Foliar boron compounds applications mitigate heat stress caused by high daytime temperatures in rice (Oryza sativa L.) Boron mitigates heat stress in rice

The objective of this work was to study the effect of foliar boric acid (BA) or sodium borate (SB) sprays on rice plants subjected to high daytime temperature. The established treatments were the following: i) seedlings with heat stress (40 degrees C) and treated with foliar SB sprays at three rates 25, 50 or 100 mg of SB L-1, ii) seedlings with heat stress and with foliar BA sprays at three rates 25, 50 or 100 mg of BA L-1, iii) seedlings without heat stress (28 degrees C) and foliar boron (B) sprays (absolute control-AB) and iv) seedlings with heat stress and any foliar B compounds treatments (HT), resulting a total of eight group of treatments (AB, HT, SB25, SB50, SB100, BA25, BA50 and BA100). The results showed that, in heat stress condition, foliar BA (25, 50 or 100 mg L-1, respectively) or SB (50 mg L-1) sprays significantly increased the values of net photosynthesis compared to untreated plants (19.7 mu mol CO2 m(-2) s(-1) with B 14.4 mu mol CO2 m(-2) s(-1) without B). The foliar B compounds sprays caused an increase on photochemical efficiency of PSII (0.76 with B vs. 0.72 without B) and lower oxidative damage (expressed as leaf malondialdehyde and proline production) in seedlings under heat stress. The results in the present study allow concluding that the use of B (BA or SB) can be considered as an agronomic strategy to mitigate the negative impact of heat stress in rice when high daytime stress periods are expected in rice areas.