Use of Pollen Vitality to Determine the Boron and Calcium Status of Flowers

Analyzing leaves, petioles, hulls and soils are cultural practices used to monitor nutrients required for “vegetative” stages of plant growth for commercially important crops. Flowers are the “reproductive” apparatus of plants and they differ concerning function and nutritional needs. Pistils support pollen germination and tube growth and very specific levels of boron and calcium are required to optimize those processes. Previously, no assays were available to categorize the status of boron and calcium in flowers to optimize fertilization of eggs. Major problems with tissue analyses involve timing. Routinely, samples are taken in summer, months after pollination occurred and when it is too late to remedy deficiencies. Another problem is that at harvest, fruit and nuts are completely removed from an orchard; consequently, nutrients contained in them are deleted from the ecosystem months before pollination begins. Later, leaves are shed and boron and calcium they contain are lost. Paradoxically, tissues routinely used to monitor the status of essential nutrients for many crops are not even present during bloom when they are needed to optimize pollen viability and pistil fecundity. Many studies involving a multitude of plant varieties showed that responses of pollen viability to different boron or calcium quantities produce a bell-shaped curve. With this knowledge, an in vitro bioassay was devised which measures pollen vitality: it involves fluorescein diacetate as a vital stain, and utilizes pollen suspended in varying concentrations of those nutrients. Relative changes in vitality revealed seven categories of boron and calcium in flowers ranging from excessive to deficient. Pollen samples obtained from flowers of commercially important fruit and nuts were about 80% deficient in boron and 75% were deficient in calcium. Importantly, orchards were deficient to considerably varying degrees. Ironically, growers indicated that tissue analyses revealed boron and calcium levels near or within recommended amounts, suggesting elements were sequestered or lost. If pollen is deficient, it is logical that flower pistils are deficient. Flowers could be “revitalized” before bloom was completed via immediate application of foliar nutrients. Only 20% to 30% of the orchards did not need boron or calcium. Thus, routine applications of those nutrients during bloom can cause toxic amounts to accumulate. A pollen bioassay accurately and rapidly measures the status of available boron and calcium in flowers so farmers can optimize flower fertility before bloom ends and their crop is set.