1 Introduction

Elephant foot yam (Amorphophallus paeoniifolius (Dennst.) Nicolson) is a tropical tuber crop commonly used as a staple food. This cash crop can be cultivated in harsh agro-climatic environment with little plant care. We consider yam plantation of ‘Bidhan Kusum’, a non-irritant high yielding variety in extreme climatic environment of high summer temperature with nominal plant care of sustainable agriculture in alluvial soil full of gravels as in Giridih, Jharkhand (23.35^oN, 85.33^oE).

Sensitivity of plants under various stimuli is well studied. Electrical signalling in the wounded plants is investigated e.g., in Wildon et al. (1992). Plant sensitivity under stress can suitably be utilized to increase crop yield see e.g., Dasgupta (2016), and the references therein. Among other stimuli, plants are sensitive to the color of light and touch. The affect of colored light on plants are studied e.g., in Zait et al. (2017). Touch sensitivity of plants are investigated e.g., in Musah et al. (2015).

Plants usually respond to changes in temperature, chemicals, touches etc., by change in growth rate or its direction of growth. Extreme climatic stress has in general a negative impact on crop yields. We consider a form of stress that may suitably be utilised for higher growth of yam. With a modification in harvesting strategy, one may substantially increase yam yield in two-stage harvesting from individual plants with interim harvest at appropriate time of growth, this plus the final yield at plant maturity; the total yield, sum of the two; is seen to be higher compared to that for other strategies including usual single harvest from plants. In this study we select appropriate seed weight and also investigate the choice of appropriate time for interim harvest of yam in order to have high yield with nominal plant care in harsh agro-climatic condition as in Giridih, Jharkhand (India). Under stress, yam plants rejuvenate to initiate retuberization. The total yield, early plus final harvest is more than usual yield due to fast growth of tuber after yam detachment; this is also evident from yam growth curve analysis. Since early harvest fetches a good price in market, the two stage harvesting procedure with interim harvest, at about four and half months after sprouting, from yam plants with seed weight 800 g is found to be conducive for maximising revenue from higher total yield in harsh agro-climatic environment and nominal plant care, with a few irrigations in extreme summer of Jharkhand.

2 Materials and Methods

We study longitudinal growth of Elephant-foot-yam for sixty plants under extreme agro-climatic stress in a field experiment conducted in the agricultural farm at Indian Statistical Institute, Giridih, with seed weights 500 g, 650 g and 800 g of yam.

The experimental layout is similar to that described in Dasgupta (2016), this consists of six columns, in each column there are ten equidistant pits at a distance of 1 m. First two columns are for seed weight 500 g, next two are for seed weight 650 g, and the last two columns are for plants with seed weight 800 g. Column to column distance is also 1 m; the plants are numbered 1-10 in the first column, 11-20 in the second etc.

Little vermicompost is given in the pits while planting the fungicide treated cut yam seed corms of specified weights at the start of the experiment on 20 March 2016. From the start of the experiment we consider a general plant stress viz., little fertilizer and scanty irrigation.

The average lifetime of yam plants in this experiment is 185.90 days. Due to plant stress induced in the experiment, the figure is less than plant lifespan average that may be up to 9 months. At about one fourth of that time period i.e., at about two and half months after sprouting we take first interim growth reading on underground yam; and at half of the time period of 9 months i.e., at about four and half months from sprouting we take second interim reading on underground yam in this longitudinal study. Plants are subjected to possible yam detachment only at the time of interim growth recordings. Uprooting and replanting are done sequentially one by one with possible yam detachment, to minimise resultant plant stress of aboveground exposure.

With 20 plants in each seed category, all yam plants are subjected to stress of harsh agro-climatic environment. Further as a severe stress, for some plants in each seed weight category, underground yam associated with root structure are detached at about two and half months after sprouting, and for some plants yam detachment is at about four and half months after sprouting, and for remaining plants no interim yam detachment is made; decision of detachment is based on an assessment of underground yam growth at the time of interim recording.

For plants with yam detached, stem structure with a few roots still attached near the base of stems are replanted immediately in the same pit to continue experiment. For plants in each seed weight category under general stress, interim yam readings are recorded by taking plants off the ground with care, and underground yam volumes are measured by Archimedean principle of water displacement and the experiment is continued after replanting in the same pit. Yam weight is indirectly estimated by multiplying the volume of yam, obtained from Archimedean principle of displaced water volume, when only the yam part of a plant is immersed in a water container. Yam volume multiplied by density (≈ 4 g /c.c) provides an estimate of yam weight in non-destructive testing.

Plants are subjected to two interim yam growth measurements, if found alive at the time of measurements i.e., after about two and half months from sprouting, and after about four and half months from sprouting. At most one yam detachment per plant during the experiment is made. Some plants are left undisturbed of yam detachment till the end for comparison with other strategies.

When yams are not detached from stems, stress is considered moderate for plants. In the present study the time of first intervention is on 30 July 2016. The second intervention is on 28 September 2016.

