The drought tolerance of six green-and yellow-podded varieties of green beans with different genetic backgrounds was tested in the phytotron. During the week prior to flowering the plants were kept either at 25/15°C (day/night) or at high temperature (30/15°C), with RH 75% and optimum water supplies. The heat-stressed plants were then divided into three groups; the first was returned to the control (25/15°C) chamber (RH 75%, optimum water supplies), while the second and third were exposed to mild drought stress (RH 60%, 50% water) at temperatures of 30/15°C and 35/25°C, respectively, throughout the flowering period.The varieties survived the short period of heat stress (30/15°C) prior to flowering without damage provided the temperature during flowering was reduced to 25/15°C and the water supplies were optimum. There was a sharp increase in the carotene level in the leaves of drought-stressed plants when the temperature during flowering was 30/15°C, but in plants exposed to 35/25°C during flowering the level dropped to near the control level. The latter group exhibited considerable damage, with a reduction in the water-soluble antioxidant content (ACW: antioxidant capacity of water-soluble substances) and the chlorophyll
content compared with the control.The antioxidant content (ACW) in the dark green leaves of green-podded varieties was lower than in the yellow-podded varieties and did not change as the result of drought and heat stress. In yellow-podded varieties, however, there was a significant decline in ACW in response to stress. Differences between the varieties in their adaptability to drought and heat could be detected as changes in the chlorophyll and carotene contents of the leaves even at 30/15°C.
Authors:J. Szarka, O. Toldi, E. Szarka, J. Remenyik, and et al.
The fact that production is often unsuccessful even when resistant varieties are selected on the basis of the hypersensitive reaction can be attributed to the lack of adequate knowledge on plant disease resistance. In addition to specific plant responses to pathogen species, plants also possess an aspecific defense reaction which, instead of causing rapid tissue destruction, is based on the opposite strategy, protecting the plant against attack by microbes through tissue compaction achieved by cell enlargement and cell division. Genetic analyses carried out in pepper revealed that the general defense reaction was inherited as a monogenic recessive trait (gds). Pathophysiological observations indicate that the stimulus threshold is lower and the reaction rate faster than for specific defense reactions. Biochemical analyses suggest that, unlike plants exhibiting rapid tissue destruction, plants containing the gds gene do not require an oxidative burst elicited by hydrogen peroxide to stimulate the defense mechanism. It was also found that the regulation of the general defense system involves metabolic pathways that are independent of salicylic acid. The general and specific plant reactions form an integrated system of plant defense.