THE DEVELOPMENTAL STAGE DURING WHICH BORON LIMITATION CAUSES STERILITY IN WHEAT GENOTYPES AND THE RECOVERY OF FERTILITY

Sterility in wheat in parts of the subtropics has been linked primaril y with low-boron soils and its variability between years and locations with variation in the weather. This paper shows, from reciprocal tran sfers of plants between adequate and zero boron root media at differen t developmental stages, that the period during which florets are steri lised by boron insufficiency can be very short. For any ear it extends from after its flag leaf tip has emerged until shortly after the flag leaf has become fully expanded. This critical period of up to 1 week in length has already passed when the ear begins to emerge. Because ti llers are sequential, the critical stage for the whole plant is longer than 1 week. It was shown that ears could also be sterilised by enclo sing the whole plant in a clear plastic bag during this critical perio d, even though the plants were growing with adequate boron provided in sub-irrigated gravel culture. It is suggested that one of the effects of enclosure is to prevent transpiration and possibly the associated uptake and movement of boron to the reproductive growth centres. The s ix genotypes tested did not differ in the developmental stage at which they were sensitive, and all were completely sterile when they were g rown without boron up to and including that stage. However, it appeare d that a prior period in adequate boron had a different effect on ster ility amongst genotypes. One genotype (Fang 60) showed evidence of a b oron reserve that could be utilised even after a period equivalent to 3 phyllochrons whereas others appeared to have no boron pool. Ears whi ch were fully sterilised by inadequate boron could have their fertilit y raised marginally by a spray of boric acid even several days after t hey had emerged. The implications of these results to grain yield in t he subtropics are discussed.