TOPIC : WATER USE EFFICIENCY - TERMINOLOGY INTRINSIC WUE – IMPORTANCE AS A DROUGHT RESISTANT TRAIT PRESENTED BY

ZUBY GOHAR ANSARI TAM/2014/26

INTRODUCTION Water is highly essential to plant growth &

development. Water constitute more than 80% of the most of plant cells and tissues in which there is active metabolism. It forms continuous liquid phase through the plant from the root epidermis and liquid phase continum generally extend into the soil or substrate in which the plant is rooted .

Various plant factors are affected by water contents in cells and tissues. Various physiological factors which are influenced by water are cell division, cell expansion, photosynthesis, transpiration, membrane permeability and N metabolism.

Different morphological factors includes plant height, root growth, No. of primary branches, leaf area, No. of flowers and final yield are also influenced by cell water status.

Available water for irrigation to agricultural purposes is depleting year by year due to increased industrialization and urbanization. Rainfed agriculture is still affected due to erratic monsoon year by year. Under these conditions, water needs to be conserved and utilized efficiently . Specially under water limited situations , crop productivity is determined by how efficiently water is utilized. Among the several physiological traits, WUE is one of the important trait, which can be explored to develop new genotypes, tolerant to situation.

WHAT IS WUE It is defined as in different context by 1. Hydrologist: It is the ratio of the volume of

water used for productivity to the volume of water potentially available for irrigation & amount of water available from soil.

2. Agronomists: Amt of economic yield produced by unit of water applied where both evaporation and transpiration are considered.

3. Physiologist: It is the amount of water used in transpiration to produce unit amount of dry matter during a particular growth period.

WUE=Dry matter produced (g) / Water lost in transpiration(Kg)

SIGNIFICANCE The widely accepted yield model of Passioura

(1986) adequately emphasis the importance of both transpiration and WUE. Accordingly, it can be visualized that increasing either total transpiration or WUE, at a given transpiration, would enhance total biomass production. Most yield improvements have been achieved by increasing the transpirational components through management and breeding. Similarly in certain grain crops yield was improved by increasing H.I.

C4 species typically having a WUE of 4-5.5 g/Kg compared to the C3 species (1.5-3 g/Kg).

Perhaps because of this trait, the C4 species have significantly higher productivity compared to C3 species under arid and semi-arid tropics.

Genetic variability of WUE in field crops: 1. Cereals: 2.25 to 5.86 g/Kg 2. Pulses: 2.26 to 3.87 g/Kg 3. Oilseeds: 1.81 to 3.71 g/Kg

FACTORS AFFECTING WUEEnvironmental factors : 1. Vapour pressure deficit ( VPD) : It depends

on atmospheric humidity. Under high RH, VPD will be low and vice versa. VPD is the driving force for transpiration, hence an increase in leaf to air vapor pressure difference substantially increases the transpiration, there by decrease in WUE. This situation occur under water limited situation or high leaf temp. hence the prevailing RH & leaf temp. influences the VPD and it has major effect on WUE. Therefore high WUE can be achieved through minimizing the VPD, by planting the crops early in the season, where VPD is generally lower.

2. Light: The solar radiation has a vital role to play in determining WUE. Optimum irradiance will cause maximum efficiency of water use. High irradiance will increasing the leaf temp. and reducing the stomatal resistance there by decrease WUE.

3.Temperature: Leaf temp. have profound effect on WUE. Leaf temp. depend on atmospheric temperature & leaf water content. High atmospheric temp. coupled with lower water uptake ( moisture limited conditions) increases high VPD , there by increases transpiration and finally WUE is affected. The genotypes maintain low leaf temp. are always good at efficient utilization of water.

4. CO2 concentration: Enhanced Co2 concentration in atmosphere will increase WUE , by higher photosynthesis, & higher dry matter accumulation. Such increased Co2 concentration is one of the component in global warming & increase in productivity of crops was estimated initially due to global warming or green house effect.

5. Moisture stress: Drought stress is a complex combination of stress because of both water deficit and high temp. moderate drought increases WUE up to 100% while extreme drought could substantially decrease WUE. A common response to water stress is simultaneous decrease in photosynthesis & transpiration & increase in leaf temp..

