Water deficiency is a major factor that determines the productivity performance of the forage plants in the tropics. Different mechanisms contribute to drought resistance in plant: Avoidance of plant water deficits by drought escape (short duration), water conservation and more efficient water uptake. Any one of these could be considered to be used as a defiance strategy by plants.

This study was initiated to identify genotypic differences for drought resistance in a range of Lablab genotypes and to verify if drought resistance is confined to any particular habit–annual or perennial–and to examine if drought resistant genotypes could be selected under laboratory conditions based on some simple physiological attributes.

It was a pot culture experiment. Four replications of 15 genotypes were planted and the plants were raised under controlled environment 30 / 25°C day/night temperature, 14 h photoperiod and 60 and 90% relative humidity during day and night, respectively.

Pots were watered to weight (total 15 kg) for four weeks. The first fully expanded leaf was sampled from one plant of each pot to measure % relative water content – initial (RWC- initial): [(fresh wt-dry wt) / (turgid wt- dry wt)] 100.

After 4 weeks of sowing, pots were watered for the final time to 15 kg and drought stress was imposed by withholding water. Water use was monitored by weighing the pots and ranked (from highest to lowest water use) at 5, 9 and 12 days after imposing water stress. At day 12 into drought stress regime, another leaf sample was taken to calculate RWC during drought stress (RWC–stress). The decline in RWC (RWC–decline) was calculated as the difference between RWC–initial and RWC- stress and rank (from highest to lowest) in each genotype at the end of the 12-day stress period.

At 9 weeks after sowing (5 weeks after withholding water or 3 weeks after most of the applied water used up by the plants) a visual assessment of foliage yield was recorded (Growth Rating). A rating of 4 indicated a high percentage of green leaf minimal leaf loss, good vigour and minimal turgor loss (visible wilting) in the leaves and rating of 1 indicated the lowest value for these parameters.

At 12 weeks after sowing (8 weeks after the start of drought stress or 6 weeks after most of the water has been used), 50% of the lines appeared dead. At this time all pots were re-watered and within two days after re-watering, surviving green leaves were harvested and dry weight recorded. Leaf survival was calculated (Green leaf wt/green+dead leaf wt) 100. The level of recovery and re-growth was monitored 2 weeks after re-watering and was used as an indicator of drought resistance and ranked from the highest to the lowest.

There were significant differences in the total water use among the 15 Lablab lines at days 5, 9 and 12 after beginning of the drought stress. Drought resistance was however, not related to whether genotype is annual or perennial. One of two commercial cultivars, 4 of 8 cultivated accessions, 1 of 3 wild accessions and 2 of 2 bred-lines were measured as drought resistant after a prolonged period of withholding water. This showed that there is high level of variation for drought resistance within the Lablab genotypes. Genotypes with the desirable level of drought resistance could be selected from out of the germplasm collections and use them in breeding programme for increasing forage yield and perenniality.

These physiological indicators, including leaf survival are laborious and need extensive experimentation. It is highly advantageous and breeding progress will be rapid if the (unknown) osmoprotectant in Lablab is identified and molecular markers are developed based on already available genetic map for Lablab. The genes responsible for the biosynthesis of the osmoprotectant could be identified for the introduction into more valuable commercial crops so as to make them more drought tolerant.

By intensive plant breeding methods carried out at the Pulse Section, Agricultural College, Coimbatore, four promising varieties of Avarai (D. Lablab), DL. 250, DL. 269, DL. 453 and DL. 692 were released for general cultivation. But the major problem facing the breeders and the farmers in this crop is the heavy flower shedding with the consequential low pod setting. This peculiar defect has a depressing effect on the total yield in this crop. To give a scientific explanation to this phenomenon and also to assess the variations, if any, among varieties, flower and pod setting was studied in four avarai varieties, viz., DL.250, DL.269, DL.453 and DL.692 at the Pulses Research Station, Jayankondam.

The inflorescences were carefully selected so that the first flower on all the inflorescences in the four varieties open on the same day. Flower opening was recorded for 22 days when all the buds on all the inflorescences in the four varieties have opened. This way, it was possible to fix the number of flowers opened daily on each of the inflorescences. The number of pods set on the inflorescence finally was recorded.

There was varietal difference in flower and pod setting. The variety DL.692 produced the maximum number of flowers followed by DL.250, DL.453, and DL.269. It was interesting that DL. 269 which produced the lowest number of flowers recorded the highest pod set (22.3%) followed by DL. 692, DL. 453 and DL. 250 with 15.9%, 5% and 4.9%, respectively.

It was also noted that the percentage of pod set alone cannot be taken as a criterion in judging the yield behavior of the variety. The ultimate yield of pods depended upon the percentage of flower dropping. The more the shedding of flowers, the less was the pod set. This was evident in the variety DL.250, while the shedding was the least in DL.269 with higher percentage of pod set with the other two varieties falling in between. Yet the variety DL.692 developed the highest number of pods per inflorescence.

Flower and fruit drop is one of the main constraints in the productivity of Dolichos Lablab. It can be attributed to lack of source or impaired translocation to the sink or very low sink capacity. Experiments were conducted to work out the source-sink relationship in a determinate variety Hebbal Avare-3.

