-N fraction increased with increasing nitrate concentration in the above range. Above 1.5 mM the effect was virtually confined to the plant nitrate concentration, and there was always a definite relationship between the nitrate content and the soluble-carbohydrate content. The optimum temp, for DM production increased with increasing light intensity.-R.B. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The growth curves of closed grassland swards obtained during the growing seasons of six successive years were compared with each other and with theoretical growth rates calculated by DE Wit's method and based on the mean light energy input data for the same years. The form of the growth curve consisted of a period with a constant rate, lasting for 6-7 weeks in May-June, then rapidly shortening to 3 weeks in August-September, after which there was a rather abrupt change to zero values. A comparison with theoretical values showed that a period of constant growth rate was possible, because the promoting influence of increasing LAI counteracted the effect of increasing respiration. Before the longest day this period was lengthened by an increasing daylength; thereafter the reverse was found. There is no ready explanation for the sudden decline to zero values. This is thought to be associated with changes in the rate of photosynthesis.

Young seedlings of Zea mays L. scarcely grow at temperatures of 15DC or lower. The leaves are yellowish, the chlorophyll concentration being lower than 10 [J.g (a-l-b) per em", and the rate of photosynthesis is negligible. When a period of low temperature is given to older plants, which already have some green leaf surface, the reduction in relative growth rate and photosynthesis is less severe. Only those leaf parts that elongate during the cold treatment are yellow.It can be shown that the day temperature is the important factor in influencing the chlorophyll concentration in growing leaf parts. Plants subjected to low night temperatures remain fully green and their growth rate is virtually unaffected. The data suggest that photosynthesis rate is reduced at concentrations of chlorophyll (a +b) below 40[J.g per crrr", When plants are brought back to temperatures of 20 0 or higher both photosynthesis rate and growth rate soon reach values normal for this temperature.

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