Influence of Light Regime on the Toxicity and Physiological Activity of Herbicides.

The responlse of plants to certain herbicides is inifltieniced by the light coniditions precedinig or followinig applicatioln of the herbicide (14, 15, 16. 18, 21t) but the physiological basis for the differential activity of certaini herbicidal compounds in relation to the light conditions tunider which they are applied is uincertaini in most cases. This paper deals witlh 2 sutch lighlt-infltuence(d responises of plants to herbicides. Onle series of experimiients was perforn,ed to study an observation first reported by Salisbury (21) that plants treated with 2 4-dinitrophenol (DNP) are considerably more injured when the plants receive darkness after treatment than when they are exposed to light following application of the compound. Another series of investigations deals with the physiological activity of sodiumii 2, 4-dichlorophenoxyacetic acid (2, 4-D) in pllants as iniflueenced by uiltraviolet light anid the light regime to which plants are subjected after UVirra(liation and auxin treatmenit. There are many reports in the literature concerninig the effects of DNP on photosynthesis and photophosphorylationl (2, 17, 26), respiration anid oxi(lative phosphorylatiol (i 5 7), and other energyrequiring processes involvingo hi-h eniergy phosphate comlpounds (1, 19). In such stutdies the toxicity of DNP anid related col)ipounids is often marked! y infltueniced by the pH at which the material is applied to plants (23), an(d the carrier in which the material is applied (8). Salisbury (21) has suggested that the physiological basis for the differential toxicity of DNP to plants suibjected to light or dlarkness after DNP treatment may lie in a difference in senisitivity of the oxi(lattive and photosynithetic phosphorylationi mechaniismiis to DNP. Sinice DNP is more phytotoxic wheni its application is followed by (larkiess, D)NP mlay be inhibiting oxi(lative phosphory! ation but lnot photoplosphorytatioll. T hus, the level of ATP in plaints left in the (lark after DNP treatment wvould be lowered anid result in observable plant damage, but plants treated anid left in the light might avoid injury. Ohmura (17) and Wessels (26) have inideed demiionstrate(d a pathway of photoIhosphorylation whiclh is inisenisitive to DNP at concenitrations which inhibit oxi(lative phosphorylationi. De Zeeuw and Leopold (10) have reported that tomato plants irradiated with UV light for short periods of time do not exhibit the normal epinastic responses following auxini (iiapthalene acetic acid) treatment. IJltraviolet radiationi given before auxini treatment was more effective in preventinig epinasty in atuxin-treated plants than UV radiation given after auxini treatmiient. In other experimiients they (letermiined that UV ra(liation did not significantly affect the uptake of auxini. They concluded that the prevention of auxin responises in UV irradiate(l plants was due to a preventioni of auxin actioni insi(le the plants. It has beeni noted freqtuently that the physiological anid histological effects of UV on plant anid animal tissues can be overcomiie or reversed by wlhite light. For example, the inhibitory effects of UN' light oni such videly diverse processes as chlorophyll destruction (25), inactivation of bacteriophages (11), and growth of fuingi (12) can be reversedl by white light followving UV radiation. The mechanism of this photoreversal is still obscure, and experimental restults are diffictult to interpret (ltle to the multiplicity of physiological processes in plants affecte(d 1w UNV radiation. The present work was conducted to elaborate uponl these light-infltuenced responises of pllants to 2. 4-D, DNP, anid related comiipotunids alld shed(l ne\\ light oln the plhysiological basis for the phytotoxicity of these compounds.