4. Discussion
4.1. Variation for drought score
Large genotypic variation for drought score existed under the different types of drought stress occurring in experiments A and B. There was no association between the drought score of the RILs determined in experiments A and B, due in part to the different patterns of development and severity of plant water deficit between the two experiments.
4.1.1. Pattern of drought development
The patterns of development of plant water deficit in experiments A and B were different, i.e. slow development in experiment A and fast development in experiment B. The slow soil water depletion in experiment A, interpreted to be due to the occurrence of lateral water movement in the field (experiment A was positioned low in the toposequence), contrasted with the quick soil drying due to a high evaporative demand (about 5 mm per day of pan evaporation and 34 °C maximum air temperature) and no problems with lateral water movement in experiment B (experiment B was positioned high in the toposequence). These differences in soil water conditions resulted in differences in the pattern of development of plant water deficit in these two experiments.
It should be noted that under the growing conditions of experiments A and B, not only was there a shortage of water but nutrients might also have been deficient and soil pH may be reduced relative to flooded conditions (Fukai et al., 1999). Thus, achievements of low drought score by RILs may reflect genotypic resistance to nutrient deficiency and soil acidity. Furthermore, during the drought period, the temperature during the daytime was high (about 34 °C in experiment B), hence low drought score may also reflect genotypic differences in heat tolerance.
4. Discussion4.1. Variation for drought scoreLarge genotypic variation for drought score existed under the different types of drought stress occurring in experiments A and B. There was no association between the drought score of the RILs determined in experiments A and B, due in part to the different patterns of development and severity of plant water deficit between the two experiments.4.1.1. Pattern of drought developmentThe patterns of development of plant water deficit in experiments A and B were different, i.e. slow development in experiment A and fast development in experiment B. The slow soil water depletion in experiment A, interpreted to be due to the occurrence of lateral water movement in the field (experiment A was positioned low in the toposequence), contrasted with the quick soil drying due to a high evaporative demand (about 5 mm per day of pan evaporation and 34 °C maximum air temperature) and no problems with lateral water movement in experiment B (experiment B was positioned high in the toposequence). These differences in soil water conditions resulted in differences in the pattern of development of plant water deficit in these two experiments.It should be noted that under the growing conditions of experiments A and B, not only was there a shortage of water but nutrients might also have been deficient and soil pH may be reduced relative to flooded conditions (Fukai et al., 1999). Thus, achievements of low drought score by RILs may reflect genotypic resistance to nutrient deficiency and soil acidity. Furthermore, during the drought period, the temperature during the daytime was high (about 34 °C in experiment B), hence low drought score may also reflect genotypic differences in heat tolerance.
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