INFLUENCE OF TEMPERATURE AND FLORET AGE ON NECTAR SECRETION IN TRIFOLIUM-REPENS L

The influence of temperature on nectar secretion in non-pollinated flo rets of Trifolium repens was investigated in growth chambers at 10, 14 , 18 and 22-degrees-C. The effect of temperature on the rate of nectar secretion was significant in all clones. The optimum temperature for secretion in three clones varied from 10-degrees-C for a clone of Icel andic origin, to 18-degrees-C in a clone selected from a Danish variet y. Similarly, the average nectar yield varied significantly among clon es of different geographical origin. One clone secreted two to four ti mes more than others at 10-degrees-C. The optimum day temperature for nectar secretion was higher when the plants were exposed to low night temperature, presumably a result of decreased night respiration. Necta r accumulated at the floret base until senescence. Evidence for reabso rption of nectar was obtained in four clones. Sucrose, fructose and gl ucose were identified as the major sugars in the nectar. High night te mperatures led to decreased sucrose percentage in favour of glucose an d fructose. The frequency of new florets opening per day was not influ enced by temperatures between 10 and 22-degrees-C in one clone, wherea s low temperatures significantly decreased the number of new florets i n another. Few or no modified stomata were observed in the epidermis o f the nectary. The high variation with respect to nectar secretion at low temperatures, along with the high heritability of this quality, su ggests that breeding for high nectar production at low temperature is plausible. The significance of nectar yield in pollination biology is discussed.