Studies On The Identification And Appraising Methods Of Heat Resistance Of Lactuca Sativa L.

Adapted to cool climate, lettuce (Lactuca sativa L.) is liable to be heat injured or grow poorly when grown in hot seasons. Heat resistance identification is the basic job in variety selection for hot seasons, as well as for breeding of heat resistant varieties. The relationship between heat resistance and morphological, physiological and biochemical indexes and seed vigor were studied. Ten head lettuce varieties and six leaf ones were used as materials in the studies. Results are the following:1. Morphology and heat resistanceMorphological indexes studied include time of bolting, leaf shape ratio, length of condensed stems, and ratio of condensed stems to net weight. Results show that these morphological indexes are significantly correlated to heat resistance, and may be applied as basic indexes in heat resistance identification of lettuce. The bolting time of heat resistant varieties was later; the condensed stem was shorter and constituted a little proportion of the head weight, in comparison to those of heat-sensitive ones. Significant correlations were observed between node length, leaf shape ratio and heat resistance in head varieties. The shorter the node was, the more resistant the germplasm was. However, the observations in leaf ones were contrary. Based on our results, node length, leaf shape ratio can be applied in heat resistance identification of lettuce.2. Commercial characters and heat resistanceThe commercial characters observed, i.e. yield, heading rate (head lettuce only), quality and ratio of condensed stems to net weight, were significantly correlated to heat resistance. There was a decrease in commercial characters of all varieties, but the extent of decrease differed among varieties. This is consistent to the results of morphological identification, thus heat-resistant varieties maintained the commercial characters less affected under thermo stress. 3. Physiological and biochemical changes and heat resistanceSome physiological and biochemical properties, such as the contents of malonaldehyde (MDA), proline, soluble carbohydrate, the root activity, activities of peroxidase (POD) and superoxide dismutase (SOD), showed significant correlation with heat resistance. These properties may be applied as accessorial identification indexes. Results showed also that there is optimal time for testing of physiological and biochemical properties (e.g. the 19th and 13th day for head and leaf lettuce, respectively). The results indicate that the lipids of heat-resistant varieties are better protected and remain more integrated than those of heat-sensitive ones.4. Seed vigor and heat resistanceSeed vigor was correlated with heat resistance in some extent. However, inconsistent behaviors were observed between head and leaf lettuce. No inevitable correlations between seed vigor and heat resistance can be drawn from the studies.