For plants with a first interim cut i.e., detachment of yam at the time of first interim growth record, no further cut is made; however a second interim growth record is taken on the plants alive, before final harvest. For plants with yams attached to stems, experiment continued after growth recording till the end of plant lifetime. Stress is severe on plants under yam detachment.

It may be mentioned that in a similar set-up under plant stress of no weeding, little fertilizer, and little irrigation before first interim reading on yam, regular plant care started only after first interim reading till the end of experiment; in that case the strategy of yam detachment made around middle of experiment is seen to be a superior strategy to maximise total yield with 650 g of seed weight, among seed weights 500 g, 650 g, 800 g; see Dasgupta (2016).

The present experiment is conducted with a little vermicompost given in the pits at start, and a few irrigations given only in extreme summer. Here the plant stress is extreme in harsh agro-climatic environment.

Individual growth curves are drawn with linear interpolation in between, from four possible readings on underground yam per plant viz., one in the beginning, two interim readings during experiment, and one at the end of plant lifetime. Overall growth curves in each categories are drawn by computing the average response (mean / median) at specific time points where at least one yam observation is available in individual growth curves and then using nonparametric lowess / spline regression on averaged points, see Dasgupta (2015a), Dasgupta (2015b). The growth curves help to understand underground yam deposition process over plant life time.

Our procedure of comparison of two strategies of yam harvest is mainly based on growth curve analysis. In addition, asymptotic normal test and Welch t test to compare harvesting strategies of yam are investigated. Quantification of the advantage in yam yield, when two strategies are compared, is made by nonparametric Hodges-Lehmann estimator, see Hodges and Lehmann (1963).

3 Results

3.1 Parametric and nonparametric techniques to compare different harvesting procedures

Consider the approximate normal test

z = (`x - `y) /√{(s²_x / m) + (s²_y / n)} =(1.3215 - 1.06033) /√(0.183/10+0.009276/3) =1.785632 for equality of means of total yield when first and second interim cut is considered for seed weight 800 g over two distinct set of yam plants; z is significant at 5% level, p value in one sided alternative that cut in second interim produces higher yield turns out to be p=0.0371. When the same comparison of first and second interim cut is made for seed weight 650 g, z = 2.446258; z is highly significant, p value in one sided alternative being p=0.0072.

If we compare total yield with second interim cut for seed weight 800 g being better than that for 650 g, then z = 2.042643, and z is significant at 5% level, p value in one sided alternative being p=0.0205.

For seed weight 500 g, growth curves of plants show downward trend till first intervention, and then these remain parallel to x axis showing a stalled growth, this indicates 500 g is minimal seed weight for plant survival in harsh agro-climatic condition of conducted experiment.

The value of z may be compared with a t variable, referred as Welch's t statistic with degrees of freedom (d.f.) computed as {(s²_x / m) + (s²_y / n)} ² / {(s²_x /m) ² / (m-1) + (s²_y /n) ² / (n-1)}. The true distribution of the test statistic depends slightly on the two unknown population variances and this is an approximate test. In the above three cases of comparison with approximate normal test, the d.f. of Welch's t statistic are approximately 11, 10 and 17, respectively. The p values are low for one-sided alternatives; the values in three cases are 0.050864, 0.017241 and 0.028458, respectively.

As already mentioned, in a similar set-up under severe plant stress of no weeding, little fertilizer, little irrigation before first interim reading on yam; detachment around middle of experiment is seen as a superior strategy to maximise total yield with 650 g seed weight, among seed weights 500 g, 650 g, 800 g; see Dasgupta (2016).

With extreme plant stress, more severe than that described in Dasgupta (2016), higher seed weight 800 g comes to the forefront of yam yield, surpassing the seed weight 650 g of the previous experiments. With interim detachment around four and half month after sprouting; plants of higher seed weight can cope better with extreme plant stress for survival, and achieve higher total yield in a harsher agro-climatic environment.

For quantification of the advantage in yam yield when two strategies are compared, one may consider nonparametric Hodges-Lehmann estimator, see Hodges and Lehmann (1963). This statistic is an estimate of the difference between two populations. For two sets of data with m and n observations, the set of two-element sets made contains (m x n) pairs of points, one from each set; each such pair defines one difference of values. The Hodges-Lehmann statistic is the median of these mn differences. This estimates the differences between the populations of the paired random variables drawn respectively from the populations.

For seed weight 500 g, total yield (in kg) i.e., interim yield plus final yield on plant maturity from 8 yam plants, with yam detached at second interim recording are: 1.186, 0.873, 0.620, 0.298, 0.787, 0.532, 0.598, 0.787.

For the same seed weight 500 g, yield (in kg) from 11 mature plants with no interim detachment are: 0.267, 0.128, 0.122, 0.12, 0.158, 0.539, 0.541, 0.502, 0.162, 0.294, 0.736.

One plant of seed weight 500 g had yam cut at first interim growth recording. The 88 values of the differences i.e., yam yield with detachment at second interim recording minus yield from undisturbed yam growth with no detachment has median value 0.407 kg. This is a substantial additional amount per plant with seed weight 500 g.

