Midday drop in leaf water content is an effective trait for evaluating germplasm for heat and drought tolerance in snap bean (Phaseolus vulgaris).

　　In the subtropical islands of Okinawa, snap bean production in summer is very difficult due to high temperatures, strong solar radiation, and drought, etc. The crop faces a water deficit due to excessive transpiration caused by high temperature s. However, the influences of high temperature and drought on pod/seed production, shoot extension, water status and photosynthesis as related to cultivar variations have hitherto been unclear. To develop simple physiological traits for heat and drought tolerance is also desired for breeders to accelerate the screening of stress-tolerant germplasm. This study was, therefore, conducted to identify the plant traits/ processes related to heat and/or drought stresses and to elucidate the mechanisms of stress tolerance in snap bean.

　　A discriminant analysis revealed that the five cultivars (Haibushi, Ishigaki-2 Kurodane-Kinugasa, Kentucky Wonder and 92783) displayed two distinct types of response (Fig. 1). One group included.

cultivars Haibushi, Ishigaki-2 and Kurodane-Kinugasa, which showed a large reduction of about 16-20% in both shoot extension and water potential under unirrigated dry conditions ; these produced a higher number of pods per plant and seed yield than cultivars Kentucky Wonder and 92783. Cultivars Kentucky Wonder and 92783, which formed a separate group, displayed a comparatively smaller reduction (4-8%) in both water potential and shoot growth under unirrigated dry conditions. On the other hand, the former group displayed a smaller reduction in leaf water content while the latter group displayed a larger reduction in leaf water content. These results indicate that the maintenance ability of relatively higher leaf water content with increasing water deficit plays an important role in terms of higher pod setting, pod retention and seed yield in snap bean under stressed conditions.
　　The leaf water content was positively correlated to photosynthetic parameters such as stomata conductance and intercellular CO2. The cultivars with a smaller midday drop in leaf water content showed a higher pod setting ratio and produced a larger number of pods per plant and consequently gave higher yields than plants with a larger midday drop in leaf water content (Fig. 2). 

　　From these results, it can be concluded that leaf water content plays an important role in maintaining better photosynthetic conditions and, thus, higher pod/seed production under heat and drought conditions, and is maintained by reduction in leaf water potential and shoot extension in response to heat and drought stresses. These traits can thus be used as a marker to screen germplasm for heat and drought tolerance.

(H. Omae, A. Kumar, K. Kahiwaba, and M. Shono)

Fig. 1. Relationship of reduction (ratio of unirrigated to irrigated control in percentage) in leaf water potential with leaf water content and shoot extension in 5 cultivars of snap bean. Each dotted circle encloses a group of cultivars/strains identified by discriminant analysis. *: P < 0.05. Fig. 2. Association of midday drop in leaf water content (ratio of leaf water content at midday to morning) with the number of pods per plant and seed yield. The measurements were taken in 8 cultivars under irrigated and unirrigated hot conditions during the reproductive stage. *: P < 0.05, ** P < 0.01.

For further details log on website :
https://www.jircas.affrc.go.jp/english/publication/highlights/2005/2005_14.html