Effect of incandescent and fluorescent lighting used in photoperiod extension on the vegetative growth and floral development of four lines of pigeonpea

Abstract

The effects of light quality, as used in photoperiod extension, on vegetative growth and floral development of pigeonpea [ Cajanus cajan (L.) Millsp.] were studied using three photoperiod-insensitive lines, QPL2, Prabhat, and Hunt, and one photoperiod-sensitive line, Royes. Plants were grown in controlled-environment cabinets under day/night temperature regimes of 24/16°C (low) and 28/24°C (high) for 72 days after emergence (DAE). The five light treatments consisted of 12 and 16-h photoperiods and a 12-h photoperiod extended to 16 h with low-intensity lighting from either incandescent (I), fluorescent (F), or incandescent plus fluorescent (I+F) lamps. Little genetic variation occurred in vegetative response (main stem length and node number, leaf area and shoot dry weight) to extension light source. Response varied with temperature, plant age and growth stage. Main stem node appearance and stem extension tended to decline following floral initiation (FI), whereas leaf area and shoot dry weight accumulation increased rapidly after 35 or 42 DAE, irrespective of whether plants were vegetative or floral. Plants were larger under the high-temperature regime. Temperature altered plant response to extension light source. I-extension promoted growth under both temperature regimes, whilst I+F promoted growth under high temperatures but was unexpectedly inhibitory under low. Under high temperatures F extension was inhibitory prior to FI, but promoted later growth during floral bud development (FBD). Under low temperatures, F-extension was not inhibitory during the pre-initiation phase. The four genotypes differed in their floral response to extension light source, the effect on initiation and development of floral primordia varying dependent upon temperature. Low temperatures resulted in sensitivity to extension light source, delaying FI and FBD. In Royes, failure to initiate under low temperatures was associated with small plant size (≤3.5 g shoot dry weight). High temperatures tended to negate the delaying effects of extension light source, but resulted in photoperiod sensitivity, delaying FI in the early-flowering lines QPL-2 and Prabhat. Under high temperatures, Royes was insensitive (FI and FBD) to extensions containing incandescent light (I, I+F). Under high temperatures, time of flowering was least affected by incandescent extensions (I, I+F), and most by F-extension, which delayed FBD in all lines. Under low temperatures, I+F extensions resulted in the greatest variation in flowering time. It was concluded that where heterogeneity exists in floral response to the intensity and/or quality of light, use of artificial lighting to select for photoperiod sensitivity may lead to misclassification.