The drought response of landraces of wheat from the northern Negev Desert in Israel

Summary Diverse landraces of wheat, collected from the semi-arid (150 to 250 mm of total annual rainfall) Northern Negev desert in Israel were considered as a potential genetic resource of drought resistance for wheat breeding. These materials were therefore evaluated for their reponses to drought stress in agronomical and physiological terms. Up to 68 landraces, comprising of Triticum durum, T. aestivum, and T. compactum were tested in two field drought environments, in one favourable field environment, under post-anthesis chemical plant desiccation which revealed the capacity for grain filling from mobilized stem reserves, under a controlled drought stress in a rainout shelter and in the growth chamber under polyethylene glycol (PEG)-induced water stress. Biomass, grain yield and its components, harvest index, plant phenology, canopy temperatures, kernel weight loss by chemical plant desiccation, growth reduction by PEG-induced drought stress and osmotic adjustment were evaluated in the various experiments.Landraces varied significantly for all parameters of drought response as measured in the different experiments, which was in accordance to their documented large morphological diversity. Variation in grain yield among landraces under an increasing drought stress after tillering was largely affected by spike number per unit area. Kernel weight contributed very little to yield variation among landraces under stress, probably because these tall (average of 131 cm) landraces generally excelled in their capacity to support kernel growth by stem reserve mobilization under stress. Yield under stress was reduced with a longer growth duration of landraces only under early planting but not under late planting. Landraces were generally late flowering but they were still considered well adapted phenologically to their native region where they were always planted late.Landraces differed significantly in canopy temperature under drought stress. Canopy temperature under stress in the rainout shelter was negatively correlated across landraces with grain yield (r=0.67^**) and biomass (r=0.64^**) under stress. Canopy temperature under stress in the rainout shelter was also positively correlated across landraces (r=0.50^**) with canopy temperature in one stress field environment. Osmotic adjustment in PEG-stressed plants was negatively correlated (r=–0.60^**) with percent growth reduction by PEG-induced water stress. It was not correlated with yield under stress in any of the experiments. In terms of yield under stress, canopy temperatures and stem reserve utilization for grain filling, the most drought resistant landrace was the Juljuli population of T.durum.     

Mass selection of wheat for grain filling without transient photosynthesis

Summary Post-anthesis chemical desiccation of wheat (Triticum aestivum L.) plants in the field eliminates transtent photosynthesis by killing all green tissues, thus revealing the plant's capacity for grain filling from stored stem reserves, as the case is for post-anthesis stress such as drought or leaf diseases. This study was conducted to investigate whether mass selection for large kernels under chemical desiccation would lead to the improve ment of grain filling in the absence of transient photosynthesis.Six crosses of common spring wheat were subjected to three cycles of mass selection from F₂ through F₁ when selection was performed for large kernels by sieving grains from plants that were erther chemically desiccated after anthesis, or not (controls). The resulting 36 bulks (six crosses by three selection cycles by two selection environments) were compared with their respective F₂ base populations, when tested with and without chemical desiccation.Selection for large kernels under potential conditions (without chemical desiccation) did not improve kernel weight under potnetial conditions, evidently because these materials were lacking in genetic variation for kernel weight under potential conditions. In four of the crosses, 3rd cycle selection for large kernels under potential conditions decreased kernel weight under chemical desiccation. On the other hand, selection for large kernels under chemical desiccation was effective in improving kernel weight and test weight under chemical desiccation, depending on the cross and the selection cycle, with no genetic shift in mean days to heading or mean plant height. Selection for large kernels under chemical desiccation was also effective in some cases in increasing kernel weight under potential conditions. The results are interpreted to show that selection under potential conditions and under chemical desiccation operate on two different sources for grain filling, namely transient photosynthesis and stem reserve utilization, respectively. In order to expose genetic variability for stem reserve utilization to selection pressure, transient photosynthesis must be eliminated, as done by chemical desiccation in this study.     

An evaluation of seed and seedling drought tolerance screening tests in wheat

Summary A series of experiments was performed in order to evaluate the significance of seed germination and seedling growth in osmotic media as screening methods for drought tolerance.Ten spring wheat (Triticum aestivum L. em Thell.) and one durum wheat (Triticum durum Desf.) were tested under controlled environments, using polyethylene glycol-6000 (PEG) solutions as the moisture stress inducing media.Tolerance in the rate of endosperm utilization, under stress, prior to the onset of germination varied among cultivars.Germination rate or injury to germination at various concentrations of PEG differed significantly among cultivars. Cultivar rating with respect to injury to germination changed with stress levels. Injury to germination did not correlate with endosperm utilization rate in PEG or in water.Germinating seedlings were tolerant to extreme desiccation up to the stage of emergence of the first leaf from the coleoptile.Growth of photosynthesizing seedlings was monitored as they were carried through an increasing concentration gradient of PEG solutions, ranging from –5.9 to –11.3 bars of water potential. Cultivars significantly differed in seedling growth tolerance to increasing levels of water stress. Seedling growth tolerance across cultivars was not correlated with their germination responses under srress.It is concluded that tolerance to water stress in growing seedlings can be screened for by using PEG-containing nutrient solutions. It can not be predicted from germination tests in osmotica.Work was done under a US-Israel Binational Science Foundation (BSF) Grant no. 1654/78.Contribution from Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 192-E, 1979 series.     

The Grain Quality of Landraces of Wheat as Compared with Modern Cultivars

Landraces of tetraploid and hexaploid wheat from the Northern Negev in Israel were evaluated over two years for their grain quality attributes. Twenty-one populations of tetraploid wheat (Triticum durum, represented by 56 accessions) and 8 populations of hexaploid wheat (T. aestivum and T. compactum, represented by 13 accessions) were compared with 3 and 4 modern improved Israeli cultivars of tetraploid and hexaploid reheat, respectively. This comparison allowed to estimate the progress made in the improvement of gram quality m present-day cultivars. Except for grain protein content in the hexaploids, significant variation was revealed among landraces in kernel weight, protein content (tetraploids), sedimentation, mixograph score and carotin content. The best modern cultivars were comparable to the best landraces in kernel weight and carotin content (tetraploids), indicating that modern wheat breeding maximized kernel weight and carotin content, as compared with the tested landraces. Gram protein content and/or quality was not maximized in modern cultivars and its improvement was deemed possible by introgression from the best landraces in this respect.