Pattern of translocation of photosynthates during bud stage showed that during this stage all the reproductive parts (floral organs) are functionally active sinks. But translocation pattern changed at later stages when the flowers in the upper whorls opened and in the lower whorls pod development had started. There were several sinks within the inflorescence, which received very low activity. Sinks, which were in advanced stages of growth, received relatively higher activity. The sinks were spread in all the whorls. The early formed inflorescences are potentially better sinks than the late formed ones.

Application of nitrogen enhanced photosynthetic 14CO2 fixations and thus increased the source capacity. When source capacity was reduced by way of defoliation, the photosynthetic 14CO2 fixation in the remaining leaves increased per unit leaf area. But when the total source was less, it resulted in lesser influx of 14C-metabolites into developing floral organs. When source availability per unit sink was enhanced by removing the sinks of the upper whorls, there was an increased influx of photosynthates per sink.

Pattern of translocation of 14C- metabolites following feeding of a single intact leaf with 14CO2 showed that the leaf on a branch contributed photosynthates to the sinks in the same branch. But when source availability was reduced on a branch due to defoliation, photosynthates from the other branches move into the inflorescence of the defoliated branches. Due to drastic reduction of sources by way of defoliation, the pattern of translocation changed. Only the sinks in the advanced stages of growth, which were in the lower whorls, received higher activity and the younger sinks of the same inflorescence received very little activity. But the potential sinks in the inflorescence other than fed leaf-inflorescence received higher activity.

The reduction in the source capacity by way of defoliation enhanced the flower drop and decreased the pod number and pod weight and ultimately yield per plant. Removal of the reproductory organs of the late formed ones situated above the sixth whorl resulted in the enhancement in pod and seed yield. But the removal of 50% flower buds in all the whorls and retaining only two flower buds in each whorl did not result in the enhanced yield, which suggests that the sink above the sixth whorl only were non-productive.

The efficient utilization of solar energy by source strength and partitioning of photoassimilates to the harvested economic sink determines the crop productivity. The source strength is the combination of size and activity and the leaf area contributes the maximum for source size. In addition, pod also plays a major role in the productivity of grain legumes similar to that of the flag leaf and inflorescence in cereals. The role of pods as a form of production unit have been studied by many researchers in peas. The present study reports the results of the assessment on the relative role of the pods in Lablab been in relation to seed development with the help of 14Co2.

The experiment was conducted with Lablab bean variety CO.8 in pot culture at TNAU, Coimbatore. Data were collected and Translocation Index was calculated as:

                 Translocation Index = Activity in seeds/Activity in pods x100

The pods fixed the carbon photosynthetically which varied with the developmental period. The incorporation was maximum in young pods (10 days) after 30 minutes of feeding compared to other feeding periods of sampling. The higher demand by developing pod as well as the seeds may be the possible reason for the higher incorporation during the earlier stages of pod and seed development.

The translocation Index was maximum at 10 days followed by 30 days feeding, the peak being at 8h (65.5%, 29.56% and 44% at 10,20 and 30 days, respectively). After 20 days of anthesis, there may be intense metabolism both in fruit walls and seeds, which may result in production of organic acids which are quite unstable and require to be assimilated immediately. Even the minimum traces fixed at later stages of pod development has been effectively translocated to the seeds which was confirmed by the higher Translocation Indices with the advancement of pod maturity, the maximum being at harvest.

The observations on ethanol insoluble fractions (starch, cellulose and protein) clearly indicated that old pods fixed less amount of 14Co2, but the rate of transfer to seeds was much faster than in young pods. Upto 8h, the insoluble fractions increased with the pod development, thereafter a general decrease was observed suggesting the conversion of assimilates into structural forms. Such transformation declined with the maturation of pods and seeds. With the advancement of maturity, all the fixed carbon in the pod was utilized by the seeds.

The seeds of legumes placed in closed system cannot photosynthesise directly though they contain chlorophyll. The seeds have to derive the assimilates either through the source of plant or pod. This study confirms that the pod fixes 14Co2 through photosynthesis, accumulates and translocates them to the developing seeds, which decreased with the age of the pod.

Measurements were made on leaves of 100 lines of D. Lablab [Lablab purpureus] chosen from the genetic stocks maintained at Jhansi, India. Tabulated data are presented on estimated leaf area calculated by 3 methods. It was concluded that specific leaf-area constants (0.6588, 0.6731 and 0.6905 for slender, medium and broad leaves, respectively) multiplied by length X width give the most accurate estimates of leaf area.

A determinate variety Lablab bean introduced from India was used in this study. In all experiments, seeds were sown in vermiculite and the germinated seeds were transplanted in plastic pots (12-15 cm in diameter), containing a mixture of pumice: vermiculite and sand (1:1:1, V/V/ V/). The plants were fed with a 0.1% solution of a compound fertilizer, OK-F-1 (N: P2 O5: K2 0, 15:8:17, Otsuka chemical Co; Tokyo). Nine plants were used in each treatment.

