Seed production in sorghum is a vital link in crop improvement through which improved cultivars reach the farmers. Hybrid Seed production in sorghum is largely confined to the states of Madhya Pradesh, Andhra Pradesh, Karnataka and Tamil Nadu.
i. A, B and R Lines:
Commercial seed production of hybrids in sorghum is accomplished through the use of cytoplasmic genetic male steriles. In this system, a male sterile line designated as A line is used as the female parent and crossed with a fertile restorer line designated as R line. Since the male sterile A line cannot produce seed by itself, a separate crossing block has to be established with its fertile counter-part known as B line or maintainer line for its multiplication.
The R line as well as B line is multiplied in the normal way like other self-propagating lines. For the production of certified hybrids, a crossing block with the A line and R line has to be established. Seed obtained from female parent in this crossing block is the certified seed from which the commercial hybrid crop is raised.
ii. Nicking:
Serious problem encountered in hybrid seed production in sorghum is proper nicking or synchronisation of flowering in the two parents. Several methods are adopted to overcome the problem depending on the situation. Most commonly followed technique is staggered planting so that they flower simultaneously. The other method is selective irrigation and fertiliser application including foliar spray of urea.
This method will only enable to bridge the gap if it is 2-4 days. However, combination of staggered planting, selective irrigation and fertiliser application can meet the requirement in most of the situations. If the gap in flowering between the parents is of greater magnitude, transplantation or ratooning of the early parent would be a better choice. As an emergency measure, dusting or spraying of pollen can be adopted.
iii. Plant Height:
Differences in height between the parents have considerable bearing on seed setting. Where the parents are of equal height or where the pollen parent is taller than the seed parent no difficulty is usually experienced for good seed set. If the pollen parent is considerably shorter than the seed parent, obtaining good seed set, even if nicking is poor, is rather difficult.
iv. Planting Ratio:
The ideal planting ratio between male and female lines is two male rows alternated by 4-6 female rows. The number of female rows has to be determined on the basis of size of male ear heads, the duration of its flowering and the amount of pollen production. Where the male line has smaller ear heads and shorter span of flowering compared to the female ones, it is desirable to allow only four female rows. At least, a five row thick border all-round the seed production plots is a must.
v. Flowering and Pollination:
If the duration of flowering in male line is prolonged, it is obviously a desirable trait as pollen is available for more number of days. As the pollen is viable only for a few hrs after flower opening, there is need to prolong flowering of male line for obtaining better seed set. The only way to achieve this objective is to adopt double staggering of the two male rows wherever the situation demands.
As regards pollination, while there is always certain amount of crossing due to wind, it is not safe to entirely depend on natural flown wind. It is desirable to adopt artificial method of pollen dispersal by tapping the male plants or flowing air through empty dusters over the male ear heads.
vi. Stigma Receptivity and Pollen Production:
Generally, the stigma retains good receptivity up to five days after flower opening. During hot summer, receptivity is lost quicker due to desiccation of stigmas. Similarly, pollen production may also very depending on variety and climate. During winter months, if the night temperatures are very low, pollen production will be reduced.