Use of Pollen Vitality to Monitor the Nutritional Status of Flowers

Analyzing tissues and soils are cultural practices routinely used to monitor the nutrient status in vegetative stages of commercially important crops. Flowers are a plant's reproductive apparatus. Following pollination and pollen germination, pistils provide nutrients to support pollen tube growth through the stigma, style, and ovary. Major problems with tissue analyses involve timing. Samples are taken in summer, months after pollination occurred and when it is too late to remedy floral deficiencies. 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. In addition, leaves are shed and the nutrients 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 their contents are needed to optimize pollen vitality and pistil fecundity. Previously, no assays were available to categorize the status of floral nutrients to optimize fertilization of eggs and, consequently, fruit or nut set. Studies involving a multitude of plant varieties indicate that responses of pollen viability to different boron or calcium concentrations produce bell-shaped curves. With this knowledge, an in vitro nutrient bioassay was devised that involves pollen vitality. Pollen was suspended in increasing concentrations of nutrients: boron, calcium, copper, iron, magnesium, manganese, molybdenum, potassium, and zinc. Next, fluorescein diacetate was added as a vital stain. The amount of florescence produced was used as a criterion of pollen vitality. If pollen vitality increased in the presence of a nutrient, compared to controls, flowers were considered deficient. If vitality decreased, flowers were considered at excessive levels. If vitality remained the same as controls, flowers were considered near optimum levels. Pollen samples from commercial orchards revealed five categories of floral nutrients: excessive, near optimum, slightly deficient, moderately deficient, and extremely deficient. If pollen is deficient in any of those nutrients, it is logical that the flower that produced them is deficient, which includes the pistil. Samples near optimum levels ranged from 30% to 79%. Samples with excessive levels ranged from 0% to 18%. Therefore, routine applications of nutrients during bloom can cause toxic amounts to accumulate. Pollen bioassays accurately and rapidly measure the status of available floral nutrients so farmers can optimize flower fertility before bloom ends and their crop is set.