If transpiration decrease faster than the photosynthesis, causes increase in WUE. Under severe moisture stress condition, leaves become less efficient with respect to water and Co2 exchange. Water lost is prevented due to stomatal closure, nut water still be lost through the cuticle, but Co2 entry through stomata is severely restricted, causing low WUE. Mid- season stress drastically affects WUE .

6. Agronomic practices & crop management: Early sown crops will escape the moisture stress while delayed sowing favors heavy weed growth which creates severe competition. Depth of sowing also influences water availability and there by seedling emergence, vigor, & final yield.

7. Antitranspirants: WUE can be improved by using antitranspirant which reduces transpiration. It may influence stomatal closure ( PMA, ABA, CCC, Salicylic acid etc.) or film forming (hexadecanol, cetyl alcohol etc.) on leaf surface or increase plant reflectivity (kaolinite) & reduce leaf temp.

8. Use of mulches: Used to conserve water up to 10 – 50 %. It depend on the crop in which it is used, rainfall, wind velocity, and temp. of soil and water. Organic mulches ( straw, rice dust, saw dust etc.) light colored & light reflecting mulches reduce soil temp. But black colored mulches such as polythene sheets, petroleum prdts increase temp. up to 5-8 degree Celsius. Plastic mulches are costly & suitable to areas where soil temp. are low & unfavorable to crop growth.

9. Use of shelter belts: Decrease the damaging effect of wind on crops and modify micro climate. Examples are cotton, onion, sweet potato, tomato & wheat.

10. Methods and quantities of water application : Frequent irrigation keep soil wet for longer time but loss of evaporation more. Heavy application of water causes deep percolation losses. Selection of proper method of irrigation is important based on soil and crop. WUE is higher with sprinkler irrigation than surface method. Drip is also increase production & decrease water use by crop. WUE of a crop increases in order of wild flooding, border strip, check basin, basin & furrow irrigation.

11. Fertilizer application: WUE of a crop invariably increases with application of fertilizers on deficit soils under adequate soil moisture conditions. This is particularly with yielding varieties and hybrids.

12. Weed control: weeds due to early establishment & a better root system are

able to exhaust soil moisture effectively than crop plants. Therefore, both yield & WUE are reduced. Controlling weeds is essential for

high WUE of crops.

PLANT FACTORS INFLUENCING WUE 1. Leaf movements: Leaf is the substrate where

both assimilation and transpiration takes place to the maximum. Transpiration normally shows a positive relationship with increasing irradiance. Hence leaf movement & surface reflectance pattern provide energy load on the leaf. Leaf pubescence helps in controlling the leaf temperature & water balance. The orientation of leaves directly towards incident irradiation results in relatively higher loss of water & hence decreasing the WUE. Nictinastic movement observed in leaves at mid day or high temp. is to conserve moisture through low transpiration.

2. Root system: The distribution of roots, its density can influence water use by crops. Thus the rate of root growth & their spread can affect WUE, particularly during early stages of crop growth. Root growth has direct relation with transpiration and it was established in several crops. Plants with deeper root growth are able to extract more soil moisture from deeper soil profiles & cause higher transpiration. Under less water stress conditions these types display lower WUE & no effect on yield.

3. Influence of nutrients: N, Mg, Fe are constituents of chlorophyll & availability of these nutrients influence leaf chlorophyll content and dry matter accumulation. Hence these nutrients influence WUE in terms of dry matter accumulation.

P being plays important role in root growth & its activity, availability of this nutrients influence transpiration, there by WUE. K plays critical role in stomatal movements & indirectly influence stomatal conductance & WUE.

Transpiration Efficiency (TE)

. Physiological methods of estimating WUE is TE, where on dry matter is calculated based on transpiration loss.

. Transpiration efficiency is synonym of WUE.

. PASSIOURA, 1986 has given an excellent model, where crop yields under field condition can be explained by following

GRAIN YIELD= T* TE*HI Where T= Total transpiration, TE=Transpiration efficiency or WUE, HI= Harvest index This relationship assumes greater importance when

the water availability is limited. Under such situations genetic variations in dry matter accumulation depend on TE & T. The total transpiration (T) from crop canopy is cumulative effects of the avg. rate of water uptake, duration of transpiration and canopy cover. TE or WUE is the amount of biomass produced per unit amount of water transpired.

