Folkard Asch

Soluble Invertase Expression Is an Early Target of Drought Stress during the Critical, Abortion-Sensitive Phase of Young Ovary Development in Maize

To distinguish their roles in early kernel development and stress, expression of soluble (Ivr2) and insoluble (Incw2) acid invertases was analyzed in young ovaries of maize (Zea mays) from 6 d before (−6 d) to 7 d after pollination (+7 d) and in response to perturbation by drought stress treatments. The Ivr2 soluble invertase mRNA was more abundant than the Incw2 mRNA throughout pre- and early post-pollination development (peaking at +3 d). In contrast,Incw2 mRNAs increased only after pollination. Drought repression of the Ivr2 soluble invertase also preceded changes in Incw2, with soluble activity responding before pollination (−4 d). Distinct profiles of Ivr2and Incw2 mRNAs correlated with respective enzyme activities and indicated separate roles for these invertases during ovary development and stress. In addition, the drought-induced decrease and developmental changes of ovary hexose to sucrose ratio correlated with activity of soluble but not insoluble invertase. Ovary abscisic acid levels were increased by severe drought only at −6 d and did not appear to directly affect Ivr2 expression. In situ analysis showed localized activity and Ivr2 mRNA for soluble invertase at sites of phloem-unloading and expanding maternal tissues (greatest in terminal vascular zones and nearby cells of pericarp, pedicel, and basal nucellus). This early pattern of maternal invertase localization is clearly distinct from the well-characterized association of insoluble invertase with the basal endosperm later in development. This localization, the shifts in endogenous hexose to sucrose environment, and the distinct timing of soluble and insoluble invertase expression during development and stress collectively indicate a key role and critical sensitivity of the Ivr2soluble invertase gene during the early, abortion-susceptible phase of development.

Leaf K/Na ratio predicts salinity induced yield loss in irrigated rice

Salinity is a major constraint to irrigated rice production, particularly in semi-arid and arid climates. Irrigated rice is a well suited crop to controlling and even decreasing soil salinity, but rice is a salt-susceptible crop and yield losses due to salinity can be substantial. The objective of this study was to develop a highly predictive screening tool for the vegetative growth stage of rice to estimate salinity-induced yield losses. Twenty-one rice genotypes were grown over seven seasons in a field trials in Ndiaye, Senegal, between 1991 and 1995 and were subjected to irrigation with moderately saline water (3.5 mS cm-1, electrical conductivity) or irrigation with fresh water. Potassium/sodium ratios of the youngest three leaves (K/NaLeaves) were determined by flame photometry at the late vegetative stage. Grain yield was determined at maturity. All cultivars showed strong log-linear correlations between K/NaLeaves and grain yield, but intercept and slope of those correlations differed between seasons for a given genotype and between genotypes. The K/NaLeaves under salinity was related to grain yield under salinity relative to freshwater controls. There was a highly significant correlation (p < 0.001) between K/NaLeaves and salinity-induced grain yield reduction: the most susceptible cultivars had lowest K/NaLeaves and the strongest yield reductions. Although there were major differences in the effects of salinity on crops in both the hot dry season (HDS) and the wet season, the correlation was equally significant across cropping seasons. The earliest possible time to establish the relationship between K/NaLeaves under salinity and grain yield reduction due to salinity was investigated in an additional trial in the HDS 1998. About 60 days after sowing, salinity-induced yield loss could be predicted through K/NaLeaves with a high degree of confidence (p < 0.01). A screening system for salinity resistance of rice, particularly in arid and semi-arid climates, is proposed based on the correlation between K/NaLeaves under salinity and salinity-induced yield losses.

Sodium and potassium uptake of rice panicles as affected by salinity and season in relation to yield and yield components

Salinity is a major yield-reducing stress in many arid and/or coastal irrigation systems for rice. Past studies on salt stress have mainly addressed the vegetative growth stage of rice, and little is known on salt effects on the reproductive organs. Sodium and potassium uptake of panicles was studied for eight rice cultivars in field trials under irrigation with saline and fresh water in the hot dry season and the wet season 1994 at WARDA in Ndiaye, Senegal. Sodium and potassium content was determined at four different stages of panicle development and related to salt treatment effects on yield, yield components and panicle transpiration. Yield and yield components were strongly affected by salinity, the effects being stronger in the HDS than in the WS. The cultivars differed in the amount of salt taken up by the panicle. Tolerant cultivars had lower panicle sodium content at all panicle development stages than susceptible ones. Panicle potassium concentration decreased with panicle development under both treatments in all cultivars, but to a lesser extent in salt treated susceptible cultivars. Grain weight reduction in the early panicle development stages and spikelet sterility increase in the later PDS were highly correlated (p < 0.01) with an increase in panicle sodium concentration in both seasons, whereas reduction in spikelet number was not. The magnitude of salt-induced yield loss could not be explained with increases in sodium uptake to the panicle alone. It is argued that the amount of sodium taken up by the panicle may be determined by two different factors. One factor (before flowering) being the overall control mechanism of sodium uptake through root properties and the subsequent distribution of sodium in the vegetative plant, whereas the other (from flowering onwards) is probably linked to panicle transpiration.

