Heat Stress Tolerance in Tomato

Tropical tomato varieties must include high levels of heat tolerance in addition to disease resistance and flood tolerance. Since the 1970’s, AVRDC – The World Vegetable Center has led initiatives to breed heat tolerant tomato for Asia. Screening of almost 6,000 tomato germplasm accessions from the AVRDC genebank resulted in 41 lines that largely maintained fruit set under elevated temperatures (35°C during the day, 25°C at night). Multi-crossing heat tolerant material originating from the Philippines and the USA resulted in heat tolerant lines that have been widely used in Asia for developing open pollinated varieties or as hybrid parents.

CLN1621L, one of the heat tolerant lines developed by AVRDC, was crossed with the heat sensitive Tomato yellow leaf curl virus (TYLCV) resistant line CA4 to establish a TYLCV-resistant recombinant inbred line (RIL) population for use in field trials to study heat tolerance mechanisms. The RIL population was exposed to natural heat conditions in the open field and in rain-out zones. Only a weak correlation between open field and rain-out zone data was found for fruit set (R = 0.27) and yield (R = 0.2). The parents of the RIL population demonstrated considerable differences in pollen viability and fruit set in repeated evaluations under heat stress; CLN1621L had high pollen viability and fruit set under heat stress, while CA4 had very low pollen viability, and the F₁ hybrid intermediate values. Among 96 individuals of the RIL population the correlation between pollen viability and fruit set was surprisingly low (R = 0.3), indicating that other factors in addition to pollen viability strongly contribute to fruit set in the RILs.

Twenty-nine putative quantitative trait loci (QTLs) associated with heat tolerance were identified in the rain-out zone and field trials. Most primary QTLs detected in the field and in the rain-out zones did not overlap, indicating that different heat stress tolerance mechanisms were active in the different environments. Two QTLs were associated with the number of viable pollen grains under heat stress in the rain-out zone. Field and screenhouse experiments are ongoing to corroborate these QTLs.