The Effect of Summer Nitrogen Applications on The Quality of Apple Blossom

A summer application of nitrogen was used to induce the production of “ strong ” flowers for comparison with the " normal ” flowers produced on trees of Worcester Pearmain given only a spring application of nitrogen.The flowers were given various pollination treatments to provide material for an assessment of flower quality. These included the following : auto and alio self-pollination and cross-pollination with diploid apple or pear at anthesis, cross-pollinations and self-pollinations at additional daily intervals subsequent to anthesis.The stigmas of strong flowers remained receptive for a longer period than those of normal flowers.After cross-pollination with a diploid apple, fertilization took place in 6-7 days. After self-pollination a typical incompatibility reaction was apparent. Pear pollen tubes showed an intermediate response.Egg sacs in strong flowers continued to enlarge after those of corresponding normal flowers had ceased to grow. The fertilized ovules and embryo sacs of strong flowers showed a more rapid acceleration in growth rate than those of normal flowers.Cell division in unfertilized ovules also continued for a longer period in strong flowers and appeared to be correlated with the behaviour of the egg sac.Ovule longevity provided the most striking difference between the two flower types. The ovules of strong flowers remained capable of fertilization for almost twice as long as those of normal flowers.Abnormalities of several types were seen in ovules. The most common was the production of secondary egg sacs, a feature of strong flowers which had been self-pollinated. Three types of secondary sac, micropylar, lateral and chalazal are described and illustrated.The effects of rate of pollen tube growth, ovule longevity and the period of stigma receptivity in limiting the effective pollination period, are discussed. It is suggested that some variations in fruit set may be due to differences in flower quality. The use of summer or early autumn nitrogen is advocated as a method of improving blossom quality in certain varieties.