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Irrigation Regime and Honeybee Activity as Related to Seed Yield in Alfalfa

A. Goldman, A. Dovrat
Agronomy journal 1980 v.72 no.6 pp. 961-965
Medicago sativa, agronomy, alfalfa, death, honey bees, irrigation management, pollen, quantitative analysis, seed yield, soil water, sprinkler irrigation
Seed yields of alfalfa (Medicago sativa L.) vary capriciously in time and space. This variation seems mainly caused by the delicate interplay between plant factors monitored by soil moisture availability and the obligate tripping requirement of alfalfa flowers by pollinators. The object of this study was to quantify the main effects and interactions between water application to the alfalfa field and tripping by honeybees, to identify mechanisms which affect the seed yield and to suggest indices for their quantitative evaluation. A wide array of conditions differing in soil moisture and in tripping level was established in the sprinkler-irrigated field. Observations were made on flowering and pod set of the plants and on the pollinating behavior of the bees. High water rates decreased seed yield. A moderate rate, applied at mid-flowering to a relatively dry soil, markedly increased seed yield; the effect diminished toward the beginning of the end of flowering. The effect of water application depended also on the degree of tripping. The tri ping pressure of honeybees (ie., their propensity to trip alfalfa flowers) was found to be inconsistent and seems to depend mainly on the availability of competing pollen sources. Dry and wet plots were equally welltripped under high tripping pressure and were neglected under low tripping pressure. Under moderate tripping pressure, pollen bees were attracted to dry plots earlier in the morning and in greater numbers than to wet plots, resulting in a higher percentage of tripped flowers, but the total number of tripped flowers was higher in the wet plots. Death of stem apices and a decrease in the rate of node formation were identified as mechanisms which regulate the duration and intensity of flowering and pod set.