Abstract
From previous glasshouse experiments, 12-18 mg B kg-1 dry wt in the youngest fully expanded leaf blade (YFEL) represents the minimum leaf boron (B) requirement for unrestricted leaf blade elongation (LBE) in black gram (Vigna mungo). A field experiment at Chiang Mai, Thailand was undertaken to test the validity of this B concentration range for diagnosis of B deficiency and to examine the possibility of using B concentration in the YFEL during growth for prognosis of seed yield at maturity. Six levels of B fertilizer (0, 0.5, 1, 2, 4 and 8 kg borax ha-1) and basal fertilizers were applied to 6 replicates. The critical B concentration for diagnosis of pod number in the YFEL at the first mature pod was 13-17 mg B kg- I dry wt being within the range established from the glasshouse experiments. Thus, the functional B requirement for LBE, 12-18 mg B kg-I dry wt in the YFEL, can be recommended for diagnosis of B deficiency in black gram. The critical B ranges in the YFEL for prognosis of seed yield were 31 ± 3, 30 ± 4, 15 ± 1, and 9 ± 1 mg B kg-1 dry wt at the first inflorescence, first pod, first full pod and first mature pod, respectively. The unusually low value of 9 ± 1 mg B kg-1 dry wt is attributed to the recovery of B deficient plants as the result of an increase in B uptake following rain after a drought period. Thus, the use of prognostic values of B deficiency can be limited by the probability of rain after sampling. Despite this limitation, critical values for the prognosis of B deficiency can be used as a guide to the possibility of B deficiency depressing final yield.