Diagnosis of boron deficiency in black gram

Noppakoonwong, Rojarae Netsangtip (1991) Diagnosis of boron deficiency in black gram. PhD thesis, Murdoch University.

Abstract

In many countries, black gram (Uigna mungo (L.) Hepper cu. Regur) is grown where there is widespread occurrence of B deficiency in field crops. Consequently, this thesis aims to establish and evaluate critical concentrations of B in leaves for diagnosis of B deficiency in black gram and to enamine the possible value of using B concentration in leaves for prognosis of seed yield at maturity.

The relationships of B concentration in young leaf blades of black gram to B deficiency during growth and seed yield at maturity were examined in four solution culture experiments at Perth, Western Australia and one field experiment at Chiang Mai, Northern Thailand.

The first experiment was undertaken in a controlled environment room to establish a critical B concentration for the diagnosis of B deficiency in black gram during vegetative growth. Seedlings were transferred 7 days after germination (D7) to non-renewed complete nutrient solutions containing 1 (B1) or 10 (B10) µM H3BO3. The lengths of leaf blades was measured daily and their leaf blade elongation rates (LBER : mm day^-1 ) were determined. Plants were harvested on D17, D18, D20 and D24 for dry matter (DM) of plants and B concentrations of roots and the first three trifoliate leaf blades (TF1, TF2 and TF3). The B requirement, or critical concentration, for the expansion of a young leaf blade was taken as the B concentration in the blade at which its LBER in B deficient plants was depressed to 90 7o of that of B adequate plants.

Within 2 days of symptoms appearing in the roots, B deficiency depressed LBER of TF1, TF2 and TF3, induced abscission of some leaflets of the youngest emerged leaf blade (TF3), depressed DM of TF3 and roots but had no effect on shoot BM. The critical B concentrations for expansion of TF1 and TF2 mere 8 and 6 mg B kg^-1 DM, respectively.

The second and third experiments mere undertaken concurrently in the glasshouse to enamine the effect of plant age on the critical B concentration in leaf blades of black gram and to compare the sensitivity to B deficiency of vegetative and reproductive organs. Boron deficiency mas induced by transferring plants from B adequate to B inadequate solutions: control plants received adequate B continuously.

In the second experiment, plants mere transferred to culture solutions with 1 and 10 µM H3BO3 on D15 and in the third experiment to solutions with 0 and 10 µM H3BO3 on D40 so that symptoms of B deficiency in root developed on D22 and D44 when the plants were in early vegetative growth and early podding, respectively. Plants of the second experiment mere germinated 21 days after those of the third experiment so that B deficiency developed in the plants of both experiments at the same time and under identical environmental conditions. In the third experiment, B mas resupplied to the nutrient solution of B deficient plants after 10 days' deprivation. The effects of deprivation and resupply of B on reproductive development mere followed by observation of the development of flowers and pods on individual inflorescences.

Within 2 days of symptoms appearing in the roots, B deficiency depressed LBER of the young leaves in both the vegetative plants of the second experiment and the reproductive plants of the third experiment. One day after LBER mas depressed in the third experiment, flowers commenced shedding from both B0 and B10 plants; 3 days later, pods commenced shedding from B0 plants and after a further 2 days from the B10 plants. B deficiency increased fourfold the number of shed flowers and pods, had no effect on the number of new flowers and slightly decreased the number of new pods. Abnormal aborted seeds were observed in pods of B0 plants. That LBER responded to B deficiency before flowers responded by shedding throws doubt on previous conclusions that the B requirement for reproductive growth is higher than that for vegetative growth. That critical B concentrations for expansion of young expanded leaf blades were 13 and 17 mg B kg^-1 DM in vegetative plants and 15 mg B kg^-1 DM in reproductive plants suggests that critical B concentrations for leaf blade expansion do not change with plant development stages from early vegetative growth to early podding. These values were nearly twice the critical B concentrations observed previously for black gram grown in the controlled environment room and the difference was tentatively attributed to the difference of light between the experiments.

The fourth experiment was undertaken to examine the effect of light on the critical B concentration for leaf blade elongation in black gram. Six days after germination (D6), seedlings were transferred from B adequate solutions to 8 pairs of pots: one pot of each pair contained 1 µM H3BO3 and the other 12 µM H3BO3. On D10, one day before the emergence of TF2, four pairs of pots mere shaded with black plastic mesh, decreasing the light intensity they received to about 35% of full sunlight (2000 (µmole m^-2 sec^-2 at noon) compared with 70% received by the unshaded plants, fill responses to B deficiency mere much less pronounced in shaded than in unshaded plants. Critical B concentrations for LBER in TF2 mere 10 and 15 mg B kg^-1 DM in shaded and unshaded plants respectively, supporting the previous suggestion that low light depressed the B requirement for leaf blade expansion. From a comparison of light environment of the present experiment with that in countries where black gram is grown commercially, it is suggested that the critical concentrations for diagnosis of B deficiency in black gram mill lie in the range 12 – 18 mg B kg^-1 DM. These values are recommended for diagnosis of B deficiency in the youngest fully expanded leaf (VFEL) in black gram.

