Photoperiod and Light Quality Effects on Cowpea Floral Development at High Temperatures

Abstract

High temperatures in tropical and subtropical zones often have detrimental effects on plants. Plants in these zones experience differences in daylength that could influence sensitivity to heat. Contrasting genotypes of cowpea, Vigna unguiculata (L.) Walp., were grown under fluorescent plus incandescent (F) or metal halide plus incandescent (MH) lamps at different daylengths (11, 12, 13, 14, or 16 h) to determine whether the sensitivity of floral development to high night temperatures is influenced by light quality and photoperiod. Floral bud development was suppressed in heat‐sensitive genotype (CB5) at a 14‐h photoperiod under MH, while a 16‐h photoperiod was required to elicit a similar response under F. Spectral analysis showed five times more ultraviolet‐A light (UV‐A) (315‐400 rim) in MH than in F, but F supplemented with UV‐A light did not elicit the same suppression of floral bud development as MH. The reproductive response to long days with hot nights (30 °C) was closer to that of field‐grown plants under MH than under F. Percent pod set of two sensitive genotypes (CB5 and 7964) subjected to high temperatures (33/30 °C day/night) was higher (23 and 19%) at an ll‐h photoperiod than at a 14‐h photoperiod (5%) under F. No pod set occurred in an ll‐h photoperiod with red light (R) during the night. Substantial pod set (41%) was observed a 14‐h photoperiod was followed by far‐red light (FR). The effect was reversed when FR was immediately followed by R. Apparently, pod set at high nightemperatures in heat‐sensitive, day‐neutral cowpea is dependent on photoperiod through a mechanism involving phytochrome.