Weight Loss At Night? Morphological and Night-Time Physiological Factors That Negatively Impact Biomass Accumulation in Rice.

Plant biomass accumulation is the culmination of many processes and current initiatives to investigate photosynthetic rates are considered a novel frontier to improve grain and biomass yields. We previously identified a negative relationship between total biomass and photosynthetic rates in diverse rice varieties. This negative correlation has been observed in other crop and weed species, however there is no mechanistic explanation of this phenomenon. We took detailed physiological measures in a growth chamber experiment of 20 diverse rice varieties known to differ in biomass. We measured gas exchange in both day and night, chlorophyll fluorescence in light- and dark-adapted plants, chlorophyll content, and stomatal density. These data revealed significant phenotypic variation among the 20 lines in all traits measured. Total biomass was negatively correlated with night-time stomatal conductance, transpiration, dark respiration and chlorophyll content. The 20 lines varied as much as 5.7-fold for night-time stomatal conductance, and night-time transpiration rates were between 4 and 26% that of day-time and thus represent a substantial fraction of total daily water loss for some lines. Our work indicates that simply increasing day-time carbon assimilation rates may not be sufficient. A more comprehensive approach will also target carbon metabolism, respiration and water losses due to night-time transpiration.