Floral nectar secretion and ploidy in Brassica rapa and B. napus (Brassicaceae). II. Quantified variability of nectary structure and function in rapid-cycling lines

Abstract

Haploid, diploid and tetraploid lines of Brassica rapa L. (syn. campestris), and allotetrapoid B. napus L., were examined to determine the influence of ploidy on floral features, particularly nectary morphology and anatomy, and to relate nectary structure to nectar production capacity. Except for haploids, all lines were rapid-cycling. Average flower dry weight, and petal length and width, were in the descending order B. napus > B. rapa (4n) > 2n > n. Pollen grains of 4n plants were larger than those of 2n plants; haploids lacked pollen. All lines developed nectaries. Typically, each flower produced two pairs of nectaries, of different types and nectar production capacity. Normally, each lateral gland was located above the base of a short stamen, and together this pair yielded most of a flower's nectar carbohydrate. Each median nectary arose at the outer junction of the bases of two adjacent long stamens. All lateral nectaries received a vascular supply of phloem alone, but median glands received reduced amounts of phloem, or lacked vasculature altogether. Most nectaries were solitary, but 14% of all flowers, and especially those of 2n B. rapa, had at least one median and lateral gland connected. Obvious variation existed in nectary morphology between ploidy levels, between flowers of the same plant, and even within flowers. Ten forms of each nectary type were recognized. Plants producing the most nectar carbohydrate had high frequencies of lateral nectaries which were symmetrical, unfurrowed swellings. Tetraploids of B. rapa had both the highest frequencies of furrowed lateral glands, and of isolated segments of nectarial tissue at that position. Even these separated nectarial outgrowths received phloem and produced a nectar droplet. At the median location, nectaries were commonly of two forms: peg- or fan-shaped. Lobes on median nectaries, up to four per nectary, were detected in almost half of glands of 4n flowers examined; lobes were absent in haploids. © 1996 Annals of Botany Company.