In phytotron glass rooms, an 8 hour–day length was achieved by exposing the plants to 8 h natural light and covering them from 17-30 h to 09-30 h of the following day. Day lengths longer than 8 h were obtained by extending the 8 h photoperiod with light from 70 k incandescent lamp (14.8 µmol m-2 s-1). The covers were ventilated by means of S-shaped pipes placed at the top of the covers in order to prevent the temperature from raising during lighting.

Seeds were sown on 30 April 1991 and germinated in green house. The seedlings were placed in the growth chambers (15-300C) from 15 May until 22 July.

Seedlings grown in the same manner as above received temperature (15–30°C) and photoperiod (15 and 8 h) treatments in the phytotron glass rooms from 20th May. Data were recorded on 27th July.

Germination was started on 26th April 1992 in plastic-film greenhouse. On 7th May, the seedlings were transferred to 12, 13 or 14 h day lengths at 25°C, or to 9, 10 or 11 h day length at 30°C in phytotron glass rooms. Measurements were made on 9th July. For the experiment at 20°C, seeds were germinated on 16th July 1992 and the seedlings received 18, 21 or 24 h day lengths from 22nd July until 18 September.

Seeds were sown on 11th January 1992 in a growth chamber at 25°C. After germination, the seedlings were placed in the growth chamber at 20 or 30°C. On 24th January (7 days after germination) or on 31st January (14 days after germination), the plants grown at 30°C were transferred to 20°C, and those grown at 20°C were transferred to 30°C. Growth was recorded on February 26th.

Seeds were sown on 26th April 1992 in a plastic-film green-house and the seedlings were grown at 25 or 30°C in the phytotron glass room under 15 or 8 h days length from 7th May to 26th June. On 22nd May, 20 days after germination, the day length regimes were changed from 8 h to 15 h from 15 h to 8 h.

Data indicate that the critical day length separating the indeterminate growth habit from determinate growth is 13 h at 25°C and is between 10 and 11 h at 30°C. The fact that there was no shift of growth habit from determinate to indeterminate in any day length at 20°C suggests the existence of a minimum temperature that permits the shift in growth habit. Relationships between temperatures and photoperiod in determining the growth habit show that if the critical day length is shorter, the higher the temperature. In a similar fashion, the short day requirement for flowering in some short day plants can be replaced by low temperatures.

Longer day permitted initiation of at least seven more stem nodes and nearly twice as many branch nodes as were produced at the shorter photoperiods in determinate soybean plants (Thomas and Raper, 1983). In Lablab bean, however, the number of nodes was increased by high temperature and internode length was longer when the plants were under long day lengths. These results may suggest that day length regulates internode length rather than number of internodes and that the main factor for determining number of nodes is temperature.

Determinate Lablab bean (Labalab purpureus) was treated with various plant hormones. Gibberellin (GA3) partly induced the indeterminate growth of plants grown under non-inductive conditions. Cytokinins (Kinetin and benzyl adenine) retained the determinate growth habit under inductive conditions for indeterminate growth. Endogenous contents of gibberellins and auxins were higher in the plants under inductive conditions than in those under non-inductive conditions, which the reverse was true for the endogenous contents of cytokinins. It is suggested that in determinate Lablab gibberellin (s) and/or auxin(s) induce indeterminate growth habit in long days and high temperatures, while cytokinin(s) sustain determinate growth in short days and low temperature conditions.

A study was carried out to derive a suitable non-destructive method of leaf area determination using length and breadth of leaflets of the trifoliate leaves of Dolichos Lablab. A random sample of 20 leaves giving 60 leaflets per variety in a group of 17 varieties was used for leaf area determination. The three models utilized were A1 = bLB, A2 = a + bLB, and A3 = aLb1Bb2 resulting in three prediction equations; A1=0.06647 LB, A2=2.3345 + 0.6321 LB and A3 = 0.1223 L0.5633 B1 2791. Although the three equations were found equally efficient, the equation A1=0.6647 LB was selected for its simplicity in calculation.

An experiment was carried out in Mymensingh, Bangladesh during May, 1991 to April, 1992 to investigate the phenology and fruiting behaviour of 3 advance lines (DSC-1-6/S1, DSC-1-6/S-2 and DSC-1-6 (S) of Lablab bean (L.purpureus) cross in F6 generation and their parents. The parental genotypes were DS-21-B (local) and DS-109 (Japanese). The test genotypes were sown on 3 dates, i.e., 15th May, 1st July and 15th August 1991. Results revealed strong interaction of sowing dates with genotypes for flowering and fruiting characteristics. The flowering time in the local parental genotypes gradually decreased with delay in sowing date due to photoperiod sensitivity and timely fixed behaviour. The exotic parent and 3 selected advance generation (F6) lines showed periodically fixed and photoperiod neutral behaviour. The effect of phenology showed that the local parent produced maximum flowers but incurred maximum flower abortion. The edible pod yield of the local genotype also varied widely with sowing dates, but remained statistically unchanged in the exotic genotype as well as in the advance lines. Among 3 sowing dates, July sowing was the best for pod production. May sowing produced the highest percentage of edible pod protein.