Analyzing drought resistance and suggesting a physiological selection criterion for drought resistance

– The need to consider cause and effect

Chlorophyll fluorescence was evaluated as a phenotypic marker for drought resistance in drought-stressed pot-grown barley varieties by using polyphasic OJIP fluorescence transient of photosystem II (PSII) in response to leaf drying (Oukarroum et al., 2007). The measurement and analysis is not simple and it was repeated over time with considerable replications. A numerical drought factor index (DFI) was developed as a final indicator of PSII response over time to leaf drying in terms of its fluorescence transient components. It was found that there was a relationship between the field response to drought of certain varieties and their DFI in this experiment indicating that DFI expressed their drought resistance. Relative water content (RWC) at peak drought stress was also measured in the experiment. In the discussion the authors mention that the highest and lowest varieties for DFI were also highest and lowest in their RWC. They concluded that DFI should be used to select barley for drought resistance since it is related to their field performance and their RWC in the pot experiment. Using the data in the report one can develop a regression of DFI over RWC (not done by the authors). It can be seen (Figure) that the association is solid. Evidently DFI is largely a function of leaf water status and varieties that maintain high RWC under stress sustain a higher DFI and lower injury to PSII performance. Therefore the conclusion from this study is that RWC and not DFI (chlorophyll fluorescence) should be used as a selection criterion in barley, for two reasons:

1. Variations in RWC among varieties are the main cause of the variations in PSII and DFI. These varieties differ in their dehydration avoidance (the ability to maintain high leaf water status under drought stress) and PSII activity as measured here is a reflection of that.

2. RWC is much simpler, faster and cheaper to measure than polyphasic OJIP fluorescence transient of PSII, which is an important consideration in selection work.

Abdallah Oukarroum, Saïd El Madidi, Gert Schansker, Reto J. Strasser. 2007. Probing the responses of barley cultivars (Hordeum vulgare L.) by chlorophyll a fluorescence OLKJIP under drought stress and re-watering. Environ. Exp. Bot. 60:438-446.

Abstract

The main objective of this study was to evaluate the effects of drought and re-watering on 10 varieties of barley (Hordeum vulgare L.) originating from Morocco. Five varieties obtained from the National Institute of Agricultural Research (INRA) of Morocco and five landraces (local varieties defined by high stress tolerance, high yield stability, an intermediate yield and low-input demand) collected at five localities in the south of Morocco were used in the present study. After 2 weeks of growth, drought stress was initiated by withholding water for 2 weeks followed by 1 week of re-watering. The polyphasic OJIP fluorescence transient was used to evaluate photosystem II (PSII) criteria at the end of the first week of drought stress (moderate drought), at the end of the second week (severe drought) and the end of the recovery phase. Drought and re-watering had little effect on the maximum quantum yield of primary photochemistry ?Po(=FV/FM). The photosynthetic performance index (PI) is the product of an antenna, reaction center and electron transport dependent parameter. It revealed differences between varieties as a function of drought and re-watering. For the screening for drought stress tolerance, changes in the PI during a 2-week drought stress treatment were analysed and a new parameter was defined: the drought factor index (DFI) = log(PIweek 1/PIcontrol) + 2 log(PIweek 2/PIcontrol). The DFI of the tested varieties correlated with their drought tolerance. Another parameter that was analysed was the relative water content. It decreased during the drought stress treatment varying between 61% and 78.2% at the end of the drought period. During the subsequent recovery period, it increased in a species-dependent manner (65.1–94.1%). A third parameter studied were changes in the initial fluorescence rise. The fluorescence rise during the first 300 ?s (L-band) can give information on the energetic connectivity between PSII units whereas changes in the rise during the first 2 ms (K-band) offer information on developing limitations on the donor side of PSII. Changes in respectively the L and K-bands of the fluorescence transients OJIP were shown to have predictive value with respect to the vitality of leaves and the tolerance of the varieties to drought stress.

UPDATE:

Same conclusion as above can be reached for the paper by Hu et al. (2009)*. They found that Bermudagrass cv. Tifway had higher net photosynthesis than cv C299 under drought stress, owing to a respective difference in photosystem metabolic activity under stress. However the two cultivars also differed in their RWC. This was not considered at all by the authors. When the reader will normalized for RWC (as above), it will be seen that there is no difference in net photosynthesis between the two cultivars for the same level of leaf dehydration. Hence, The difference between the two cultivars was in their capacity for dehydration avoidance rather than their capacity for photosynthesis in the dehydrated state.

(*) Longxing Hu, Zhaolong Wang and Bingru Huang. 2009. Photosynthetic Responses of Bermudagrass to Drought Stress Associated with Stomatal and Metabolic Limitations. Crop Sci.49:1902-1909.