Climatic determinants of irrigated rice performance in the Sahel — I. Photothermal and micro-climatic responses of flowering

Abstract In the Sahel, variable crop duration of irrigated rice poses serious timing problems for intensification of production. Photothermal effects on phenology have been studied to develop simulation tools for breeding and cropping systems research. Forty-nine genotypes were planted at monthly intervals in various rice-garden trials. Environment variability among seasons, sites and within the crop canopy was characterized to develop a field-based, photothermal model for flowering. Basic concepts were summation of heat units and a linear thermal response of development rate having upper (Topt) and lower (Tbase) response limits. Photoperiodism was modeled by a slope constant (CPP) and a basic vegetative phase (BVP). Photoperiodism and transplanting shock acted as modifiers of heat requirements (Tsum), thereby having greater effects on duration at low than at high temperatures. Tbase, Topt, Tsum, BVP and CPP were considered genotypic constants and calibrated by optimization. Daily input temperature for the model was the physiologically relevant temperature Tphys at the shoot apex. Tphys depended on apex submergence, water temperature and diurnal temperature patterns. Diurnal temperature segments exceeding the Tbase-Topt range were disregarded. Mean water temperature was below air temperature, particularly at high leaf area index and on dry days. Mean air temperature was closer to the minimum than to the maximum when amplitudes were high or days short. Minimum temperatures below 18°C at booting stage resulted in near total spikelet sterility and a specific delay in heading. The model was validated for a site thermally different from the site of calibration.

Salinity increases CO2 assimilation but reduces growth in field-grown, irrigated rice

Salinity is a major yield-reducing factor in coastal and arid, irrigated rice production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf area (SLA), sodium concentration and leaf CO2 exchange rates (CER) at photosynthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Senegal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 under irrigation with fresh or saline water (flood water electrical conductivity = 3.5 mS cm-1). Relative leaf chlorophyll content (SPAD method) and root, stem, leaf blade and panicle dry weight were measured at weekly intervals throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (topmost) and second leaf. Salinity reduced yields to nearly zero and dry-matter accumulation by 90% for the susceptible cultivar in the HDS, but increased leaf chlorophyll content and CER at PAR- saturation. The increase in CER, which was also observed in the other cultivars and seasons, was explained by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf senescence and chlorophyll degradation. The growth reductions were attributed to loss of assimilates (mechanisms unknown) that must have occurred after export from the sites of assimilation. The apparent, recurrent losses of assimilates, which were between 8% and 49% according to simulation with the crop model for potential yields in irrigated rice, ORYZA S, might be partly due to root decomposition and exudation. Possibly more importantly, energy-consuming processes, such as osmoregulation, interception of sodium and potassium from the transpiration stream in leaf sheaths and their subsequent storage, drained the assimilate supply.

