Effect of heat stress on the growth, root sugars, acid invertase and protein profile of pepper seedlings following transplanting

The development of pepper transplants is sensitive to the prevailing temperature in the field. High temperatures immediately after transplanting pepper seedlings delay their development in the field. During nine days of post-transplanting heat stress (30/25°C or 35/25°C, day/night temperature regimes), root and shoot growth were inhibited compared to the control transplants (25/18°C) with a significant decline in total protein and soluble acid invertase activity in the roots of the heat stressed plants. Acid invertase activity was less sensitive to the heat stress in the leaves than in the roots. A significant reduction in the root invertase activity occurred as early as two days after the beginning of the stress. Transplanting caused a 72% inhibition in the carbon allocation to the roots, compared with the non-transplanted seedlings. Exposure to heat after transplanting did not further reduce carbon allocation to the roots. The protein profile on SDS-PAGE from leaves shows that during the heat stress, some heat related proteins were synthesized concomitant with the reduction in the synthesis of constitutive proteins. In the roots, the synthesis of heat related proteins was barely detectable with no visible changes in the protein profile. The results suggest that roots are more sensitive to heat stress than leaves, possibly because they are less able to synthesize heat related proteins. The causal relationship between protein synthesis during heat stress and the performance of the transplants is not well understood.