Thus for the plants with seed weight 500 g, the gain in yield on an average for following the strategy of interim yam detachment at the time of second growth recording i.e., at about four and half months from sprouting; compared to usual yield from single harvest at the end of plant maturity is 407 g, per plant.

For seed weight 650 g, total yield with cut at second interim minus yield with cut at first interim has median value 0.213 kg; for same seed weight total yield with cut at second interim minus yield of undisturbed growth with no cut has median value 0.0935 kg.

For seed weight 800 g, total yield with cut at second interim minus yield with cut at first interim has median value 0.3645 kg; for same seed weight total yield with cut at second interim minus yield of undisturbed growth with no cut has median value 0.2715 kg.

The agro-climatic environment is harsher in the present experiment in comparison with that considered in Dasgupta (2016) where seed weight 650 g with interim yam detachment in the middle of experimental duration showed an additional yield of 457 g per plant, compared to conventional single harvest on plant maturity.

It appears that second interim cut is a good strategy to increase yield in all the seed weights we considered, the additional amount of yam yield per plant going up to 407 g, when interim cut for yam at the time of second reading is taken recourse to, compared with other harvesting strategies.

3.2 Superior status of yield with yam detachment at the time of second interim growth recording

Yam growth curves for all seed weights and for each seed weight separately are shown in Figure 1, Figure 2, Figure 3, and Figure 4.

Except for Figure 2 with seed weight 500 g, a general upward trend in growth is seen in these figures.

Figure 5, Figure 6, and Figure 7 show the growth curves of yam in three situations with first interim cut, second interim cut, and for undisturbed yam with no interim cut respectively, for plants with seed weight 500 g. Figure 5 has a single curve from a plant with first interim cut, as no substantial yam deposition was seen at that early stage in other plants with low seed weight. The figures indicate that the response curve with second interim cut in Figure 6, based on mean /median of observations y for fixed time x, is superior among these three situations.

For seed weight 650 g, growth curves are shown in Figure 8, Figure 9, Figure 10. The same phenomenon of superior status of yam detachment at second interim growth recording is reflected. Figure 9 with second interim cut show superior response curve for seed weight 650 g.

In Figure 11, Figure 12, Figure 13, we consider seed weight 800 g in three different harvesting strategies, the response curve in Figure 12 with second interim cut seems to be stable towards end.

Figure 14 explains the median growth curve for different seed weights under three situations of yam detachments viz., first interim cut, second interim cut, and undisturbed with no interim cut. The curve for second interim cut of yam with seed weight 800 g seems superior.

Figure 15 explains the above scenario in terms of mean growth curve. The curve for second interim cut of yam with seed weight 800 g is superior, if we incorporate the criterion of growth stability towards the far end of the growth curves. In Figure 16, the assertion that the cut at the time of second interim growth recording of yam with seed weight 800 g is superior becomes clearer from lowess regression of the curves with f = 0.35. Yam cut at second interim growth recording with seed weight 800 g corresponds to higher yam yield. The corresponding curve reaches a stable value of higher yield towards the end of plant lifetime viz., beyond 160 days.

Figures on growth curve indicate the following. For maximising the total yield, an interim detachment of yam may be made during the season; but no further uprooting for interim records should be made to these plants till maturity, as a downward slope of growth is then observed in general. Corm weight of 500 g is the minimal seed weight for plant survival under the condition of nominal care of plants; some growth curves show no indication of change over time, although downward slope of growth is arrested by uprooting and replanting plants after interim growth record. Uprooting and replanting the plants during the experiment served as a favorable stimulus for growth, when plant growth becomes inert under nominal plant care.

3.3 Sensitivity of plants under stress, affecting yam production in a positive manner

Curves with interim of yam detachment show an upper trend, indicating plants’ alertness to counter the extreme stress by gathering resources for subsequent higher growth. Even when the plants are just uprooted and replanted without detachment of underground yam, the plants are subjected to stress to an extent, and this helps the plants to come out of inert state of growth as seen in reverting from the downward trend in curves, and then the curves turn towards a favorable change in growth slope showing improved status.

4 Discussions

In the present investigation we see that a two stage harvest of yam with first harvest at about four and half month from sprouting produces significantly higher yield. Study methods practiced a procedure on application of growth curve analysis. With extreme plant stress, higher seed weight 800 g comes to the forefront of yam yield scenario surpassing the seed weight 650 g of the previous experiments. With interim yam detachment around four and half month after sprouting; plants with higher seed weight 800 g can achieve higher total yield in a harsh agro-climatic environment and are able to cope with extreme plant stress for survival and growth.

5 Conclusions

Crop yield in general may decrease under plant stress. Plant response under stress may suitably be utilised to increase yield of yam. In harsh agro-climatic conditions with severe plant stress, 800 g of yam seed weight is recommended to farmers for high yield of Elephant-foot-yam with interim harvest of yam prescribed after four and half months from sprouting, and final harvest at the end of season on plant maturity from replanted stem structure, leading to high revenue.

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