Skyfarming an ecological innovation to enhance global food security

Population growth increases the demand for food and thus leads to expansion of cultivated land and intensification of agricultural production. There is a definite limit to both of these options for food security and their multiple negative effects on the environment undermine the aim for sustainability. Presently the impact of the Green Revolution on crop production is levelling off at high yields attained and even the potential of large scale irrigation programmes and transgenic crops seem to be limited in view of the expected increase in demand for food. Moreover, climate change threatens to affect agricultural production across the globe. Skyfarming represents a promising approach for food production that is largely environment independent and therefore immune to climate change. Optimal growing conditions, shielded from weather extremes and pests are aimed at raising plant production towards the physiological potential. Selecting rice as a pioneer crop for Skyfarming will not only provide a staple for a large part of the global population, but also significantly reduce the greenhouse gas emission caused by paddy cultivation. Multiplication of the benefits could be achieved by stacking production floors vertically. In Skyfarming the crop, with its requirements for optimal growth, development and production, determines the system’s design. Accordingly, the initial development must focus on the growing environment, lighting, temperature, humidity regulation and plant protection strategies as well as on the overall energy supply. For each of these areas potentially suitable technologies are presented and discussed.ZusammenfassungDer steigende Bedarf an Nahrungsmitteln infolge einer weiterhin exponentiell wachsenden Weltbevölkerung erfordert enorme Anstrengungen seitens der Agrarwirtschaft, die bislang mit Ausdehnung der Anbauflächen und mit Intensivierung der landwirtschaftlichen Produktion reagierte. Beiden Optionen zur Sicherung der Welternährung sind physische und biologische Grenzen gesetzt und die landwirtschaftlichen Aktivitäten führen ihrerseits zu mannigfaltigen Umweltwirkungen, die die ökologische Tragfähigkeit der Erde mindern können. Durch die Grüne Revolution konnte ein rasanter Anstieg der Nahrungsmittelproduktion erreicht werden, jedoch ist in einigen Regionen das Ertragspotential bestimmter Feldfrüchte nahezu erreicht und die Erwirtschaftung zusätzlicher Ertragssteigerungen wird zunehmend schwieriger. Auch der mögliche Beitrag von Bewässerungsprogrammen und seitens genetisch veränderter Varietäten scheint angesichts des zu erwartenden Anstiegs beim Nahrungsmittelbedarf begrenzt. Darüber hinaus sind negative Auswirkungen des Klimawandels auf die landwirtschaftliche Produktion zu erwarten. Skyfarming ist ein Ansatz zur weitgehend umwelt- und somit klimaunabhängigen Nahrungsmittelproduktion im Hochhaus. Unter optimalen Wachstumsbedingungen, geschützt vor Wetterextremen und Schadorganismen soll das physiologische Produktionspotential der Pflanzen weitestgehend ausgeschöpft werden. Mit Reis als Modellpflanze für Skyfarming wird einerseits ein wichtiges Grundnahrungsmittel bereitgestellt und andererseits die Möglichkeit eröffnet die durch den Naßreisanbau verursachte Emissionen klimarelevanter Gase signifikant zu reduzieren. Eine Vervielfachung des Ertrages ließe sich durch die vertikale Anordnung mehrerer Produktionsebenen erreichen. Bei Skyfarming steht die Kulturpflanze, mit ihren spezifischen Ansprüchen für optimale Entwicklung und Wachstum im Vordergrund eines systemischen Ansatzes. Dementsprechend muss der Fokus zu Beginn der Technologieentwicklung auf folgende Bereiche gerichtet sein: Wachstumsraum, Beleuchtung, Temperatur, Luftfeuchteregulierung, Pflanzenschutzstrategien, sowie – übergeordnet – Energieversorgung. Für jeden dieser Bereiche werden entsprechende Technologien vorgestellt und diskutiert.

Response of rice varieties to soil salinity and air humidity : a possible involvement of root-borne ABA

In a phytotron experiment four rice varieties (Pokkali, IR 28, IR 50, IR 31785-58-1-2-3-3) grown in individual pots were subjected to low (40/55% day/night) and high (75/90%) air humidity (RH), while soil salinity was gradually increased by injecting 0, 30, 60 or 120 mM NaCl solutions every two days. Bulk root and stem base water potential (SWP), abscisic acid (ABA) content of the xylem sap and stomatal resistance (rs) of the youngest fully expanded leaf were determined two days after each salt application. The SWP decreased and xylem ABA and rs increased throughout the 8 days of treatment. The effects were amplified by low RH. A chain of physiological events was hypothesized in which high soil electric conductivity (EC) reduces SWP, followed by release of root-borne ABA to the xylem and eventually resulting in stomatal closure. To explain varietal differences in stomatal reaction, supposed cause and effect variables were compared by linear regression. This revealed strong differences in physiological reactions to the RH and salt treatments among the test varieties. Under salt stress roots of IR 31785-58-1-2-3-3 produced much ABA under low RH, but no additional effect of low RH on rs could be found. By contrast, Pokkali produced little ABA, but rs was strongly affected by RH. RH did not affect the relationships EC vs. SWP and SWP vs. ABA in Pokkali, IR 28, and IR 50, but the relationship ABA vs. rs was strongly affected by RH. In IR 31785-58-1-2-3-3 RH strongly affected the relationship SWP vs. ABA, but had no effect on ABA vs. rs and EC vs. rs. The results are discussed regarding possible differences in varietal stomatal sensitivity to ABA and their implications for varietal salt tolerance.

