Physiological Responses of Cassava (Manihot esculenta Crantz) to Drought Stress

Abstract

Cassava is the drought tolerant crop, however the heat and drought stresses in Thailand during 2013-2015 had caused the delay of crop calendar, replanting, shortening of growing period, and eventual yield variation among cultivars. To understand the responses of cassava varieties under water stress conditions basic information is needed. Three  cassava varieties: Rayong 11, Rayong 86-13 and CMR43-08-89 were used in the study of relationship between environmental elements (solar radiation, temperature and relative humidity) and photosynthesis parameters under water stress condition in Kamphaeng Phet Province in 2015. Results showed that, under water stress conditions all varieties had reached maximum net photosynthetic rate (P_n) at 8.00-9.00 a.m. with the values of 14.3- 18.7 µmolCO₂ m^-2 s^-1. After watering, all had reached maximum Pn at the same period but values were higher at 20.8- 24.8 µmolCO₂ m^-2 s^-1. The period of photosynthesis was limited with water stress as confirmed by 6 MAP data, that photosynthesis period was expanded to 10.00 a.m. during rainy season and all varieties showed higher Pn 22.5-26.1 µ molCO₂ m^-2 s^-1.  Rayong 86-13 with high starch content, showed smallest T_r during the day and stomatal conductance (g_s) highest in early morning and stomata close before the others. Pn showed positive correlation with transpiration rate(T_r), whereas T_r showed negative relation with relative humidity (RH), Photosynthetic Photon Flux Density (PPFD) showed positive relation with air temperature and Tr but negative relation with RH, significantly. Photosynthesis occurred during PPFD 150 – 2,300 mµol m^-2 s^-1. Under water stress condition, photosynthesis  occurred during low PPFD and high RH, but would occur for longer period in non-water stress condition, although low RH 32-40%.