Annals of Botany

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Late arrivals are welcome in the almond orchard

An almond orchard in full bloom is indeed a pretty sight, one that for me is associated with spring holidays in the Mediterranean region. However, it is a sad fact for almond growers that, as pointed out by Yi et al., Athens, Georgia and Bakersfield, California, USA (pp. 57–63), percentage fruit set from those flowers may be as low as 30 %. What is it that limits fruit set in almond? One clue is that almond is almost completely self-incompatible and therefore requires cross-pollination. Following this up, the authors examined the effectiveness of pollination in relation to the developmental stage of the flower receiving the non-self pollen. Flowers were examined by light microscopy and by SEM; pollinations with non-self pollen were done by hand over the period from flower opening through to petal abscission. During the life of an individual flower, the stigmatic papillae elongate and the amount of stigmatic exudate increases. Intuitively we may expect that these changes are associated with increased receptivity of the stigma and the authors’ observations show that expectation to be correct. However, what is unexpected is the timing of this maximum receptivity: it occurs late in the life of the flower, after the flower is fully open, and receptivity continues even after petals start to abscind and fall off. There is no loss of ovule viability in this period since almond is one of those species in which ovule maturation occurs only after successful pollination. At the most receptive stages, pollen germination was at a maximum and pollen tube growth was fastest. In the hand-pollination experiments, maximum fruit set varied between 44 and 75 % in different cultivars, significantly higher than that usually obtained in commercial orchards. The authors suggest that almond growers should bring honeybee hives into the orchards throughout the flowering period in order to maximize nut production—a practical step that, fortunately, will not decrease the aesthetic appeal of almond orchards in bloom.