It can be visualized that increasing either T & TE, at a given transpiration would enhance the biomass production.

Increase in T cannot be done under water limited and dry land conditions. However T can be increased with introgression of better root traits, which tap higher water uptake.

T.E. or WUE is the prime physiological traits, which can be exploited to improve high dry matter accumulation. It can be computed that at a rainfall of 800mm and 40% of which is available for transpiration, 0.1g/kg increase in TE or WUE would results in 0.3t/ha increase in total biomass. Hence, TE is important physiological trait that would determine total dry matter production.

INTRINSIC WUE WUE can be measured at whole plant level

using “gravimetric method” & at a single leaf level using ‘gas exchange studies’. The physiological methods of measuring WUE at single leaf level by measuring Co2 fixation & stomatal conductance is called intrinsic WUE.

Biomass produced is a function of photosynthetic rate. Hence IWUE is the ratio of carbon assimilation rate (A) to transpiration (T). Transpiration rate is determined by the intrinsic stomatal conductance & the existing leaf to air vapor pressure difference.

If the plants being studied in a similar environment, it can be expected the leaf to air vapor pressure will be similar and hence, the major factors that determines transpiration that the intrinsic stomatal conductance factor (gs), therefore,

IWUE=A/gs= carbon assimilation rate/stomatal conductance factor

IWUE can be used as useful trait to detect genotypic variability. However it has limitations, as the plants has to be grown in glass house condition only to get reliable data.

In the field conditions variability in the data observed due to dynamic vapor pressure. It can be measured with the equipment “infra red gas analyzer”.

Scope for plant improvement and crop yields:

For the improvement of crop yields under water limited situations, improving WUE is the potential option. However for any successful breeding programme, existence of genetic variability for WUE in a crop is essential. As several report says, in most of the crops, genetic variability exists for WUE.

Some of the variabilities are Rice – 2.49 – 5.41 g/Kg Wheat – 4.60 – 5.27 g/Kg Cowpea – 2.74 – 3.00 g/Kg Soybean – 1.66 – 2.44 g/Kg Chickpea – 1.61 – 2.23 g/Kg Groundnut – 1.41 – 3.30 g/KgBreeding for high WUE is it possible?

Though WUE is an important component of yield in the model proposed by Passioura (1986)

The existing genetic variability in WUE could not be exploited through breeding. Many such attempts were not successful since any improvement in WUE was often associated with reduction in dry matter accumulation and yield. Because most often plants have evolved to maximize the WUE through a reduction of transpiration and there by CGR & dry matter accumulation is reduced. Since stomatal conductance (gs) is associated with transpiration & internal Co2 partial pressure (Pi), WUE & transpiration are strongly interdependent. This interdependency is the major reason for the lack of success in breeding for increased WUE

Success in breeding for high WUE depend on

gm = Mesophyll efficiency gs = stomatal diffusion factorPi = internal Co2 partial pressure 1. The intrinsic mesophyll efficiency &

the Co2 diffusion process are associated with stomata that regulate carbon assimilation.

2. Transpiration rate on the other hand is controlled by the differences in gs at a given vapor pressure deficit(VPD).

3. These two physiological traits (gm and gs) determines Pi i.e. internal Co2 partial pressure, which directly influences the change in WUE.

4. Depending on contribution of gm or gs to Pi, genotypes and species are classified as gs dependent ( conductance type) or gm ( capacity type) dependent.

In capacity types, mesophyll factors will determine carbon assimilation. So in these types, WUE is interdependent of gs & hence transpiration will not be associated with WUE.

Selection for high WUE from these capacity types will result in high CGR, high dry matter and final yields. Hence screening the germplasm for capacity types and using as donor parents in a breeding programme, will lead to enhanced crop yields. G’nut, beans, wheat, grasses have capacity type genotypes. The capacity types can be determined by measuring WUE through IRGA or carbon isotope discrimination (CID).the success in developing g’nut genotype with high WUE and high yields occurred in our university at RARS, Tirupati, where dept. of plant physiology evolved and release 2 g’nut genotypes Abhaya and Greeshana become popular in farming community.