Florence Grovida Gardening

BROCCOLI BREEDING Heat tolerance

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Susceptibility to high temperature damage is one of the major problems in broccoli. This makes the crop agronomically more suitable for cultivation in cool, moist climates. Heat at harvest time is not critical, except for the fact that high temperatures will reduce the period over which the crop is marketable. About 3-4 weeks prior to the head being marketable is the critical period when the growing point is differentiating to become reproductive (Chapter 4). Bjorkman and Pearson (199 7) studied the development stage at which the temperature response is most critical. The cross-section of the developing reproductive structure is shown in Fig. 2.4. It is most sensitive at the straightened and bowed stages, which is a relatively short period of 4-5 days. Leaf bracts can enlarge and grow through the head.

When the plant is sensitive to heat, the buds will become enlarged and grow through the head, due to elongation of the sepals, and the degree of enlargement is quantitative and influenced by the amount of heat (Fig. 2.5). Additionally, the secondary inner whorls of buds may turn yellow, especially the inner buds of the whorl, causing a condition called 'yellow eye' or 'starring'. Yellow eye tends to be dominant to normal bud formation. Also, leaflet extension may be encouraged in the head. Non-leafiness tends to be dominant to leafy. All three defects are enhanced or triggered by heat, and can render the crop unmarketable. The best screening method is to grow the plants under normal ideal conditions. They are then moved to a hot environment (35-25°C day/night) for 10 days when the growing point is at the critical 'straightened and bowed' stage before returning the plants to a normal growing regime at 20-25°C. Efforts are being made to map and find

1. 2.4. External appearance of the shoot tip of broccoli (calabrese, Brassica oleracea var. italica) during the transition from vegetative to reproductive growth. The plants can be screened for heat tolerance in the straightened (b) or bowed stages (c). When the plants reach the crown (d) and head (e) stages, it is too late for effective screening and the vegetative stage (a) is too early (M.H. Dickson).
2. 2.4. External appearance of the shoot tip of broccoli (calabrese, Brassica oleracea var. italica) during the transition from vegetative to reproductive growth. The plants can be screened for heat tolerance in the straightened (b) or bowed stages (c). When the plants reach the crown (d) and head (e) stages, it is too late for effective screening and the vegetative stage (a) is too early (M.H. Dickson).

Fig. 2.5. Illustration of 'yellow eye' or 'starring' in broccoli (calabrese, Brassica oleracea var. italica) (M.H. Dickson).

quantitative trait loci (QTLs) for heat tolerance or susceptibility to permit breeding for heat tolerance.

The plants can be grown in the field in a location where high temperatures are expected when the plants are maturing, and subsequently field selections are made for resistance to heat injury. Some seed companies have bred vigorously for heat tolerance, and selections show considerable tolerance compared with most broccoli hybrids. Yang et al. (1998) developed hybrid 'Ching-Long 45' with heat tolerance from the cross of broccoli and an inbred involving Chinese kale (B. oleracea var. alboglabra).