A field experiment mas undertaken to test the validity of this critical range of B concentrations in the VFEL for deficiency diagnosis in black gram and to examine the possible value of using B concentration in the VFEL during growth for prognosis of seed yield at maturity.

Black gram seeds mere sown in September, 1987 (D0) at Chiang Mai, Thailand, on a silty loam with initial hot water soluble B level of 0.08 mg kg^-1 to which 6 levels of B fertilizer (0, 0.5, 1, 2, 4 and 8 kg borax ha^-1 referred to as B0, B0.5, B1, B2, B4 and B8, respectively) and basal fertilizer mere applied to 6 replicate blocks. The plots mere not irrigated. During vegetative growth from D4 to D29, 292 mm of rain fell, 11 mm fell from late vegetative growth (D30) to late pod development (D65), and 139 mm fell from then until final harvest at maturity (D92). The mean gravimetric soil water contents of all plots on D42, D50, D57 and D92 were 15, 11, 11 and 16%, respectively.

Symptoms of B deficiency developed in some plants of all treatments except B8. On the basis of their B deficiency symptoms on D48, replicate blocks were divided into 2 groups. In one group of 3 replicate blocks, no B deficiency symptom was observed in plants except in one plot. In the other group of 3 replicate blocks, B deficiency symptoms were observed in four treatments of each replicate block; within these replicate blocks, the severity of symptoms was not consistent with B treatment.

For each treatment, the average B concentration in the VFEL was highest at first inflorescence (D35), declined by 50% to first full pod (D58), remained steady to first mature pod (D70) and then increased by 50% to maturity (D90). Within each treatment at each harvest, the B concentrations of the VFEL of individual replicates varied widely. However, when all plots were rated for severity of B deficiency symptoms during pod development, the B concentrations in the VFEL of individual plots correlated closely with the severity of their symptoms.

Variations in soil water contents appeared to explain much of the variation in leaf B concentrations and B deficiency symptoms. The changes in mean leaf B concentrations in each treatment with time showed a pattern similar to that for the mean soil water content of all plots. With the exception of one replicate block on D42, the mean soil water contents of each of the 3 replicate blocks with widespread B deficiency symptoms were lower at every sampling than those of each of the 3 replicate blocks with few or no symptoms. With the exception of B8, within each treatment, the B concentration of the VFEL of the replicate plots at the first pod (D42) increased with increasing soil water content. These three phenomena suggest that low soil water depressed B uptake and enhanced B deficiency.

Boron deficiency depressed seed yield by increasing shedding of flowers and pods. Although seed yield at maturity increased progressively with increasing B supply, seed yield was very variable within each treatment. For all B treatments other than B8, this variability was attributed to the influence of soil water content on B uptake. However, seed yield was closely related to B concentration in the VFEL and B deficiency symptoms during plant development.

Critical values for both diagnosis and prognosis of B deficiency were derived from the relationship of leaf B concentrations to pod number and seed yield, respectively. The critical range of B concentrations in the VFEL for diagnosis of pod number at the first mature pod was 15 ± 2 mg B kg^-1 DM. At all developmental stages sampled, B deficiency symptoms did not appear in any plant with a B concentration in its VEEL of 14 or more mg B kg^-1 DM; all plants with 10 or less mg B kg^-1 DM had symptoms. These results from Chiang Mai support the suggestion from the glasshouse experiments that critical values for diagnosis of B deficiency in commercial black gram crops mould lie in the range 12 to 18 mg B kg^-1 DM of the VFEL. T

he critical B concentration in the VFEL for prognosis of maximum seed yield at maturity were 31 ± 3, 30 ± 4, 15 ± 1 and 9 ± 1 mg B kg^-1 DM at the first inflorescence, first pod, first full pod and first mature pod, respectively. The unusually low prognostic value of 9 ± 1 mg B kg^-1 DM in the VFEL at the first mature pod is attributed to the recovery of plants from B deficiency after sampling as the result of an increase in B uptake following rain after a drought period. This suggests that a dry period during crop growth will upset the value of leaf B concentrations for prediction of seed yield. Therefore, any critical concentration for prognosis of B deficiency affecting seed yield must be used with caution since the present results suggest that plant B status responds strongly and rapidly to soil mater conditions. However, for crops grown under continuously well watered condition, it is likely that leaf B concentrations mill be a useful guide for the prediction of B deficiency limiting seed yield.

Item Type:	Thesis (PhD)
Murdoch Affiliation(s):	School of Biological and Environmental Sciences
Notes:	Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s):	Loneragan, Jack, Bell, Richard and Dell, Bernard
URI:	http://researchrepository.murdoch.edu.au/id/eprint/51898

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