Ovary abscisic acid concentration does not induce kernel abortion in field-grown maize subjected to drought

This study investigated the effects of drought of different duration and severity on ovary abscisic acid (ABA) concentration and yield components in field-grown maize (Zea mays L. cv. Loft). The study was conducted in a field lysimeter of the Royal Veterinary and Agricultural University (KVL) in Hojbakkegaard (55°40′N; 12°18′E; 28 masl), Denmark in 1997. Irrigation was withheld at four different dates to induce drought of different duration and severity at the reproductive stage of the plants. Plots were re-watered shortly after silking and kept at field capacity for the remainder of the season. Soil water status, plant height, and early morning leaf water potential were monitored during the treatment. Ovary ABA concentration was determined at four dates before and after fertilization. Final grain yield, total DM, harvest index (HI), mean kernel weight, kernel weight distribution, and kernel number per cob were determined at maturity. Plant height was significantly (P<0.05) reduced by 40 and 25%, respectively, in the two most severe drought treatments. In the two shorter drought treatments no effect of drought stress on plant height or biomass was observed. Leaf water potential decreased slowly as a function of relative available soil water content and resulted in −0.4 MPa at the end of the longest and −0.12 MPa at the end of the shortest stress period. Under fully watered conditions, plot yields averaged 1400 g m−2 for total dry matter (DM) and 700 g m−2 for grain yield, with a HI of about 0.5. Initiation of a drying cycle close to flowering did not change yields. Long drying cycles resulted in significant (P<0.05) yield reductions up to 70% of the fully watered controls. Kernel number per cob was reduced up to 60% under long drought conditions and not affected under short-term drought. Drought imposed about two weeks prior to fertilization resulted in 30% reduction in kernel number per cob, but this effect was balanced by an increase of 25% in mean kernel weight. Long and severe drought increased ovary ABA concentration prior to fertilization, whereas short-term drought did not. At fertilization no increase of ovary ABA as compared to fully watered controls was found in any treatment. It is concluded that drought induced grain yield losses in field grown maize cannot be attributed to kernel size reduction or kernel abortion due to ovary ABA concentrations as reported by some authors for studies on maize and wheat under controlled conditions, as ovary ABA concentrations peaked before zygote formation and endosperm development.

Chlorophyll index, photochemical reflectance index and chlorophyll fluorescence measurements of rice leaves supplied with different N levels

Rapid and non-destructive diagnosis of plant N status is highly required in order to optimise N fertilizer management and use-efficiency. Additionally to handheld devices for measurements of chlorophyll indices (e.g., SPAD meter) parameters of canopy reflectance via remote sensing approaches are intensively investigated and the photochemical reflectance index (PRI) appears to be a reliable indicator for changes of the epoxidation state of xanthophyll cycle pigments. In order to assess the suitability of a handheld PRI as an additional tool for N diagnosis, rice plants were grown in a nutrient solution experiment with seven N-supply levels (0.18-5.71 mM) and CI (SPAD) and PRI values and chlorophyll fluorescence parameters measured 20 and 28 days after onset of treatments. N-supply had effects on both CI (SPAD) and PRI values with a more reliable differentiation between levels. Maximum quantum yield of PSII (F(v)/F(m)), actual efficiency of PSII photochemistry (Ф(PSII)) and regulated non-photochemical quenching (Ф(NPQ)) did not differ significantly between N levels. Non-photochemical quenching (NPQ) and fast- relaxing NPQ (NPQ(F)) were significantly affected by N-supply. NPQ and NPQ(F), but not the slow-relaxing component (NPQ(S)), were correlated with CI (SPAD) and PRI values. This finding which has not been reported for N-supply effects so far is indirect evidence that low N-supply induced xanthophyll cycle activity and that PRI values are able to indicate this at least in plants subject to severe N deficiency.

A conceptual model for sodium uptake and distribution in irrigated rice

Sodium uptake and distribution in rice was investigated in various field and screenhouse trials at WARDA’s research station in Ndiaye, Senegal. A conceptual model for sodium uptake and distribution was developed on the basis of the following results: (i) sodium uptake to the plant was transpiration driven; (ii) varieties differed in the way they regulated their stomata in relation to relative humidity and salt stress; (iii) sodium uptake was modulated at the root level by a ‘root filter’; and (iv) sodium was taken out of the transpiration stream and retained in the sheaths with the daily retention capacity as a varietal constant. The model describes the passive uptake of sodium to the plant and its distribution as a function of several varietal constants and transpiration. The root filter function for sodium, the maximum stem sodium retention and the sodium toxicity threshold in the leaves vary among varieties. Varietal differences in stomatal reactions to relative humidity and salt stress were implemented in the ORYZA_W evapotranspiration routine to simulate these reactions. The interactions between sodium and potassium are discussed in relation to available information from literature and preliminary results obtained from screenhouse studies. A concept of potassium distribution in the plant and its interaction with sodium uptake and distribution was included in the model structure.